1WTNT44 KNHC 082048 2TCDAT4 3 4Tropical Storm Isaac Discussion Number 5 5NWS National Hurricane Center Miami FL AL092018 6500 PM AST Sat Sep 08 2018 7 8The structure of the cyclone, while still a shear pattern, is 9improving, with the low-level center now near the convective 10overcast and improved outflow in the eastern semicircle. Satellite 11intensity estimates have increased to the 30-45 kt range, with the 12CIMSS satellite consensus technique now suggesting winds near 35 13kt. Based on these estimates, the initial intensity is increased to 1435 kt and the depression is upgraded to Tropical Storm Isaac. 15 16Isaac is now moving slowly westward, with the initial motion 275/6. 17The subtropical ridge to the north of Isaac is forecast to steer the 18system westward with an increasing forward speed for at least the 19next 2-3 days. After that time, there is some uncertainty in the 20track forecast due to the formation of a mid- to upper-level trough 21over the central Atlantic, with some of the track guidance showing a 22decrease in forward speed and a more northerly motion near the end 23of the forecast period. The GFS and ECMWF models show a continued 24westerly motion through that time, and the official forecast follows 25that scenario, although with a slower forward speed than in the 26previous forecast. The new forecast is again similar to the 27earlier forecast and lies near the various consensus models. 28 29Conditions appear favorable for gradual strengthening during the 30next 72 h as Isaac moves westward across the warm waters of the 31tropical Atlantic. In best agreement with the SHIPS model for the 32next three days, the new intensity forecast is raised from the 33previous forecast during this time. From 72-120 h, the system is 34expected to encounter westerly wind shear, which is expected to 35cause some weakening. 36 37 38FORECAST POSITIONS AND MAX WINDS 39 40INIT 08/2100Z 14.5N 36.6W 35 KT 40 MPH 41 12H 09/0600Z 14.6N 37.7W 40 KT 45 MPH 42 24H 09/1800Z 14.7N 39.5W 50 KT 60 MPH 43 36H 10/0600Z 14.7N 42.0W 60 KT 70 MPH 44 48H 10/1800Z 14.7N 44.6W 65 KT 75 MPH 45 72H 11/1800Z 14.5N 50.0W 75 KT 85 MPH 46 96H 12/1800Z 14.5N 55.5W 75 KT 85 MPH 47120H 13/1800Z 15.0N 61.0W 70 KT 80 MPH 48 49$$ 50Forecaster Beven 51 52 53 54------------=_1536439724-46358-5315 55Content-Type: text/plain; charset="US-ASCII" 56Content-Disposition: inline 57Content-Transfer-Encoding: 7BIT 58 59= = = == = = 60WTNT44 KNHC 090242 61TCDAT4 62 63Tropical Storm Isaac Discussion Number 6 64NWS National Hurricane Center Miami FL AL092018 651100 PM AST Sat Sep 08 2018 66 67Isaac is strengthening this evening. Satellite images indicate that 68the deep convection has been increasing in intensity and coverage 69with better defined banding features. The center is not located in 70the center of the convection, however, due to some easterly shear. 71An ASCAT pass around 00Z showed maximum winds in the 40-45 kt 72range. These data are also in line with the latest Dvorak 73classifications of 3.0/45 kt from TAFB and SAB. Based on these 74estimates, the initial intensity is raised to 45 kt. 75 76Isaac will likely continue to strengthen during the next few days as 77the storm remains over warm waters and moves into an environment of 78decreasing wind shear. Beyond a few days, the SHIPS model shows a 79notable increase in shear, in part due to the outflow from Florence, 80which should end the strengthening trend and cause some weakening. 81With the exception of the HMON and COAMPS-TC models, the remainder 82of the intensity guidance is higher this cycle. The NHC intensity 83forecast is raised from the previous one, but it is a little lower 84than the HCCA and IVCN guidance. 85 86The storm is moving due westward at 7 kt. The track forecast seems 87fairly straightforward. A strengthening subtropical ridge to the 88north of the system should cause Isaac to move westward at an 89increasing forward speed during the next several days. This 90scenario is supported by the usually more reliable GFS and ECMWF 91models, and the NHC track forecast is near a blend of those aids. 92Based on the current forecast, Isaac will be near the Lesser 93Antilles in 4 to 5 days and interests there should monitor the 94progress of this system. 95 96FORECAST POSITIONS AND MAX WINDS 97 98INIT 09/0300Z 14.4N 37.5W 45 KT 50 MPH 99 12H 09/1200Z 14.4N 38.7W 50 KT 60 MPH 100 24H 10/0000Z 14.5N 40.9W 60 KT 70 MPH 101 36H 10/1200Z 14.5N 43.5W 70 KT 80 MPH 102 48H 11/0000Z 14.5N 46.1W 75 KT 85 MPH 103 72H 12/0000Z 14.5N 51.5W 85 KT 100 MPH 104 96H 13/0000Z 14.6N 57.0W 80 KT 90 MPH 105120H 14/0000Z 15.0N 62.9W 75 KT 85 MPH 106 107$$ 108Forecaster Cangialosi 109 110 111 112------------=_1536460933-46358-5502 113Content-Type: text/plain; charset="US-ASCII" 114Content-Disposition: inline 115Content-Transfer-Encoding: 7BIT 116 117= = = == = = 118WTNT44 KNHC 090840 119TCDAT4 120 121Tropical Storm Isaac Discussion Number 7 122NWS National Hurricane Center Miami FL AL092018 123500 AM AST Sun Sep 09 2018 124 125The cloud pattern of Isaac has changed significantly during the past 126several hours. The storm no longer has a sheared appearance, and 127instead microwave images reveal a well-defined curved band that 128wraps a little more than halfway around the center. Despite the 129change in the storm's structure, the Dvorak classifications are 130unchanged at 3.0/45 kt. Based on that data, the initial intensity 131is held at 45 kt for this advisory. 132 133Now that the shear has lessened, Isaac should steadily strengthen 134during the next few days as the storm remains over warm waters and 135in a low wind shear environment. Beyond a few days, however, the 136SHIPS model shows a notable increase in shear, in part due to the 137outflow from Florence, which should end the strengthening trend and 138induce some weakening. There was no significant change in the 139intensity guidance this cycle, so the official forecast is largely 140an update of the previous one. This prediction lies fairly 141close to the IVCN and HCCA models. 142 143Isaac remains on the forecast track and the initial motion is the 144same as before, 270/7. A strengthening subtropical ridge to the 145north of the system should cause Isaac to move westward at an 146increasing forward speed during the next several days. Although 147there is some spread in the models, the usually more reliable GFS 148and ECMWF models have been consistent in showing a due westward 149motion through the forecast period. The NHC track forecast leans 150toward those models, and it is fairly similar to the previous 151prediction. Based on the current forecast, Isaac will be near the 152Lesser Antilles in 4 to 5 days and interests there should monitor 153the progress of this system. 154 155FORECAST POSITIONS AND MAX WINDS 156 157INIT 09/0900Z 14.5N 38.1W 45 KT 50 MPH 158 12H 09/1800Z 14.6N 39.6W 55 KT 65 MPH 159 24H 10/0600Z 14.7N 41.9W 60 KT 70 MPH 160 36H 10/1800Z 14.7N 44.6W 70 KT 80 MPH 161 48H 11/0600Z 14.7N 47.5W 80 KT 90 MPH 162 72H 12/0600Z 14.7N 53.1W 85 KT 100 MPH 163 96H 13/0600Z 14.9N 58.9W 80 KT 90 MPH 164120H 14/0600Z 15.3N 64.5W 70 KT 80 MPH 165 166$$ 167Forecaster Cangialosi 168 169 170 171------------=_1536482450-46358-5638 172Content-Type: text/plain; charset="US-ASCII" 173Content-Disposition: inline 174Content-Transfer-Encoding: 7BIT 175 176= = = == = = 177WTNT44 KNHC 091433 178TCDAT4 179 180Tropical Storm Isaac Discussion Number 8 181NWS National Hurricane Center Miami FL AL092018 1821100 AM AST Sun Sep 09 2018 183 184WindSat microwave data from several hours ago show that Isaac has 185developed a well-defined low-level ring in the 37-GHz channel, and 186the center is embedded in the middle of a small central dense 187overcast. TAFB and SAB Dvorak estimates have increased to T3.5 188given Isaac's improved structure, so the initial intensity is raised 189to 55 kt. 190 191Isaac's future intensity is the most challenging part of the 192forecast. Since sea surface temperatures will be warm (between 19327-28 degrees Celsius) and vertical shear will be almost 194non-existent (less than 10 kt) for the next 36-48 hours, the tiny 195storm will be prone to a possibly significant increase in intensity 196during the next couple of days. For that period, the NHC intensity 197forecast is above the guidance, and above the previous forecast, due 198to relatively high numbers from the various rapid intensification 199indices. However, in 2-3 days, the global models are showing 200northwesterly shear steadily increasing and potentially reaching a 201maximum of around 30 kt by Thursday. The big question is the 202timing of this shear and how much it will affect Isaac's intensity 203before the cyclone reaches the Lesser Antilles. The HWRF, which 204brings Isaac to major hurricane intensity despite the forecast 205shear, appears to be an outlier, and is therefore pulling up the 206ICON intensity consensus and HCCA. As a result, the NHC intensity 207forecast is below ICON on days 4 and 5. This new forecast shows a 208more peaked maximum intensity, with Isaac possibly increasing and 209then decreasing in strength at faster rates than shown here. 210 211While the microwave data indicate that Isaac's center is slightly 212south of previous estimates, the current motion estimate is 270/8 213kt. A low- to mid-level ridge is expected to move westward in 214tandem with Isaac for much of the forecast period, causing the 215cyclone to accelerate toward the west, reaching a forward speed of 216around 15 kt by 36 hours, and then holding that speed through day 2175. With the exception of the UKMET and HWRF models, the guidance 218envelope is tightly packed from north to south, and the speed 219differences noted over the past few days have decreased. The 220updated NHC track forecast is just a little south of the previous 221one to account for the adjusted initial position, and lies slightly 222south of the multi-model consensus in the direction of HCCA and the 223Florida State Superensemble. 224 225FORECAST POSITIONS AND MAX WINDS 226 227INIT 09/1500Z 14.3N 39.1W 55 KT 65 MPH 228 12H 10/0000Z 14.4N 40.8W 65 KT 75 MPH 229 24H 10/1200Z 14.4N 43.3W 75 KT 85 MPH 230 36H 11/0000Z 14.4N 46.3W 85 KT 100 MPH 231 48H 11/1200Z 14.3N 49.1W 85 KT 100 MPH 232 72H 12/1200Z 14.2N 54.8W 80 KT 90 MPH 233 96H 13/1200Z 14.5N 60.5W 70 KT 80 MPH 234120H 14/1200Z 15.0N 66.5W 60 KT 70 MPH 235 236$$ 237Forecaster Berg 238 239 240 241------------=_1536503597-46358-5815 242Content-Type: text/plain; charset="US-ASCII" 243Content-Disposition: inline 244Content-Transfer-Encoding: 7BIT 245 246= = = == = = 247WTNT44 KNHC 092032 248TCDAT4 249 250Tropical Storm Isaac Discussion Number 9 251NWS National Hurricane Center Miami FL AL092018 252500 PM AST Sun Sep 09 2018 253 254Isaac is a tiny tropical cyclone, but even still, recent SSMIS 255data showed that it has developed a mid-level microwave eye 256feature. At the time of the pass (1800 UTC), the convection was a 257little thin on the southern side, but geostationary satellite images 258suggest that it has filled in since that time. Dvorak intensity 259estimates are T4.0 from TAFB and T3.5 from SAB, so the maximum 260winds are estimated to be 60 kt. 261 262The intensity forecast philosophy has not changed since this 263morning. Isaac is strengthening in an environment of low shear and 264over warm sea surface temperatures, and for the next 36-48 hours, 265the NHC intensity forecast is above the bulk of the intensity 266models. The main reason for this is that Isaac's tiny size could 267allow the intensity to increase quickly within the favorable 268environment. However, the cyclone's tiny size will likely also be 269to its detriment after 48 hours when northwesterly vertical shear is 270expected to develop and increase to 20-30 kt. The shear will 271probably easily decouple the small system, causing the intensity to 272decrease much faster than suggested by most of the intensity models. 273For that reason, the official NHC intensity forecast is below the 274intensity consensus on days 3 through 5. It should be noted that 275both the GFS and ECMWF models show a weakening cyclone moving into 276the eastern Caribbean Sea by days 4 and 5, with the GFS even making 277the system an open wave by the end of the forecast period. These 278global model solutions lend credence to the belief that it may be 279difficult for Isaac to maintain hurricane status while it approaches 280the Lesser Antilles. 281 282Isaac is accelerating toward the west with an initial motion of 283275/10 kt. Ridging to the north is expected to keep the cyclone on 284a westward trajectory for the entire forecast period, with 285acceleration continuing for the next 36 hours. The UKMET remains 286the biggest outlier, showing Isaac turning northwestward and 287northward into the central Atlantic after 48 hours. That still 288appears to be unlikely at this time, and the NHC track forecast is 289still close to the center of the guidance envelope. Only a slight 290northward shift was made to the new forecast based on the latest 291model solutions. 292 293 294Key Messages: 295 2961. Isaac is expected to become a hurricane tonight or on Monday 297while it moves westward across the tropical Atlantic Ocean. 298 2992. While Isaac is forecast to begin weakening by Tuesday while it 300approaches the Lesser Antilles, it is still expected to be at or 301near hurricane intensity when it reaches the islands, and the 302uncertainty in the intensity forecast is higher than usual. 303Interests in the Lesser Antilles should continue to monitor Isaac 304during the next few days. 305 306 307FORECAST POSITIONS AND MAX WINDS 308 309INIT 09/2100Z 14.5N 40.3W 60 KT 70 MPH 310 12H 10/0600Z 14.6N 42.2W 70 KT 80 MPH 311 24H 10/1800Z 14.8N 45.0W 80 KT 90 MPH 312 36H 11/0600Z 14.9N 48.0W 85 KT 100 MPH 313 48H 11/1800Z 14.9N 50.9W 85 KT 100 MPH 314 72H 12/1800Z 15.0N 56.5W 70 KT 80 MPH 315 96H 13/1800Z 15.5N 63.0W 60 KT 70 MPH 316120H 14/1800Z 15.5N 69.5W 50 KT 60 MPH 317 318$$ 319Forecaster Berg 320 321 322 323------------=_1536525171-46358-6065 324Content-Type: text/plain; charset="US-ASCII" 325Content-Disposition: inline 326Content-Transfer-Encoding: 7BIT 327 328= = = == = = 329WTNT44 KNHC 100256 330TCDAT4 331 332Hurricane Isaac Discussion Number 10 333NWS National Hurricane Center Miami FL AL092018 3341100 PM AST Sun Sep 09 2018 335 336Isaac's IR satellite presentation has generally improved since the 337last advisory. While there is no evidence of an eye in conventional 338satellite imagery, the most recent microwave imagery around 2100 UTC 339indicated the presence of a pinhole eye. Dvorak intensity estimates 340are T4.0 from both TAFB and SAB, and on this basis Isaac has been 341upgraded to a hurricane, the 5th of the 2018 Atlantic season and 3rd 342in the past few days. 343 344I must stress that all aspects of the forecast are highly uncertain 345at this time. The dynamical track model spread is nearly 1000 miles 346at day 5, while the intensity guidance spread is nearly 100 kt. 347Although the NHC track and intensity forecasts have not been changed 348very much, this should not be interpreted as increasing confidence 349in the forecast. 350 351With regard to track, there is a clear split in the model guidance 352regarding the speed of Isaac. The GFS and ECMWF models forecast that 353a mid-level ridge to the north will accelerate the hurricane 354westward for the next 36 h, with a westward motion continuing 355through day 5 as the ridge is reinforced in the wake of Florence. On 356the other hand, the UKMET and HWRF models show a weakness developing 357in the ridge which causes Isaac to move much slower, and eventually 358turn northward. The NHC forecast has been favoring the former 359solution, and continues to do so with this advisory since I see no 360reason to make a drastic change to the forecast at this point. 361 362Isaac is a very small hurricane, which makes the intensity forecast 363particularly tricky since small storms are susceptible to rapid 364changes of intensity, up and down. Since the wind shear is expected 365to remain low, and Isaac is forecast to remain over fairly warm SSTs 366for the next 48 h, the current forecast of further intensification 367seems reasonable, and the NHC forecast is still at the top end of 368the guidance envelope. From 72 h onward, there could be a sudden, 369and substantial increase in wind shear associated first with an 370upper-level trough to the north, and later with the outflow of 371Hurricane Florence. Assuming this shear affects Isaac as expected, 372the tiny hurricane would likely weaken quickly, perhaps even faster 373than currently shown in the forecast. By the end of the forecast 374period, the NHC forecast remains a fair amount below the intensity 375consensus, closer to the weakening solutions of the GFS and ECMWF. 376 377Key Messages: 3781. Isaac is a small hurricane and uncertainty in the forecast is 379higher than normal. Although Isaac is forecast to begin weakening 380by Tuesday when it approaches the Lesser Antilles, it is still 381expected to be at or near hurricane intensity when it reaches the 382islands. 383 3842. Interests in the Lesser Antilles should continue to monitor Isaac 385during the next few days. 386 387FORECAST POSITIONS AND MAX WINDS 388 389INIT 10/0300Z 14.5N 41.6W 65 KT 75 MPH 390 12H 10/1200Z 14.7N 43.5W 75 KT 85 MPH 391 24H 11/0000Z 14.9N 46.3W 80 KT 90 MPH 392 36H 11/1200Z 15.0N 49.3W 85 KT 100 MPH 393 48H 12/0000Z 15.0N 52.0W 85 KT 100 MPH 394 72H 13/0000Z 15.2N 57.3W 75 KT 85 MPH 395 96H 14/0000Z 15.5N 63.5W 60 KT 70 MPH 396120H 15/0000Z 15.5N 70.0W 50 KT 60 MPH 397 398$$ 399Forecaster Zelinsky 400 401 402 403------------=_1536548182-46358-6326 404Content-Type: text/plain; charset="US-ASCII" 405Content-Disposition: inline 406Content-Transfer-Encoding: 7BIT 407 408= = = == = = 409WTNT44 KNHC 100840 410TCDAT4 411 412Hurricane Isaac Discussion Number 11 413NWS National Hurricane Center Miami FL AL092018 414500 AM AST Mon Sep 10 2018 415 416Isaac has generally changed little during the past several hours. 417The small hurricane continues to have a central dense overcast 418feature with some fragmented bands to the south of the center. The 419Dvorak classifications remain 4.0/65 kt from both TAFB and SAB, and 420therefore, the initial intensity is held at 65 kt. 421 422Isaac is moving westward at 11 kt. The models remain in poor 423agreement this cycle as they are split into two camps. The UKMET, 424HWRF, and a few other models show Isaac recurving near or to the 425east of the Lesser Antilles toward a developing trough over the 426central Atlantic. Conversely, the GFS and ECMWF have been very 427consistent in showing a due westward motion across the Lesser 428Antilles and into the Caribbean as the cyclone moves on the south 429side of a strengthening mid-level ridge. The NHC track forecast 430continues to favor the usually more reliable GFS and ECMWF models, 431and this forecast is near a blend of those aids. It should be 432noted, however, that confidence in this track forecast is low given 433the model spread. 434 435The intensity forecast is also quite uncertain. One of the complex 436factors is Isaac's compact size as small hurricanes like this one 437are more prone to quick intensity changes, both up and down. In 438addition, the intensity guidance is in poor agreement as solutions 439range from steady weakening from this point onward to the HWRF that 440shows Isaac becoming a major hurricane. Given that the wind shear 441is expected to remain less than 10 kt for another day or two, 442strengthening is forecast during that time period. After that, 443however, an increase in shear first from a trough over the central 444Atlantic and later from the outflow from Florence should result in 445weakening when the system moves across the Lesser Antilles and into 446the Caribbean Sea. The NHC intensity forecast is not too different 447from the previous one and lies above the model consensus in the 448short term, but below it at the later forecast times. This 449forecast is in best agreement with the latest LGEM guidance. 450 451Key Messages: 452 4531. Isaac is a small hurricane and uncertainty in the forecast is 454higher than normal. Although Isaac is forecast to begin weakening 455when it approaches the Lesser Antilles, it is still expected to be 456at or near hurricane intensity when it reaches the islands. 457 4582. Interests in the Lesser Antilles should continue to monitor Isaac 459during the next few days. 460 461FORECAST POSITIONS AND MAX WINDS 462 463INIT 10/0900Z 14.7N 42.7W 65 KT 75 MPH 464 12H 10/1800Z 14.8N 44.6W 75 KT 85 MPH 465 24H 11/0600Z 15.0N 47.4W 75 KT 85 MPH 466 36H 11/1800Z 15.0N 50.3W 80 KT 90 MPH 467 48H 12/0600Z 15.0N 53.0W 75 KT 85 MPH 468 72H 13/0600Z 15.3N 58.0W 65 KT 75 MPH 469 96H 14/0600Z 15.5N 64.0W 50 KT 60 MPH 470120H 15/0600Z 15.6N 69.2W 40 KT 45 MPH 471 472$$ 473Forecaster Cangialosi 474 475 476 477------------=_1536568855-46358-6460 478Content-Type: text/plain; charset="US-ASCII" 479Content-Disposition: inline 480Content-Transfer-Encoding: 7BIT 481 482= = = == = = 483WTNT44 KNHC 100914 CCA 484TCDAT4 485 486Hurricane Isaac Discussion Number 11...Corrected 487NWS National Hurricane Center Miami FL AL092018 488500 AM AST Mon Sep 10 2018 489 490Corrected intensity forecast in table below. 491 492Isaac has generally changed little during the past several hours. 493The small hurricane continues to have a central dense overcast 494feature with some fragmented bands to the south of the center. The 495Dvorak classifications remain 4.0/65 kt from both TAFB and SAB, and 496therefore, the initial intensity is held at 65 kt. 497 498Isaac is moving westward at 11 kt. The models remain in poor 499agreement this cycle as they are split into two camps. The UKMET, 500HWRF, and a few other models show Isaac recurving near or to the 501east of the Lesser Antilles toward a developing trough over the 502central Atlantic. Conversely, the GFS and ECMWF have been very 503consistent in showing a due westward motion across the Lesser 504Antilles and into the Caribbean as the cyclone moves on the south 505side of a strengthening mid-level ridge. The NHC track forecast 506continues to favor the usually more reliable GFS and ECMWF models, 507and this forecast is near a blend of those aids. It should be 508noted, however, that confidence in this track forecast is low given 509the model spread. 510 511The intensity forecast is also quite uncertain. One of the complex 512factors is Isaac's compact size as small hurricanes like this one 513are more prone to quick intensity changes, both up and down. In 514addition, the intensity guidance is in poor agreement as solutions 515range from steady weakening from this point onward to the HWRF that 516shows Isaac becoming a major hurricane. Given that the wind shear 517is expected to remain less than 10 kt for another day or two, 518strengthening is forecast during that time period. After that, 519however, an increase in shear first from a trough over the central 520Atlantic and later from the outflow from Florence should result in 521weakening when the system moves across the Lesser Antilles and into 522the Caribbean Sea. The NHC intensity forecast is not too different 523from the previous one and lies above the model consensus in the 524short term, but below it at the later forecast times. This 525forecast is in best agreement with the latest LGEM guidance. 526 527Key Messages: 528 5291. Isaac is a small hurricane and uncertainty in the forecast is 530higher than normal. Although Isaac is forecast to begin weakening 531when it approaches the Lesser Antilles, it is still expected to be 532at or near hurricane intensity when it reaches the islands. 533 5342. Interests in the Lesser Antilles should continue to monitor Isaac 535during the next few days. 536 537 538FORECAST POSITIONS AND MAX WINDS 539 540INIT 10/0900Z 14.7N 42.7W 65 KT 75 MPH 541 12H 10/1800Z 14.8N 44.6W 70 KT 80 MPH 542 24H 11/0600Z 15.0N 47.4W 75 KT 85 MPH 543 36H 11/1800Z 15.0N 50.3W 80 KT 90 MPH 544 48H 12/0600Z 15.0N 53.0W 80 KT 90 MPH 545 72H 13/0600Z 15.3N 58.0W 70 KT 80 MPH 546 96H 14/0600Z 15.5N 64.0W 60 KT 70 MPH 547120H 15/0600Z 15.6N 69.2W 50 KT 60 MPH 548 549$$ 550Forecaster Cangialosi 551 552 553 554------------=_1536570895-46358-6496 555Content-Type: text/plain; charset="US-ASCII" 556Content-Disposition: inline 557Content-Transfer-Encoding: 7BIT 558 559= = = == = = 560WTNT44 KNHC 101436 561TCDAT4 562 563Hurricane Isaac Discussion Number 12 564NWS National Hurricane Center Miami FL AL092018 5651100 AM AST Mon Sep 10 2018 566 567Isaac's cloud pattern is currently a bit ragged-looking, with an 568irregular CDO and limited convective banding. Dvorak intensity 569estimates from both TAFB and SAB are 65 kt, and that value will be 570used for the advisory. There is significant uncertainty in the 571intensity forecast for this system. Vertical wind shear over the 572hurricane is forecast to be weak to moderate during the next day or 573so, which should allow Isaac to strengthen somewhat. By 48 hours, 574however, the global models depict significantly stronger 575northwesterly shear, associated with an upper-level trough over the 576central Atlantic, affecting the tropical cyclone. Most of the 577guidance does not show Isaac strengthening over the Caribbean. The 578HWRF model is an outlier, strengthening Isaac into a major hurricane 579later in the period, and it is the main contributor to high 580consensus forecasts at days 3-5. The NHC intensity forecast is 581above the model consensus through 72 hours and below it by the end 582of the period. 583 584Isaac continues to move westward, or 275/12 kt. A subtropical 585ridge should be maintained to the north of the tropical cyclone for 586the next several days, so a continued westward track seems likely. 587The U. K. Met. Office model is an outlier and shows Isaac turning 588northwestward and northward to the northeast of the Leeward Islands. 589The official track forecast, like the previous one, follows the 590other reliable global models and is also close to the HFIP corrected 591consensus guidance. 592 593Key Messages: 594 5951. Isaac is a small hurricane and uncertainty in the forecast is 596greater than normal. Although Isaac is forecast to begin weakening 597when it approaches the Lesser Antilles, it is still expected to be 598at or near hurricane intensity when it reaches the islands. 599 6002. Interests in the Lesser Antilles should continue to monitor Isaac 601during the next few days. 602 603FORECAST POSITIONS AND MAX WINDS 604 605INIT 10/1500Z 14.7N 43.9W 65 KT 75 MPH 606 12H 11/0000Z 14.9N 45.9W 70 KT 80 MPH 607 24H 11/1200Z 15.0N 48.7W 75 KT 85 MPH 608 36H 12/0000Z 15.0N 51.4W 75 KT 85 MPH 609 48H 12/1200Z 15.1N 54.0W 75 KT 85 MPH 610 72H 13/1200Z 15.4N 59.4W 70 KT 80 MPH 611 96H 14/1200Z 15.5N 64.5W 60 KT 70 MPH 612120H 15/1200Z 15.7N 70.0W 50 KT 60 MPH 613 614$$ 615Forecaster Pasch 616 617 618 619------------=_1536590201-46358-6660 620Content-Type: text/plain; charset="US-ASCII" 621Content-Disposition: inline 622Content-Transfer-Encoding: 7BIT 623 624= = = == = = 625WTNT44 KNHC 102031 626TCDAT4 627 628Hurricane Isaac Discussion Number 13 629NWS National Hurricane Center Miami FL AL092018 630500 PM AST Mon Sep 10 2018 631 632Isaac has not become better organized today. The system continues 633to display a ragged CDO with limited banding features. Dvorak 634T-numbers from both TAFB and SAB continue to support an intensity of 63565 kt. It is not clear why Isaac has failed to strengthen. Cirrus 636motions and microwave imagery suggest some shear over the tropical 637cyclone, which may have had some influence. Isaac should not 638experience a significant increase in shear until about 48 hours, so 639some strengthening is called for up to that time. Thereafter, the 640dynamical guidance shows a significant increase in shear, which 641should reverse the intensity trend. The HWRF model continues to be 642an outlier in predicting significant intensification over the 643Caribbean, whereas the other models do not. The official intensity 644forecast is above the model consensus over the first half of the 645period, and below the consensus during the latter part of the 646period, considering the HWRF could be making the consensus too high 647at days 3-5. 648 649Isaac continues moving westward or 270/12 kt. There continue to be 650a few differences among the track guidance, in particular the U.K. 651Met, which shows the system turning northwestward to northward well 652to the northeast of the Leeward Islands. The GFS takes the 653system into the Caribbean but shows a more west-northwestward 654motion. The official forecast stays close to the ECMWF and the 655corrected consensus predictions, which is very similar to the 656previous NHC track. Given the spread in the guidance, the 657confidence in the details of the track forecast beyond the first 658couple of days is larger than usual. 659 660Key Messages: 661 6621. Isaac is a small hurricane and uncertainty in the intensity 663forecast is greater than normal. Although Isaac is expected to 664begin weakening when it approaches the Lesser Antilles, it is still 665likely to be at or near hurricane intensity when it reaches the 666islands. 667 6682. Interests in the Lesser Antilles should continue to monitor 669Isaac during the next few days. Watches will likely be required for 670portions of the Lesser Antilles tomorrow. 671 672FORECAST POSITIONS AND MAX WINDS 673 674INIT 10/2100Z 14.4N 45.0W 65 KT 75 MPH 675 12H 11/0600Z 14.5N 47.1W 70 KT 80 MPH 676 24H 11/1800Z 14.6N 49.8W 75 KT 85 MPH 677 36H 12/0600Z 14.6N 52.3W 75 KT 85 MPH 678 48H 12/1800Z 14.8N 54.7W 75 KT 85 MPH 679 72H 13/1800Z 15.3N 60.2W 70 KT 80 MPH 680 96H 14/1800Z 15.5N 65.5W 60 KT 70 MPH 681120H 15/1800Z 15.5N 70.5W 50 KT 60 MPH 682 683$$ 684Forecaster Pasch 685 686 687 688------------=_1536611522-46358-6866 689Content-Type: text/plain; charset="US-ASCII" 690Content-Disposition: inline 691Content-Transfer-Encoding: 7BIT 692 693= = = == = = 694WTNT44 KNHC 110243 695TCDAT4 696 697Tropical Storm Isaac Discussion Number 14 698NWS National Hurricane Center Miami FL AL092018 6991100 PM AST Mon Sep 10 2018 700 701Several microwave images that arrived after the issuance of the last 702advisory revealed that the low-level center of Isaac has raced ahead 703of the main convective mass, and the system is poorly vertically 704aligned. The system also recently passed very near NOAA buoy 41041, 705which confirmed that the center is on the western side of the 706central dense overcast. The maximum winds observed by the buoy were 707only about 30 kt, with a minimum pressure of near 1001 mb. Due to 708the western adjustment of the center location, the latest Dvorak 709classification from TAFB is lower, and a blend of the TAFB final-T 710and CI-numbers supports a lower initial intensity of 60 kt. It is 711worth noting that this estimate is more uncertain than normal due to 712limitations of the Dvorak technique, and most of the other intensity 713estimates are higher. Given the high pressure and light winds at the 714buoy, I am inclined to favor the lower estimate at this time. 715 716Confidence in the track forecast is slightly higher now, and the 717UKMET is the only outlier that does not bring Isaac westward into 718the Caribbean within the next 72 h or so. There is still some 719disagreement on the speed of Isaac, especially once it crosses the 720Leeward Islands, and this seems largely related to the cyclone's 721intensity. The ECMWF and GFS depict a weakening storm that simply 722continues westward, while the HWRF and CTCI show strengthening as 723Isaac slows down and turns more west-northwestward. The NHC track 724forecast for now continues to favor the typically more reliable 725global models and shows a steady westward motion through day 5. The 726updated NHC forecast is a little faster than the previous advisory, 727due primarily to the westward adjustment of the initial position, 728and remains close to HCCA. 729 730A slight adjustment has been made to the intensity forecast, but 731confidence in the intensity forecast remains quite low. Due to the 732current structure of Isaac, it seems less likely that the tropical 733storm will intensify more than what is shown by the intensity 734guidance over the next 24-48 h. The NHC forecast has therefore been 735lowered slightly, but it is still near the top end of the guidance, 736and calls for Isaac to be a hurricane when it approaches the 737Leeward Islands later this week. It is also worth noting that the 738small size of Isaac could make it more susceptible to sudden 739short-term changes in intensity that are nearly impossible to 740forecast. The intensity spread is tremendous by the end of the 741forecast period, with multiple models showing Isaac becoming a major 742hurricane around day 4 or 5, and others showing outright 743dissipation. The NHC forecasts have been favoring a weaker solution 744thus far due to an expected increase in shear beginning in a couple 745of days, and I see no reason to make a dramatic change at this 746point. 747 748Key Messages: 749 7501. Isaac is expected to be at or near hurricane intensity when it 751approaches the Lesser Antilles. However, confidence in the forecast 752is lower than normal. 753 7542. Interests in the Lesser Antilles should continue to monitor 755Isaac during the next few days. Watches will likely be required for 756portions of the Lesser Antilles tomorrow. 757 758FORECAST POSITIONS AND MAX WINDS 759 760INIT 11/0300Z 14.5N 46.9W 60 KT 70 MPH 761 12H 11/1200Z 14.5N 49.0W 60 KT 70 MPH 762 24H 12/0000Z 14.6N 51.7W 65 KT 75 MPH 763 36H 12/1200Z 14.8N 54.3W 65 KT 75 MPH 764 48H 13/0000Z 15.0N 56.9W 65 KT 75 MPH 765 72H 14/0000Z 15.4N 62.4W 60 KT 70 MPH 766 96H 15/0000Z 15.5N 68.0W 55 KT 65 MPH 767120H 16/0000Z 15.5N 72.5W 50 KT 60 MPH 768 769$$ 770Forecaster Zelinsky 771 772 773 774------------=_1536633840-46358-7009 775Content-Type: text/plain; charset="US-ASCII" 776Content-Disposition: inline 777Content-Transfer-Encoding: 7BIT 778 779= = = == = = 780WTNT44 KNHC 110831 781TCDAT4 782 783Tropical Storm Isaac Discussion Number 15 784NWS National Hurricane Center Miami FL AL092018 785500 AM AST Tue Sep 11 2018 786 787Isaac has not changed much overnight. Satellite images indicate 788that the strong tropical storm still has a central dense overcast 789pattern and a limited amount of outer bands. Earlier microwave 790data indicated that the center was not located in the middle of the 791convection, however, likely due to some westerly shear. Since 792the cyclone has changed little during the past several hours, the 793initial intensity is held at 60 kt, which is an average of the TAFB 794and SAB Dvorak final T- and CI-numbers. A nearby NOAA buoy shows 795that the wind field of Isaac is very compact, with tropical-storm- 796force winds only extending up to 40 n mi from the center. 797 798Satellite fixes suggest that Isaac continues to move westward at 13 799kt. Although there remains a fair amount of spread in the models, 800the usually more reliable GFS and ECMWF models continue to show a 801westward motion during the next several days as the storm moves on 802the south side of a strengthening mid-level ridge. The NHC track 803forecast continues to follow that theme, and this forecast lies 804near a blend of the GFS and ECMWF aids. This forecast is a little 805slower than the previous one, and takes Isaac across the Lesser 806Antilles and into the eastern Caribbean in 2 to 3 days. 807 808Predicting Isaac's intensity has been challenging. Isaac appeared 809to have been in relatively favorable conditions during the past day 810or so, but it did not strengthen. The upper-level pattern should 811remain relatively favorable for another day, so modest strengthening 812back to a hurricane is possible during that time. Thereafter, 813however, an increase in shear first from a trough in the central 814Atlantic and then from the outflow of Florence should cause some 815weakening when the storm moves into the Caribbean. The models 816are in a little better agreement this cycle as the HWRF is no 817longer showing Isaac becoming a major hurricane. The NHC intensity 818forecast is very similar to the previous one and is fairly close to 819the consensus aids. 820 821Key Messages: 822 8231. Isaac is expected to be at or near hurricane intensity when it 824approaches the Lesser Antilles. However, confidence in the forecast 825is lower than normal. 826 8272. Interests in the Lesser Antilles should continue to monitor 828Isaac during the next couple of days. Watches will likely be 829required for portions of the Lesser Antilles later today. 830 831FORECAST POSITIONS AND MAX WINDS 832 833INIT 11/0900Z 14.6N 48.1W 60 KT 70 MPH 834 12H 11/1800Z 14.6N 50.1W 60 KT 70 MPH 835 24H 12/0600Z 14.6N 52.6W 65 KT 75 MPH 836 36H 12/1800Z 14.8N 55.0W 65 KT 75 MPH 837 48H 13/0600Z 15.1N 57.6W 60 KT 70 MPH 838 72H 14/0600Z 15.5N 62.8W 55 KT 65 MPH 839 96H 15/0600Z 15.6N 67.4W 55 KT 65 MPH 840120H 16/0600Z 15.8N 72.0W 50 KT 60 MPH 841 842$$ 843Forecaster Cangialosi 844 845 846 847------------=_1536654722-46358-7122 848Content-Type: text/plain; charset="US-ASCII" 849Content-Disposition: inline 850Content-Transfer-Encoding: 7BIT 851 852= = = == = = 853WTNT44 KNHC 111458 854TCDAT4 855 856Tropical Storm Isaac Discussion Number 16 857NWS National Hurricane Center Miami FL AL092018 8581100 AM AST Tue Sep 11 2018 859 860One-minute visible satellite data from GOES-16 indicate that 861the center of Isaac remains on the westward edge of the central 862dense overcast. The initial wind speed is kept at 60 kt, which is a 863compromise between the lower subjective estimates and higher ADTs 864and microwave estimates. This is a more uncertain estimate than 865normal due to the spread of the intensity fixes. The initial wind 866and 12 ft seas radii have been expanded out to the north based on 867TAFB estimates and microwave data. 868 869The intensity forecast continues to be tricky. Westerly shear 870should increase over the next couple of days, which would normally 871cause some weakening. However this effect could be tempered by 872increasing SSTs, mid-level moisture and instability in the path of 873the storm. Given our limited skill in forecasting intensity changes 874in marginal environments, plus the 5 or 10 kt of initial wind speed 875uncertainty, the official forecast will show no significant change 876in intensity for the next couple of days. Because of these 877uncertainties, a Hurricane Watch has been issued for many of the 878Leeward Islands, even though Isaac is not explicitly forecast to be 879a hurricane. Some weakening is indicated at long range after 880considering the global model trends in intensity, which have done 881fairly well for Isaac. 882 883The latest initial motion estimate is 270/14. Isaac is forecast to 884move westward at about the same speed for the next few days due to 885a mid-level ridge over the central Atlantic Ocean. Other than the 886UKMET model, there is not much spread in the guidance, except with 887the forward speed. Given the strength of the ridge, the cyclone 888isn't likely to slow down much, so the official forecast is 889basically along the previous forecast track but faster. The 890uncertainty increases at long range, and is somewhat dependent on 891the intensity of Isaac, but faster seems to be way to go given the 892weakening trend anticipated at that time. 893 894Key Messages: 895 8961. Isaac is expected to be at or near hurricane intensity when it 897approaches the Lesser Antilles on Thursday, and hurricane and 898tropical storm watches have been issued for several islands. 899Interests in those areas should follow any advice given by their 900local officials. 901 9022. Interests elsewhere in the Lesser Antilles should continue to 903monitor Isaac during the next couple of days, as additional watches 904could be needed for other islands. 905 906FORECAST POSITIONS AND MAX WINDS 907 908INIT 11/1500Z 14.6N 49.7W 60 KT 70 MPH 909 12H 12/0000Z 14.6N 51.8W 60 KT 70 MPH 910 24H 12/1200Z 14.7N 54.4W 65 KT 75 MPH 911 36H 13/0000Z 14.9N 57.1W 60 KT 70 MPH 912 48H 13/1200Z 15.1N 59.9W 60 KT 70 MPH 913 72H 14/1200Z 15.5N 65.5W 55 KT 65 MPH 914 96H 15/1200Z 15.5N 71.0W 50 KT 60 MPH 915120H 16/1200Z 15.5N 75.0W 50 KT 60 MPH 916 917$$ 918Forecaster Blake 919 920 921 922------------=_1536677917-46358-7308 923Content-Type: text/plain; charset="US-ASCII" 924Content-Disposition: inline 925Content-Transfer-Encoding: 7BIT 926 927= = = == = = 928WTNT44 KNHC 112033 929TCDAT4 930 931Tropical Storm Isaac Discussion Number 17 932NWS National Hurricane Center Miami FL AL092018 933500 PM AST Tue Sep 11 2018 934 935Microwave and visible satellite data show that the center of Isaac 936is partially exposed on the western edge of the central dense 937overcast. Perhaps Isaac is a little weaker than earlier, but a 938blend of the latest subjective and objective estimates still gives 939an uncertain initial wind speed of about 60 kt. We should have a 940better idea of the maximum winds by tomorrow morning when a NOAA 941Hurricane Hunter enters the storm, or if we get lucky with a 942scatterometer pass this evening. 943 944Isaac is barely holding on in the face of increasing shear, which 945is much more evident in the 300-500 mb layer than a conventional 946analysis at 200 mb. However, convection could redevelop near the 947center while the storm moves over warming SSTs into a more moist and 948unstable environment east of the Lesser Antilles. These 949conflicting factors lead to a forecast of little change in the winds 950of Isaac until the cyclone moves into the eastern Caribbean. I 951should mention that the short-term trend on satellite makes me think 952the intensity forecast is too high over the next 36 hours. Yet, 953given our limited skill in forecasting intensity changes in marginal 954environments, plus the 5 or 10 kt of initial wind speed uncertainty, 955this is the course of least regret. We can see if this trend 956becomes more consistent before making substantial changes on the 957next advisory. Model guidance is very divergent in the long range 958for Isaac, with the regional hurricane and statistical models 959showing intensification, while the global models show continued 960weakening. The global models have done fairly well with this 961cyclone, so the forecast will lean more heavily on their weaker 962solutions. 963 964The latest initial motion estimate is 270/15, a little faster than 965before. Isaac is forecast to move westward at roughly the same 966speed for the next few days due to a mid-level ridge over the 967central Atlantic Ocean. Model guidance is tightly clustered on this 968cycle, with the UKMET now coming into good agreement with the rest 969of the guidance. The only change to this forecast is to accelerate 970Isaac roughly 1 kt over the course of the prediction, which over the 971course of a 5-day forecast results in a sizable westward shift. 972This forecast could still be too slow, since the GFS and ECMWF are 973a bit ahead of the new NHC track prediction. 974 975Key Messages: 976 9771. Isaac is expected to be at or near hurricane intensity when it 978approaches the Lesser Antilles on Thursday, and hurricane and 979tropical storm watches have been issued for several islands. 980Interests in those areas should follow any advice given by their 981local officials, and warnings will likely be issued this evening. 982 9832. Interests elsewhere in the Lesser Antilles should continue to 984monitor Isaac during the next couple of days. 985 986 987FORECAST POSITIONS AND MAX WINDS 988 989INIT 11/2100Z 14.6N 51.3W 60 KT 70 MPH 990 12H 12/0600Z 14.6N 53.4W 60 KT 70 MPH 991 24H 12/1800Z 14.8N 56.2W 60 KT 70 MPH 992 36H 13/0600Z 15.0N 59.2W 60 KT 70 MPH 993 48H 13/1800Z 15.3N 62.3W 55 KT 65 MPH 994 72H 14/1800Z 15.5N 68.5W 50 KT 60 MPH 995 96H 15/1800Z 15.5N 73.5W 45 KT 50 MPH 996120H 16/1800Z 15.5N 77.5W 45 KT 50 MPH 997 998$$ 999Forecaster Blake 1000 1001 1002 1003------------=_1536698045-46358-7445 1004Content-Type: text/plain; charset="US-ASCII" 1005Content-Disposition: inline 1006Content-Transfer-Encoding: 7BIT 1007 1008= = = == = = 1009WTNT44 KNHC 120259 1010TCDAT4 1011 1012Tropical Storm Isaac Discussion Number 18 1013NWS National Hurricane Center Miami FL AL092018 10141100 PM AST Tue Sep 11 2018 1015 1016A slew of recent microwave passes have revealed that Isaac's center 1017is outrunning the deep convection by about a degree due to 1018strengthening westerly shear. With the degradation in structure, 1019Dvorak estimates have fallen to T3.0/3.5 from both TAFB and SAB. 1020The initial intensity is being set at 55 kt to match the CI number 1021(3.5), but I wouldn't be surprised if the actual intensity is 1022closer to the final-T number. 1023 1024Isaac is moving quickly westward, or 270/14 kt. Ridging to the 1025north should keep the cyclone on a similar speed and path for much 1026of the forecast period. The only main difference among the track 1027models is Isaac's future speed, and the NHC official forecast 1028continues to favor the faster models--the GFS, ECMWF, and HCCA 1029models. Since the forecast thinking is unchanged, the updated NHC 1030forecast is not too different from the previous one. Isaac's 1031center is expected to move across the Lesser Antilles near 1032Martinique and Dominica on Thursday. 1033 1034Vertical shear is expected to increase and turn more northwesterly 1035during the next 36 hours as Isaac moves closer to the base of an 1036upper-level trough which extends northeast of the Leeward Islands. 1037Therefore, deep convection is expected to remain decoupled from the 1038low-level center, and Isaac's structure could degrade further over 1039the next day or two. The cyclone's maximum winds are also expected 1040to decrease, and the NHC official forecast is close to the HCCA, 1041Florida State Superensemble, and ICON intensity consensus aids. 1042Even though the statistical-dynamical models and the HWRF 1043restrengthen Isaac by days 4 and 5 when the shear decreases, there 1044may not be much left of the cyclone for any strengthening to occur. 1045Both the GFS and ECMWF global models show Isaac weakening to a 1046remnant low or opening up into a trough by days 4 or 5, which is 1047the scenario favored by the NHC official forecast. As such, 1048dissipation is now shown in the forecast at day 5. 1049 1050 1051Key Message: 1052 10531. Isaac is expected to remain at tropical storm intensity when it 1054moves across the Lesser Antilles on Thursday, and tropical storm 1055warnings have been issued for Martinique, Dominica, and Guadeloupe. 1056Tropical storm watches are in effect for Montserrat, St. Kitts 1057and Nevis, Antigua, and Saba and St. Eustatius. Interests on those 1058islands should follow any advice given by their local officials. 1059 1060 1061FORECAST POSITIONS AND MAX WINDS 1062 1063INIT 12/0300Z 14.5N 52.3W 55 KT 65 MPH 1064 12H 12/1200Z 14.6N 54.7W 50 KT 60 MPH 1065 24H 13/0000Z 14.7N 57.4W 50 KT 60 MPH 1066 36H 13/1200Z 14.9N 60.4W 45 KT 50 MPH 1067 48H 14/0000Z 15.1N 63.7W 45 KT 50 MPH 1068 72H 15/0000Z 15.2N 69.7W 40 KT 45 MPH 1069 96H 16/0000Z 15.0N 74.5W 35 KT 40 MPH 1070120H 17/0000Z...DISSIPATED 1071 1072$$ 1073Forecaster Berg 1074 1075 1076 1077------------=_1536721182-46358-7580 1078Content-Type: text/plain; charset="US-ASCII" 1079Content-Disposition: inline 1080Content-Transfer-Encoding: 7BIT 1081 1082= = = == = = 1083WTNT44 KNHC 120834 1084TCDAT4 1085 1086Tropical Storm Isaac Discussion Number 19 1087NWS National Hurricane Center Miami FL AL092018 1088500 AM AST Wed Sep 12 2018 1089 1090Isaac is a poorly organized tropical storm. Geostationary satellite 1091and microwave images indicate that the center of the cyclone is 1092located near the western edge of the main area of deep convection. 1093This asymmetric cloud pattern is due to about 20 kt of 1094west-northwesterly shear. The satellite intensity estimates have 1095been decreasing, and an average of the Dvorak CI-numbers supports 1096lowering the initial wind speed slightly to 50 kt. The NOAA 1097Hurricane Hunters are scheduled to investigate Isaac later this 1098morning, and that data should provide a better assessment of the 1099storm's intensity and structure. 1100 1101Isaac continues to move due westward at about 13 kt. The track 1102forecast reasoning remains unchanged. A fairly strong mid-level 1103ridge to the north of Isaac should steer the system westward at 1104about the same forward speed during the next several days. The 1105models are now in fairly good agreement, and the NHC track forecast 1106is just a little north of the previous one to come in line with the 1107latest guidance. Based on this forecast, Isaac is expected to cross 1108portions of the Leeward and Windward Islands in about 36 hours. 1109 1110Isaac is feeling the effects of wind shear, and the global models 1111suggest that the shear will likely remain moderate to strong during 1112the next couple of days. These hostile winds combined with nearby 1113dry air will likely cause Isaac to gradually weaken during the next 1114several days. The intensity models are in fair agreement, and this 1115forecast is close to the model consensus, except a little lower than 1116that aid at 96 hours. The latest runs of the GFS and ECMWF models 1117show Isaac opening up into a trough by day 5, and the NHC forecast 1118follows those models. 1119 1120Key Message: 1121 11221. Isaac is expected to remain at tropical storm intensity when it 1123moves across the Lesser Antilles on Thursday, and tropical storm 1124warnings are in effect for Martinique, Dominica, and Guadeloupe. 1125Tropical storm watches are in effect for Montserrat, St. Kitts and 1126Nevis, Antigua, and Saba and St. Eustatius. Interests on those 1127islands should follow any advice given by their local officials. 1128 1129FORECAST POSITIONS AND MAX WINDS 1130 1131INIT 12/0900Z 14.5N 53.5W 50 KT 60 MPH 1132 12H 12/1800Z 14.8N 55.7W 50 KT 60 MPH 1133 24H 13/0600Z 15.0N 58.6W 45 KT 50 MPH 1134 36H 13/1800Z 15.2N 61.5W 45 KT 50 MPH 1135 48H 14/0600Z 15.4N 64.8W 40 KT 45 MPH 1136 72H 15/0600Z 15.3N 70.6W 40 KT 45 MPH 1137 96H 16/0600Z 15.2N 75.0W 35 KT 40 MPH 1138120H 17/0600Z...DISSIPATED 1139 1140$$ 1141Forecaster Cangialosi 1142 1143 1144 1145------------=_1536741253-46358-7682 1146Content-Type: text/plain; charset="US-ASCII" 1147Content-Disposition: inline 1148Content-Transfer-Encoding: 7BIT 1149 1150= = = == = = 1151WTNT44 KNHC 121459 1152TCDAT4 1153 1154Tropical Storm Isaac Discussion Number 20 1155NWS National Hurricane Center Miami FL AL092018 11561100 AM AST Wed Sep 12 2018 1157 1158GOES-16 1-minute satellite imagery along with NOAA Hurricane Hunter 1159data show that the center of Isaac has become fully exposed on the 1160northwestern side of a shrinking area of deep convection. There is 1161still a lot of wind there, however, with the NOAA plane observing 1162SFMR and adjusted flight-level winds to support 50 kt as an initial 1163intensity. 1164 1165An upper-level trough is dropping southward over the central 1166Atlantic, which has increased the shear this morning over Isaac. 1167This high shear is forecast to remain for the next day or two, and 1168it is possible that Isaac will degrade into a tropical wave during 1169that time, although the area of tropical-storm-force winds will 1170likely not go away for a while. In a couple of days, while the 1171wind shear could decrease over the east-central Caribbean Sea, there 1172might not be much of a system left to take advantage of the more 1173conducive conditions. As a compromise, the official forecast 1174shows a slow weakening, in the direction of the model consensus, 1175and continues to show dissipation after 96 hours in line with the 1176GFS/ECMWF guidance. I wouldn't pay much attention to the long 1177range forecast for now, given the uncertainties in Isaac surviving 1178during the next day or two. 1179 1180The low-level center of Isaac has recently accelerated, moving 1181westward at 15 kt, as it decoupled from the slower mid-level flow. 1182Assuming the system stays vertically coherent, a large 1183mid-tropospheric high is forecast to persist for the next several 1184days. This should keep the system moving westward at a somewhat 1185slower pace throughout the forecast period. There isn't much spread 1186in the guidance, and the official forecast is close to the previous 1187one after accounting for the initial position. The long-range track 1188forecast highly depends on the intensity forecast, so it should be 1189considered low confidence. 1190 1191Key Messages: 1192 11931. Isaac is expected to remain at tropical storm intensity when it 1194moves across the Lesser Antilles on Thursday, and tropical storm 1195warnings are in effect for Martinique, Dominica, and Guadeloupe. 1196Tropical storm watches are in effect for Montserrat, St. Kitts and 1197Nevis, Antigua, and Saba and St. Eustatius. Interests on those 1198islands should follow any advice given by their local officials. 1199 12002. Life-threatening flash flooding is possible with Isaac. The 1201storm is expected to produce total rainfall accumulations of 2 to 4 1202inches with isolated amounts up 8 inches across Martinique, 1203Dominica and Guadeloupe. Rainfall amounts of 1 to 2 inches with 1204isolated amounts to 4 inches are expected across Puerto Rico and the 1205southern United States Virgin Islands. 1206 1207 1208FORECAST POSITIONS AND MAX WINDS 1209 1210INIT 12/1500Z 15.0N 54.7W 50 KT 60 MPH 1211 12H 13/0000Z 15.1N 56.7W 50 KT 60 MPH 1212 24H 13/1200Z 15.3N 59.4W 45 KT 50 MPH 1213 36H 14/0000Z 15.3N 62.6W 45 KT 50 MPH 1214 48H 14/1200Z 15.4N 65.6W 40 KT 45 MPH 1215 72H 15/1200Z 15.5N 71.0W 40 KT 45 MPH 1216 96H 16/1200Z 15.5N 75.0W 35 KT 40 MPH 1217120H 17/1200Z...DISSIPATED 1218 1219$$ 1220Forecaster Blake 1221 1222 1223 1224------------=_1536764381-46358-7819 1225Content-Type: text/plain; charset="US-ASCII" 1226Content-Disposition: inline 1227Content-Transfer-Encoding: 7BIT 1228 1229= = = == = = 1230WTNT44 KNHC 122058 1231TCDAT4 1232 1233Tropical Storm Isaac Discussion Number 21 1234NWS National Hurricane Center Miami FL AL092018 1235500 PM AST Wed Sep 12 2018 1236 1237The satellite presentation of Isaac hasn't changed much in the past 1238several hours, with the center exposed on the west side of a 1239re-developing band of convection. On the last pass of the NOAA 1240Hurricane Hunter aircraft, peak flight-level winds were 61 kt and 1241SFMR values were about 50 kt. These data support an initial wind 1242speed estimate of 50 kt. It is interesting to note that the 1243surface center is estimated to be a tenth of a degree or two south 1244of the well-defined swirl on satellite--a function of the fast 1245forward speed and high shear. 1246 1247The shear is forecast to continue for the next day or two, and 1248gradual weakening is anticipated, in line with the model 1249guidance. Given the strong shear, it is possible that Isaac could 1250degenerate into a wave near the Lesser Antilles. After that time, 1251Isaac could escape the upper-level trough over the central Atlantic 1252that has been shearing the cyclone, and find a more conducive 1253environment. More of the model guidance is suggesting some 1254reintensification of the cyclone in the western Caribbean Sea, with 1255the biggest hold-out being the ECMWF. Continuity dictates that the 1256forecast not change as much as the guidance, most of which is 1257showing a hurricane at long range, so the compromise solution is no 1258longer dissipating the cyclone and flat-lining the intensity at 1259long range. It almost goes without saying that this is a 1260low-confidence intensity forecast. 1261 1262Isaac has accelerated to near 17 kt over the past 12 hours, and the 1263shorter-term motion is closer to 20 kt with the convection-less 1264center. A mid-tropospheric high is forecast to persist for the next 1265several days, which should keep the cyclone moving westward, albeit 1266at a slightly slower pace. The model guidance has shifted a bit to 1267the north on this cycle, but is mostly in response to the initial 1268latitude. The NHC track forecast is moved northward as well but 1269still lies on the south side of the consensus. At long range, it 1270makes sense when forecasting a weaker system to stay on the left 1271and faster side of the consensus, so that's where the new NHC 1272prediction lies. 1273 1274 1275Key Messages: 1276 12771. Isaac is expected to remain at tropical storm intensity when it 1278moves across the Lesser Antilles on Thursday, and tropical storm 1279warnings are in effect for Martinique, Dominica, and Guadeloupe. 1280Tropical storm watches are in effect for Montserrat, St. Kitts and 1281Nevis, Antigua, and Saba and St. Eustatius. Interests on those 1282islands should follow any advice given by their local officials. 1283 12842. Life-threatening flash flooding is possible with Isaac. The 1285storm is expected to produce total rainfall accumulations of 2 to 4 1286inches with isolated amounts up 8 inches across Martinique, 1287Dominica and Guadeloupe. Rainfall amounts of 1 to 2 inches with 1288isolated amounts to 4 inches are expected across Puerto Rico and the 1289southern United States Virgin Islands. 1290 1291 1292FORECAST POSITIONS AND MAX WINDS 1293 1294INIT 12/2100Z 15.4N 56.6W 50 KT 60 MPH 1295 12H 13/0600Z 15.6N 59.0W 50 KT 60 MPH 1296 24H 13/1800Z 15.8N 61.8W 45 KT 50 MPH 1297 36H 14/0600Z 16.0N 64.8W 45 KT 50 MPH 1298 48H 14/1800Z 16.0N 67.6W 40 KT 45 MPH 1299 72H 15/1800Z 16.0N 72.5W 40 KT 45 MPH 1300 96H 16/1800Z 16.5N 76.5W 40 KT 45 MPH 1301120H 17/1800Z 17.5N 80.5W 40 KT 45 MPH 1302 1303$$ 1304Forecaster Blake 1305 1306 1307 1308------------=_1536785929-46358-7967 1309Content-Type: text/plain; charset="US-ASCII" 1310Content-Disposition: inline 1311Content-Transfer-Encoding: 7BIT 1312 1313= = = == = = 1314WTNT44 KNHC 130253 1315TCDAT4 1316 1317Tropical Storm Isaac Discussion Number 22 1318NWS National Hurricane Center Miami FL AL092018 13191100 PM AST Wed Sep 12 2018 1320 1321A NOAA P-3 Hurricane Hunter plane has been investigating Isaac this 1322evening, and the data show that the cyclone has weakened a bit. 1323The maximum 850-mb flight-level wind measured by the plane was 47 1324kt, and the highest SFMR winds not coincident with a rain spike were 1325around 45 kt. Based on these wind data, the initial intensity is 1326lowered to 45 kt, and the minimum pressure is up to 1006 mb based on 1327dropsonde data. Additionally, a superposition of the flight-level 1328wind and dropsonde surface wind data suggest that Isaac may not have 1329a closed circulation at 850 mb but is still hanging on to one at the 1330surface. 1331 1332All of Isaac's deep convection is displaced 60-120 n mi to the 1333northeast and southeast of the low-level center due to around 30 kt 1334of westerly shear. This magnitude of the shear is not expected to 1335decrease during the next 12-24 hours, so at the very least, gradual 1336weakening is anticipated. With the circulation so fragile and 1337limited to below 850 mb, however, it's entirely possible that Isaac 1338could open up into a wave at any time. Even if degeneration into a 1339wave occurs, the system would likely carry tropical-storm-force 1340winds across the Leeward Islands on Thursday. After Isaac moves 1341into the eastern Caribbean Sea, there is a lot of uncertainty 1342regarding its future. The 18Z GFS has come back in line with the 1343ECMWF, showing Isaac opening up into a trough over the central and 1344western Caribbean Sea, but the environmental conditions (lower 1345shear, warm sea surface temperatures, etc.) would suggest that the 1346system would have an opportunity to restrengthen. For now, the new 1347NHC intensity forecast maintains continuity from the previous 1348advisory, showing gradual weakening through 48 hours and then 1349holding the system at 35 kt through day 5. This remains a low 1350confidence forecast until we know if Isaac survives the next couple 1351of days. 1352 1353Isaac continues to move quickly westward with an initial motion of 1354270/17 kt. The track guidance remains tightly clustered on a 1355nearly due westward motion for much of the forecast period, with 1356some of the same speed differences noted in previous forecasts. 1357Especially since Isaac's speed has been faster than forecast, the 1358NHC track forecast continues to favor the faster guidance, in 1359particular the ECMWF, GFS, and HCCA models. 1360 1361Reconnaissance and scatterometer data suggest that there are no 1362tropical-storm-force winds within the southern semicircle, but the 1363radii we've been carrying within the northern semicircle appear 1364reasonable. The wind radii forecast have been adjusted to account 1365for the new initial radii. 1366 1367Key Messages: 1368 13691. Isaac is expected to still be producing tropical-storm-force 1370winds when it moves across the Lesser Antilles on Thursday, and 1371tropical storm warnings are in effect for Martinique, Dominica, and 1372Guadeloupe. Tropical storm watches are in effect for Montserrat, 1373St. Kitts and Nevis, Antigua, Saba and St. Eustatius, St. Maarten, 1374and St. Martin. Interests on those islands should follow any advice 1375given by their local officials. 1376 13772. Life-threatening flash flooding is possible with Isaac. The 1378storm is expected to produce total rainfall accumulations of 2 to 4 1379inches with isolated amounts up 8 inches across Martinique, 1380Dominica and Guadeloupe. Rainfall amounts of 1 to 2 inches with 1381isolated amounts to 4 inches are expected across Puerto Rico and the 1382southern United States Virgin Islands. 1383 1384 1385FORECAST POSITIONS AND MAX WINDS 1386 1387INIT 13/0300Z 15.3N 58.0W 45 KT 50 MPH 1388 12H 13/1200Z 15.5N 60.4W 45 KT 50 MPH 1389 24H 14/0000Z 15.6N 63.4W 40 KT 45 MPH 1390 36H 14/1200Z 15.7N 66.4W 40 KT 45 MPH 1391 48H 15/0000Z 15.7N 69.2W 35 KT 40 MPH 1392 72H 16/0000Z 15.6N 73.9W 35 KT 40 MPH 1393 96H 17/0000Z 16.5N 77.5W 35 KT 40 MPH 1394120H 18/0000Z 17.5N 82.0W 35 KT 40 MPH 1395 1396$$ 1397Forecaster Berg 1398 1399 1400 1401------------=_1536807228-46358-8103 1402Content-Type: text/plain; charset="US-ASCII" 1403Content-Disposition: inline 1404Content-Transfer-Encoding: 7BIT 1405 1406= = = == = = 1407WTNT44 KNHC 130859 1408TCDAT4 1409 1410Tropical Storm Isaac Discussion Number 23 1411NWS National Hurricane Center Miami FL AL092018 1412500 AM AST Thu Sep 13 2018 1413 1414Data from a NOAA P-3 Hurricane Hunter aircraft that departed the 1415storm shortly before 0600 UTC indicated that the tropical storm has 1416weakened a little more. Maximum flight-level winds were around 51 1417kt, while maximum SFMR winds were only 41 kt. Both of these 1418support an initial intensity of 40 kt. Wind measurements from the 1419plane also suggested that Isaac is close to opening up into a 1420tropical wave and its circulation is likely barely closed at the 1421surface. Another P-3 mission is scheduled for this morning. 1422 1423Isaac is nearly completely devoid of deep convection at the moment, 1424and is not trackable in radar data from Guadeloupe and Martinique. 1425Given the lack of deep convection to sustain the cyclone, continued 1426gradual weakening is expected. However, given the extent of 142740 mph winds observed by the NOAA plane, Isaac is still likely to 1428bring gusty conditions to many of the Leeward Islands through 1429this afternoon. The dynamical guidance is now in fairly good 1430agreement on the intensity of Isaac for the next few days, and 1431nearly all of the models depict Isaac opening into a trough between 143248 and 96 h. The NHC forecast conservatively maintains Isaac 1433through day 4 in an attempt to maintain some continuity, but it 1434is very possible that Isaac will open into a trough sooner than 1435indicated. Some of the dynamical models suggest that regeneration 1436could occur in the western Caribbean, however the predictability of 1437such an event is too low to explicitly show in the forecast at this 1438point. 1439 1440Isaac is still moving quickly westward, with an initial motion of 1441275/15 kt. Confidence in the track forecast remains high, and the 1442track guidance is tightly clustered through the forecast period. 1443Very little change was made to the track forecast, which continues 1444to closely follow the HCCA model. As long as Isaac remains a 1445shallow cyclone, it should continue to be steered westward by the 1446low-level tradewind flow, even if, or when, it opens up into a 1447trough. 1448 1449 1450Key Messages: 1451 14521. Isaac is expected to produce tropical-storm-force winds across 1453portions of the Lesser Antilles this morning and afternoon, and 1454tropical storm warnings are in effect for Martinique, Dominica, and 1455Guadeloupe. 1456 14572. Tropical storm watches are in effect for Montserrat, St. Kitts 1458and Nevis, Antigua, Barbuda, Saba and St. Eustatius, St. Maarten, 1459and St. Martin. Tropical storm conditions are possible on these 1460islands today and interests on those islands should follow any 1461advice given by their local officials. 1462 1463 1464FORECAST POSITIONS AND MAX WINDS 1465 1466INIT 13/0900Z 15.4N 59.7W 40 KT 45 MPH 1467 12H 13/1800Z 15.5N 61.8W 35 KT 40 MPH 1468 24H 14/0600Z 15.5N 64.8W 35 KT 40 MPH 1469 36H 14/1800Z 15.5N 67.8W 35 KT 40 MPH 1470 48H 15/0600Z 15.4N 70.4W 35 KT 40 MPH 1471 72H 16/0600Z 15.6N 74.8W 35 KT 40 MPH 1472 96H 17/0600Z 16.9N 78.5W 30 KT 35 MPH 1473120H 18/0600Z...DISSIPATED 1474 1475$$ 1476Forecaster Zelinsky 1477 1478 1479 1480------------=_1536829163-46358-8229 1481Content-Type: text/plain; charset="US-ASCII" 1482Content-Disposition: inline 1483Content-Transfer-Encoding: 7BIT 1484 1485= = = == = = 1486WTNT44 KNHC 131441 1487TCDAT4 1488 1489Tropical Storm Isaac Discussion Number 24 1490NWS National Hurricane Center Miami FL AL092018 14911100 AM AST Thu Sep 13 2018 1492 1493Satellite imagery indicate that Isaac remains a poorly organized 1494tropical cyclone, with the low-level center mostly exposed during 1495the past several hours. The center of Isaac passed between Dominica 1496and Martinique around 1300 UTC, with westerly winds reported on both 1497St. Lucia and Martinique, indicating that Isaac is still a tropical 1498storm. Data from a NOAA P-3 Hurricane Hunter aircraft indicate that 1499maximum winds remain near 40 kt, although the area of tropical- 1500storm-force winds has shrunk since yesterday. 1501 15021200 UTC radiosonde data from the eastern Caribbean and Hurricane 1503Hunter aircraft data indicate that the circulation of Isaac is 1504fairly shallow and small. It is entirely possible that Isaac will 1505degenerate into a tropical wave during the next day or two due to 1506the continued effects of strong shear. However, the environment 1507becomes much more conducive for increased organization in a few days 1508when the cyclone, or its remnants, moves across the west-central 1509Caribbean Sea. Model guidance is not consistent at all on whether 1510Isaac will survive as a tropical cyclone, which frankly might not be 1511a very predictable event given the initial uncertainties. The best 1512course of action is to let the full 1200 UTC guidance suite ingest 1513all of the latest radiosonde and other data before making any 1514changes, so this advisory holds the status quo for now. 1515 1516Isaac is moving a little south-of-west and a bit faster, since it 1517was devoid of convection most of night, at about 265/17. A mid-level 1518ridge is forecast to persist over the southwestern Atlantic, 1519steering Isaac westward for the next few days. The track forecast 1520is fairly dependent on the intensity forecast, since a deeper 1521tropical cyclone will move slower due to slightly weaker winds in 1522the middle part of the atmosphere. The forecast has been too slow 1523so far, and with the current forecast showing little strengthening, 1524the official track prediction is shifted west of the previous one, 1525close to the NOAA corrected consensus aid. At the end of the 1526forecast, if Isaac remains a tropical cyclone, it could start moving 1527more toward the west-northwest, but I would place little emphasis on 1528the 96-hour point until we get a better handle on the future 1529structure of Isaac. 1530 1531Key Messages: 1532 15331. Isaac is expected to produce tropical-storm-force winds across 1534portions of the Lesser Antilles into this afternoon, and tropical 1535storm warnings are in effect for Martinique, Dominica, and 1536Guadeloupe. 1537 15382. Tropical storm watches are in effect for Montserrat, St. Kitts 1539and Nevis, Antigua, Barbuda, Saba and St. Eustatius, St. Maarten, 1540and St. Martin. Tropical storm conditions are possible on these 1541islands today and interests on those islands should follow any 1542advice given by their local officials. 1543 1544FORECAST POSITIONS AND MAX WINDS 1545 1546INIT 13/1500Z 14.9N 61.8W 40 KT 45 MPH 1547 12H 14/0000Z 14.9N 64.2W 35 KT 40 MPH 1548 24H 14/1200Z 14.9N 67.0W 35 KT 40 MPH 1549 36H 15/0000Z 14.9N 69.8W 35 KT 40 MPH 1550 48H 15/1200Z 15.0N 72.3W 35 KT 40 MPH 1551 72H 16/1200Z 15.5N 76.5W 35 KT 40 MPH 1552 96H 17/1200Z 17.0N 80.5W 30 KT 35 MPH 1553120H 18/1200Z...DISSIPATED 1554 1555$$ 1556Forecaster Blake 1557 1558 1559 1560------------=_1536849684-46358-8329 1561Content-Type: text/plain; charset="US-ASCII" 1562Content-Disposition: inline 1563Content-Transfer-Encoding: 7BIT 1564 1565= = = == = = 1566WTNT44 KNHC 132044 1567TCDAT4 1568 1569Tropical Storm Isaac Discussion Number 25 1570NWS National Hurricane Center Miami FL AL092018 1571500 PM AST Thu Sep 13 2018 1572 1573The center of Isaac has been mostly exposed during the past several 1574hours as temporary bursts of convection fire just east of the 1575center. Assuming some spin down of the circulation after the plane 1576left, the initial wind speed is set to 35 kt. Since there haven't 1577been any reports of sustained tropical-storm-force winds in the 1578Lesser Antilles and the center is moving away, all watches and 1579warning have been lowered. However, there remains a threat of flash 1580flooding due to a large area of rain that has yet to move through 1581the Lesser Antilles. 1582 1583Strong shear and mid-level dry air is forecast to persist for the 1584next day or so, which should cause Isaac to drop back to tropical 1585depression status on Friday. However, the shear is forecast to 1586weaken by the weekend, and moisture values are expected to rise as 1587the system gets deeper into the Caribbean Sea. These conditions 1588would normally allow for some restregthening, but the initial 1589character of the system is pretty weak, and it is uncertain whether 1590there will be much left of Isaac to take advantage of the more 1591conducive conditions. Model guidance has been inconsistent on the 1592future of Isaac, which speaks to the lack of predictability of this 1593situation. The bulk of the guidance show Isaac staying weak or 1594degenerating into a tropical wave over the next few days, so the 1595official forecast follows that consensus. At this point it is 1596just best to keep an eye on the long-range forecast on each 1597advisory to see if there are any changes. 1598 1599Isaac is moving westward at about 15 kt. A mid-level ridge is 1600forecast to persist over the southwestern Atlantic, steering Isaac 1601westward for the next few days, with a turn to the west-northwest 1602possible at long range. The future track is very uncertain at long 1603range because it is dependent on the intensity. If Isaac 1604degenerates into a wave, the remnants will likely move south and 1605west of the guidance mean, and if the cyclone is stronger, it will 1606probably move slower and more to the right. The new NHC forecast 1607assumes that Isaac will stay weak, and therefore is on the left and 1608faster side of the guidance. 1609 1610 1611FORECAST POSITIONS AND MAX WINDS 1612 1613INIT 13/2100Z 14.9N 63.2W 35 KT 40 MPH 1614 12H 14/0600Z 14.9N 65.4W 35 KT 40 MPH 1615 24H 14/1800Z 14.9N 68.2W 30 KT 35 MPH 1616 36H 15/0600Z 14.9N 70.8W 30 KT 35 MPH 1617 48H 15/1800Z 15.0N 73.0W 30 KT 35 MPH 1618 72H 16/1800Z 16.2N 77.4W 30 KT 35 MPH 1619 96H 17/1800Z 17.5N 81.5W 30 KT 35 MPH 1620120H 18/1800Z...DISSIPATED 1621 1622$$ 1623Forecaster Blake 1624 1625 1626 1627------------=_1536871464-46358-8493 1628Content-Type: text/plain; charset="US-ASCII" 1629Content-Disposition: inline 1630Content-Transfer-Encoding: 7BIT 1631 1632= = = == = = 1633WTNT44 KNHC 140237 1634TCDAT4 1635 1636Tropical Storm Isaac Discussion Number 26 1637NWS National Hurricane Center Miami FL AL092018 16381100 PM AST Thu Sep 13 2018 1639 1640A burst of deep convection has developed and covered the previously 1641exposed center of Isaac this evening. However, there is still 1642evidence of northwesterly shear over the cyclone as there is a 1643rather sharp edge to the convective mass and the cirrus is quickly 1644blowing off toward the southeast. A recent ASCAT pass shows that 1645the circulation is not particularly well defined, but it did show 1646some light west or west-southwesterly winds, enough to keep the 1647system classified as a tropical cyclone. The ASCAT also revealed 1648winds of 30-32 kt, so the initial wind speed is held at 35 kt, which 1649is also an average of the latest Dvorak current intensity numbers. 1650 1651Moderate to strong northwesterly shear and dry mid-level air are 1652likely to cause Isaac to weaken to a tropical depression or even 1653become an open wave within the next day or so. When Isaac reaches 1654the west-central Caribbean Sea in a couple of days, the shear may 1655relax somewhat and the global models suggest that there will be an 1656increase in mid-level moisture. These conditions could allow for 1657regeneration of the system, but given the uncertainty in whether 1658Isaac will survive long enough to take advantage of these 1659conditions, the official forecast still calls for dissipation like 1660most of the guidance. 1661 1662Isaac continues moving westward at about 14 kt. The system should 1663move generally westward over the next couple of days as it is 1664steered by a low- to mid-level ridge to the north. Later in the 1665period, the models show that Isaac could gain some latitude, but 1666the official forecast is along the southern side of the guidance 1667since a weaker system is likely to move more westward. The new NHC 1668track forecast is essentially an update of the previous advisory. 1669 1670FORECAST POSITIONS AND MAX WINDS 1671 1672INIT 14/0300Z 14.9N 64.4W 35 KT 40 MPH 1673 12H 14/1200Z 15.0N 66.5W 35 KT 40 MPH 1674 24H 15/0000Z 15.0N 69.1W 30 KT 35 MPH 1675 36H 15/1200Z 15.0N 71.4W 30 KT 35 MPH 1676 48H 16/0000Z 15.2N 73.3W 30 KT 35 MPH 1677 72H 17/0000Z 16.3N 77.5W 30 KT 35 MPH 1678 96H 18/0000Z 17.7N 82.0W 30 KT 35 MPH 1679120H 19/0000Z...DISSIPATED 1680 1681$$ 1682Forecaster Brown 1683 1684 1685 1686------------=_1536892650-46358-8644 1687Content-Type: text/plain; charset="US-ASCII" 1688Content-Disposition: inline 1689Content-Transfer-Encoding: 7BIT 1690 1691= = = == = = 1692WTNT44 KNHC 140832 1693TCDAT4 1694 1695Tropical Depression Isaac Discussion Number 27 1696NWS National Hurricane Center Miami FL AL092018 1697500 AM AST Fri Sep 14 2018 1698 1699Northwesterly shear continues to take a toll on Isaac. Although the 1700system is still producing areas of deep convection, an ASCAT 1701overpass that arrived shortly after the release of the previous 1702advisory, indicated that the circulation had become even less 1703defined. The ASCAT data also showed that the winds are below 1704tropical storm strength, therefore the initial intensity has been 1705reduced to 30 kt. 1706 1707Moderate to strong northwesterly shear and dry mid-level air are 1708likely to cause additional weakening, and Isaac is forecast to 1709degenerate into an open wave within the next several days, but this 1710could occur much sooner if the current trends continue. 1711When the system reaches the western Caribbean Sea the shear may 1712relax somewhat and moisture levels are expected to be higher. These 1713conditions could allow for regeneration of the system, but the ECMWF 1714and GFS both show Issac weakening and moving over the western 1715Caribbean as an open wave, and the NHC forecast follows this 1716scenario. 1717 1718A low- to mid-level ridge over the southwestern Atlantic continues 1719to steer Isaac westward, and this general motion should continue 1720until dissipation occur. The new track forecast is once again 1721essentially an update of the previous advisory. 1722 1723FORECAST POSITIONS AND MAX WINDS 1724 1725INIT 14/0900Z 15.0N 65.5W 30 KT 35 MPH 1726 12H 14/1800Z 15.1N 67.3W 30 KT 35 MPH 1727 24H 15/0600Z 15.2N 69.7W 30 KT 35 MPH 1728 36H 15/1800Z 15.2N 72.0W 25 KT 30 MPH 1729 48H 16/0600Z 15.4N 73.8W 25 KT 30 MPH 1730 72H 17/0600Z 16.4N 77.7W 25 KT 30 MPH 1731 96H 18/0600Z...DISSIPATED 1732 1733$$ 1734Forecaster Brown 1735 1736 1737 1738------------=_1536913979-46358-8756 1739Content-Type: text/plain; charset="US-ASCII" 1740Content-Disposition: inline 1741Content-Transfer-Encoding: 7BIT 1742 1743= = = == = = 1744WTNT44 KNHC 141432 1745TCDAT4 1746 1747Tropical Depression Isaac Discussion Number 28 1748NWS National Hurricane Center Miami FL AL092018 17491100 AM AST Fri Sep 14 2018 1750 1751Deep convection has been increasing near the center of Isaac, but 1752visible imagery indicates that the circulation has become 1753more elongated from northeast to southwest. It is possible that 1754Isaac is in the process of losing a well-defined center, but we will 1755know more about the wind structure in a few hours since the NOAA 1756Hurricane Hunter aircraft should be in the area. The initial wind 1757speed is held at 30 kt. 1758 1759Vertical shear has decreased near Isaac, with some outflow in the 1760western semicircle noted and consolidation of the convection. 1761Still, this area of the eastern Caribbean isn't known as the 1762graveyard for nothing, and the system might just be too shallow and 1763disorganized to take advantage of the changing environment. Almost 1764all of the dynamical models, save the UKMET, show the cyclone moving 1765westward to west-northwestward at a slower forward speed during the 1766next few days and degenerating into an open wave in the central 1767Caribbean Sea. This solution is also supported by fewer members of 1768the GFS and ECMWF ensemble now maintaining a coherent system in the 1769long range. Little change is made to the previous track and 1770intensity forecasts, and we will see what the aircraft finds this 1771afternoon and if the 1200 UTC model cycle maintains the dissipation 1772trend. 1773 1774FORECAST POSITIONS AND MAX WINDS 1775 1776INIT 14/1500Z 14.9N 67.3W 30 KT 35 MPH 1777 12H 15/0000Z 15.0N 69.0W 30 KT 35 MPH 1778 24H 15/1200Z 15.1N 71.3W 30 KT 35 MPH 1779 36H 16/0000Z 15.2N 73.3W 25 KT 30 MPH 1780 48H 16/1200Z 15.6N 75.3W 25 KT 30 MPH 1781 72H 17/1200Z 17.0N 79.5W 25 KT 30 MPH 1782 96H 18/1200Z...DISSIPATED 1783 1784$$ 1785Forecaster Blake 1786 1787 1788 1789------------=_1536935552-46358-8941 1790Content-Type: text/plain; charset="US-ASCII" 1791Content-Disposition: inline 1792Content-Transfer-Encoding: 7BIT 1793 1794= = = == = = 1795WTNT44 KNHC 142051 1796TCDAT4 1797 1798Tropical Storm Isaac Discussion Number 29 1799NWS National Hurricane Center Miami FL AL092018 1800500 PM AST Fri Sep 14 2018 1801 1802It has been a very interesting flight of the NOAA Hurricane Hunter 1803aircraft this afternoon. Dropsonde and SFMR data indicate that the 1804maximum winds are around 35 kt again, and the pressure has fallen 1805to about 1002 mb. However, the system is tilted southward with 1806vertical height due to northerly shear, and scatterometer and 1807Hurricane Hunter tail-Doppler radar data indicate the system is 1808elongated from northeast to southwest, albeit closed. Note that the 1809plane was unable to sample the southwestern quadrant well because of 1810strong convection and intense lightning, so the flight-level winds 1811showing no circulation were unrepresentative. 1812 1813The intensity forecast hasn't gotten any easier with Isaac. 1814Vertical shear is expected to decrease a bit more, which might 1815diminish the effects of dry air seen on water vapor images around 1816the cyclone. However, the GFS and ECMWF continue to insist that 1817Isaac will degenerate into a wave within the next 2 or 3 days, which 1818is almost unanimously supported by their respective ensemble 1819members. This is the solution that the official forecast follows. 1820It should be noted that none of the global models show pressures 1821this low now with Isaac, except the UKMET, and that model is the one 1822that holds onto the circulation the longest. The reality of the 1823situation is that the long-term future of Isaac is cloudy, and the 1824best advice is just to follow NHC advisories every 6 hours to see 1825if we gain any confidence. 1826 1827Isaac is moving westward at about 12 kt. A ridge over the 1828southwestern Atlantic should steer the cyclone westward over the 1829weekend then west-northwestward early next week as Isaac reaches a 1830weaker portion of the ridge. Just like yesterday, a stronger system 1831would likely turn more to the west-northwest earlier, moving 1832closer to Jamaica. Since the system is forecast to be weaker, the 1833forecast lies on the westward and faster side of the guidance, and 1834should also be considered low confidence since it depends on the 1835intensity. Interests in Jamaica should monitor the progress of the 1836Isaac. 1837 1838 1839FORECAST POSITIONS AND MAX WINDS 1840 1841INIT 14/2100Z 15.3N 68.2W 35 KT 40 MPH 1842 12H 15/0600Z 15.4N 70.1W 35 KT 40 MPH 1843 24H 15/1800Z 15.4N 72.2W 35 KT 40 MPH 1844 36H 16/0600Z 15.7N 74.2W 35 KT 40 MPH 1845 48H 16/1800Z 16.2N 76.3W 30 KT 35 MPH 1846 72H 17/1800Z 17.5N 80.0W 25 KT 30 MPH 1847 96H 18/1800Z...DISSIPATED 1848 1849$$ 1850Forecaster Blake 1851 1852 1853 1854------------=_1536958325-46358-9128 1855Content-Type: text/plain; charset="US-ASCII" 1856Content-Disposition: inline 1857Content-Transfer-Encoding: 7BIT 1858 1859= = = == = = 1860WTNT44 KNHC 150238 1861TCDAT4 1862 1863Tropical Storm Isaac Discussion Number 30 1864NWS National Hurricane Center Miami FL AL092018 18651100 PM AST Fri Sep 14 2018 1866 1867Isaac has rapidly lost organization since the last advisory. The 1868cyclone has been devoid of any deep convection for most of the 1869evening, with only a small burst recently forming well to the west 1870of where Isaac's low-level center is believed to be located. While 1871it is possible, if not likely, that the maximum winds have decreased 1872a little since this afternoon due to the lack of convection, the 1873initial intensity has been held at 35 kt based on earlier aircraft 1874data. A NOAA Hurricane Hunter aircraft is scheduled to investigate 1875Isaac again overnight and will provide a better estimate of the 1876strength and status of Isaac. 1877 1878The global models continue to insist that Isaac could open into a 1879trough at any time, and based on the current state of the cyclone, 1880this could be imminent. The regional hurricane models, however, show 1881the development of one or more mesoscale lows embedded within a 1882larger cyclonic envelope that cause Isaac to persist for at least a 1883few more days in those models. While the global model solution still 1884seems more likely, I can't rule out that Isaac will persist a little 1885longer, so the NHC forecast still carries a 72 h forecast point. 1886Regardless of whether Isaac is a tropical storm or a tropical wave 1887over the next few days, heavy rains and occasional gusty winds are 1888still likely across much of the Greater Antilles through the next 1889couple of days. 1890 1891It has been very difficult to try to pinpoint a low-level center of 1892Isaac over the past few hours, but based on microwave data it 1893appears to still be moving west at about 13 kt. The GFS and ECMWF 1894are in very good agreement and show Isaac, or its remnants, moving 1895steadily westward for the next several days. The regional models on 1896the other hand show a farther north motion, in part due to one or 1897more reformations of the center. The NHC forecast continues to favor 1898the global model solutions and is on the south side of the guidance 1899envelope, however if it becomes clear that Isaac will survive 1900longer, a northward adjustment may be required in later advisories. 1901 1902FORECAST POSITIONS AND MAX WINDS 1903 1904INIT 15/0300Z 15.4N 69.5W 35 KT 40 MPH 1905 12H 15/1200Z 15.5N 71.3W 35 KT 40 MPH 1906 24H 16/0000Z 15.7N 73.3W 35 KT 40 MPH 1907 36H 16/1200Z 16.2N 75.3W 35 KT 40 MPH 1908 48H 17/0000Z 16.7N 77.4W 30 KT 35 MPH 1909 72H 18/0000Z 18.7N 81.4W 30 KT 35 MPH 1910 96H 19/0000Z...DISSIPATED 1911 1912$$ 1913Forecaster Zelinsky 1914 1915 1916 1917------------=_1536979145-46358-9268 1918Content-Type: text/plain; charset="US-ASCII" 1919Content-Disposition: inline 1920Content-Transfer-Encoding: 7BIT 1921 1922= = = == = = 1923