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| #521861 (Received by flhurricane at: 4:38 PM 27.Jun.2012) TCDAT4 POST-TROPICAL CYCLONE DEBBY DISCUSSION NUMBER 18 NWS NATIONAL HURRICANE CENTER MIAMI FL AL042012 500 PM EDT WED JUN 27 2012 STRONG WESTERLY SHEAR AND DRY AIR CONTINUE TO TAKE A TOLL ON DEBBY. ASCAT AMBIGUITY VECTORS FROM A 1527 UTC OVERPASS SUGGEST THAT THE LOW-LEVEL CIRCULATION HAS BECOME ELONGATED TO THE POINT THAT IT IS NOW A SHARP TROUGH AXIS WITHOUT A WELL DEFINED CENTER. IN ADDITION...THE SYSTEM LACKS ENOUGH ORGANIZED DEEP CONVECTION TO BE CLASSIFIABLE. ON THIS BASIS...DEBBY IS NOW DESIGNATED AS A POST- TROPICAL CYCLONE. THE EARLIER ASCAT PASS HAD UNCONTAMINATED WINDS TO 32 KT...AND NOAA BUOY 41010 HAD 1-MINUTE WINDS AS HIGH AS 31 KT. IT IS ASSUMED THAT HIGHER WINDS COULD BE OCCURRING IN THE REMAINING CONVECTION WELL REMOVED FROM THE CENTER...SO INTENSITY OF THE SYSTEM IS ESTIMATED TO BE 35 KT. BASED UPON RECENT SATELLITE FIXES...THE INITIAL MOTION ESTIMATE IS 095/11...ALTHOUGH THIS IS DIFFICULT TO DETERMINE PRECISELY DUE TO THE POOR DEFINITION OF THE CENTER. DEBBY SHOULD BE STEERED BY THE FLOW ON THE SOUTH SIDE OF A DEEP TROUGH COVERING THE WESTERN ATLANTIC...EVEN AS THIS WEATHER SYSTEM BEGINS TO LIFT OUT. ONCE REACHING A HIGHER LATITUDE...DEBBY IS EXPECTED TO FEEL THE INFLUENCE OF THE MID-LATITUDE WESTERLY FLOW AND ACCELERATE NORTHEASTWARD DURING THE REMAINDER OF THE FORECAST PERIOD. GLOBAL MODELS ARE IN GOOD AGREEMENT ON THIS SCENARIO BUT STILL DIFFER ON THE TIMING ASPECTS OF THE FORECAST. THE GFS IS SUBSTANTIALLY SLOWER THAN THE ECMWF/UKMET SINCE ITS MORE SOUTHERN INITIAL HEADING KEEPS IT EQUATORWARD OF THE STRONGER WESTERLY FLOW. THE NEW TRACK FORECAST IS SLOWED A BIT TO ACCOUNT FOR THIS POSSIBILITY...BUT IS ESSENTIALLY SIMILAR TO THE PREVIOUS ONE. LARGE-SCALE MODELS CONTINUE TO SHOW A RELATIVE DECREASE IN THE SHEAR OVER DEBBY AS IT TRAVERSES THE CENTRAL ATLANTIC OVER THE NEXT DAY OR TWO. ALTHOUGH AN ABUNDANCE OF VERY DRY AND STABLE AIR OVER THIS REGION AND DECREASING SEA SURFACE TEMPERATURES COULD BE FACTORS THAT PREVENT DEBBY FROM REGENERATING AS A TROPICAL CYCLONE...GLOBAL MODELS CONTINUE TO INDICATE THIS TO BE A POSSIBILITY. THE OFFICIAL NHC FORECAST CARRIES DEBBY AS A POST- TROPICAL CYCLONE THROUGHOUT THE FORECAST PERIOD...THOUGH DEBBY COULD REACQUIRE TROPICAL CHARACTERISTICS AT SOME POINT. FORECAST POSITIONS AND MAX WINDS INIT 27/2100Z 29.5N 78.3W 35 KT 40 MPH...POST-TROPICAL 12H 28/0600Z 29.4N 76.5W 35 KT 40 MPH...POST-TROPICAL 24H 28/1800Z 29.7N 73.8W 35 KT 40 MPH...POST-TROPICAL 36H 29/0600Z 30.7N 71.0W 35 KT 40 MPH...POST-TROPICAL 48H 29/1800Z 32.1N 68.5W 35 KT 40 MPH...POST-TROPICAL 72H 30/1800Z 35.7N 64.2W 35 KT 40 MPH...POST-TROPICAL 96H 01/1800Z 40.0N 59.0W 40 KT 45 MPH...POST-TROPICAL 120H 02/1800Z 46.0N 48.0W 40 KT 45 MPH...POST-TROPICAL $$ FORECASTER KIMBERLAIN |
