1WTNT42 KNHC 240858 2TCDAT2 3 4TROPICAL DEPRESSION SEVENTEEN DISCUSSION NUMBER 1 5NWS NATIONAL HURRICANE CENTER MIAMI FL AL172011 6500 AM AST SAT SEP 24 2011 7 8THE SMALL AREA OF LOW PRESSURE SEVERAL HUNDRED MILES SOUTH OF THE 9CAPE VERDE ISLANDS HAS DEVELOPED ENOUGH ORGANIZED CONVECTION TO BE 10CONSIDERED A TROPICAL CYCLONE. MICROWAVE DATA FROM TRMM AND AMSR-E 11OVER THE PAST FEW HOURS SHOWED DISTINCT CURVED BANDS...AND EARLY 12MORNING VISIBLE IMAGES SUGGEST THAT THE CENTER IS WELL EMBEDDED 13WITHIN THE CLOUD CANOPY. AN EARLIER ASCAT PASS INDICATED THAT THE 14SYSTEM WAS CLOSED...AND HAD WINDS OF ABOUT 30 KT...SO THAT WILL BE 15THE INITIAL INTENSITY. ENVIRONMENTAL CONDITIONS APPEAR TO BE 16FAVORABLE FOR STRENGTHENING FOR THE NEXT COUPLE OF DAYS WHILE THE 17DEPRESSION IS OVER WARM WATERS WITH LIGHT SHEAR. AFTER THAT TIME... 18THE CYCLONE COULD WEAKEN SOMEWHAT DUE TO INCREASING SOUTHWESTERLY 19SHEAR FROM A LARGE UPPER-LEVEL TROUGH OVER THE CENTRAL ATLANTIC 20OCEAN AND DECREASING SSTS. THE NHC INTENSITY FORECAST IS ON THE 21CONSERVATIVE SIDE OF THE GUIDANCE ENVELOPE...BUT FOLLOWS THE TREND 22OF THE LGEM MODEL. 23 24BEST ESTIMATE OF THE INITIAL MOTION IS 280/12. A MID-LEVEL RIDGE TO 25THE NORTH OF THE DEPRESSION IS FORECAST TO WEAKEN...WHICH SHOULD 26CAUSE THE CYCLONE TO GRADUALLY TURN TOWARD THE WEST-NORTHWEST AND 27NORTHWEST OVER THE NEXT FEW DAYS. SOME OF THE GUIDANCE TURNS THE 28SYSTEM MORE TOWARD THE NORTH BY THE END OF THE FORECAST PERIOD DUE 29TO A STRONG MID-LATITUDE TROUGH. FOR NOW...MOST OF THE RELIABLE 30GLOBAL MODELS SHOW THE TROUGH STAYING FAR ENOUGH AWAY FROM THE 31DEPRESSION TO AVOID RECURVATURE...AND THE NHC FORECAST IS ON THE 32LEFT SIDE OF THE GUIDANCE ENVELOPE AND NOT TOO FAR FROM THE ECMWF 33MODEL. 34 35 36FORECAST POSITIONS AND MAX WINDS 37 38INIT 24/0900Z 10.7N 25.2W 30 KT 35 MPH 39 12H 24/1800Z 11.1N 27.2W 35 KT 40 MPH 40 24H 25/0600Z 11.8N 29.3W 40 KT 45 MPH 41 36H 25/1800Z 12.7N 31.4W 50 KT 60 MPH 42 48H 26/0600Z 13.7N 33.1W 60 KT 70 MPH 43 72H 27/0600Z 16.0N 35.5W 60 KT 70 MPH 44 96H 28/0600Z 18.0N 37.5W 55 KT 65 MPH 45120H 29/0600Z 20.0N 39.5W 50 KT 60 MPH 46 47$$ 48FORECASTER BLAKE 49 50 51WTNT42 KNHC 241458 52TCDAT2 53 54TROPICAL DEPRESSION SEVENTEEN DISCUSSION NUMBER 2 55NWS NATIONAL HURRICANE CENTER MIAMI FL AL172011 561100 AM EDT SAT SEP 24 2011 57 58AFTER AN EARLIER INCREASE IN DEEP CONVECTION AND BANDING 59FEATURES...THE CONVECTIVE ORGANIZATION OF THE DEPRESSION HAS 60DECREASED A LITTLE SINCE THE 12Z FIXES. INTENSITY ESTIMATES ARE 61T2.8/41 KT FROM UW-CIMSS ADT...T2.5/35 KT FROM TAFB...AND T1.5/25 62KT FROM SAB. HOWEVER...THE CURVED BAND TO THE NORTH OF CENTER HAS 63FRACTURED AND DEGENERATED AND THE ADT ESTIMATES ARE BASED ON AN 64UNREPRESENTATIVE SHEAR PATTERN. AS A RESULT...THE INITIAL INTENSITY 65WILL REMAIN AT 30 KT FOR THIS ADVISORY...WHICH IS CONSISTENT WITH 66SEVERAL 30-KT WIND VECTORS NOTED IN A 1058Z ASCAT OVERPASS. 67 68THE INITIAL MOTION ESTIMATE IS 285/14. THE CYCLONE IS EXPECTED TO 69MOVE IN A GENERAL WEST-NORTHWESTWARD MOTION FOR THE NEXT 3 DAYS OR 70SO UNDER THE INFLUENCE OF A STRONG SUBTROPICAL RIDGE ORIENTED 71EAST-WEST ALONG 20N LATITUDE. HOWEVER...BY 72 HOURS A BREAK IN THE 72RIDGE IS FORECAST TO DEVELOP BETWEEN 40-45W LONGITUDE AS A BROAD 73LOW PRESSURE TROUGH DIGS SOUTHEASTWARD OVER THE EASTERN ATLANTIC. 74THIS SHOULD RESULT IN THE CYCLONE SLOWING DOWN AND TURNING TOWARD 75THE NORTHWEST INTO THE WEAKNESS. THE OFFICIAL FORECAST IS JUST AN 76EXTENSION OF THE PREVIOUS ADVISORY TRACK AND LIES CLOSE TO THE 77CONSENSUS MODELS TVCN AND TVCA. 78 79ENVIRONMENTAL CONDITIONS WILL BE MAXIMIZED DURING THE NEXT 48 HOURS 80OR SO FOR INTENSIFICATION TO OCCUR OVER THE NEXT COUPLE OF DAYS. 81AFTER THAT...VERTICAL SHEAR IS EXPECTED TO INCREASE...ALBEIT TO 82ONLY ABOUT 15 KT...FROM THE SOUTHWEST AHEAD OF THE AFOREMENTIONED 83MID/UPPER-LEVEL TROUGH. THIS IS EXPECTED TO RESULT IN A SLOWER RATE 84OF INTENSIFICATION. THE OFFICIAL INTENSITY FORECAST REMAINS ON THE 85CONSERVATIVE SIDE OF THE INTENSITY GUIDANCE ENVELOPE AND FOLLOWS A 86BLEND OF THE SHIPS AND LGEM MODELS. 87 88FORECAST POSITIONS AND MAX WINDS 89 90INIT 24/1500Z 11.0N 26.7W 30 KT 35 MPH 91 12H 25/0000Z 11.4N 28.3W 35 KT 40 MPH 92 24H 25/1200Z 12.2N 30.5W 40 KT 45 MPH 93 36H 26/0000Z 13.2N 32.3W 50 KT 60 MPH 94 48H 26/1200Z 14.2N 33.8W 60 KT 70 MPH 95 72H 27/1200Z 16.4N 36.1W 60 KT 70 MPH 96 96H 28/1200Z 18.4N 38.1W 55 KT 65 MPH 97120H 29/1200Z 20.5N 39.5W 50 KT 60 MPH 98 99$$ 100FORECASTER STEWART 101 102 103