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| #520594 (Received by flhurricane at: 10:59 PM 23.Jun.2012) TCDAT4 TROPICAL STORM DEBBY DISCUSSION NUMBER 2 NWS NATIONAL HURRICANE CENTER MIAMI FL AL042012 1000 PM CDT SAT JUN 23 2012 DEBBY IS GRADUALLY SHOWING SIGNS OF IMPROVED ORGANIZATION AS THE UPPER-LEVEL SHEAR SHIFTS FROM SOUTHWESTERLY TO MORE OF A SOUTHERLY COMPONENT. SHIP ZCDJ2 LOCATED ABOUT 80 NMI EAST OF THE CENTER AND NEAR NOAA BUOY 42003 REPORTED A PRESSURE OF 1001.6 MB AND 38 KT WINDS AT 24/00Z. BASED ON THIS INFORMATION...THE CENTRAL PRESSURE IS CONSERVATIVELY ESTIMATED TO BE 998 MB. AIRCRAFT RECONNAISSANCE DATA AT 23/2020Z INDICATED 850 MB FLIGHT-LEVEL WINDS OF 61 KT IN THE NORTHEAST QUADRANT...WHICH IS EQUAL TO ABOUT 49 KT SURFACE WINDS. CONVECTION WAS SOMEWHAT MEAGER AT THAT...BUT SINCE THEN HAS INCREASED RATHER MARKEDLY IN THAT AREA. THEREFORE THE INITIAL INTENSITY ESTIMATE IS A SOLID 45 KT. DEBBY HAS BEEN NEARLY STATIONARY DURING THE PAST 6 HOURS. MULTIPLE LOW-LEVEL VORTICES HAVE BEEN OUT FROM UNDERNEATH THE EASTERN SEMICIRCLE CONVECTIVE CLOUD MASS ABOUT A MEAN CENTER OF ROTATION OF A LARGER GYRE...AND THE LOCATION OF THE LARGER STATIONARY GYRE IS THE ADVISORY POSITION. ALTHOUGH THERE MAY BE SOME SLIGHT NORTHEASTWARD MOTION DUE TO REDEVELOPMENT OF THE CENTER CLOSER TO THE DEEP CONVECTION...THE GENERAL MOTION FOR THE NEXT 24 HOURS OR SO SHOULD BE SLOWLY POLEWARD. THE LONGWAVE PATTERN ACROSS THE U.S. HAS A DEEPENING TROUGH OFF THE WEST COAST...WHICH IS CREATING DOWNSTREAM RIDGING ACROSS THE CENTRAL AND EASTERN UNITED STATES. THIS INCREASED RIDGING IS REFLECTED BY 20-40 METER 500 MB HEIGHT RISES AT 00Z ACROSS THE CENTRAL AND SOUTHERN PLAINS...AND EXTENDING AS FAR EAST AS THE CAROLINA COASTS. THIS BLOCKING RIDGE PATTERN TO THE NORTH OF DEBBY IS EXPECTED TO GRADUALLY TURN THE CYCLONE WESTWARD ACROSS THE NORTH-CENTRAL GULF OF MEXICO BY 36-48 HOURS...WITH A CONTINUED WESTWARD MOTION ACROSS THE NORTHWESTERN GULF AFTER THAT. ALL OF THE AVAILABLE NHC MODEL GUIDANCE...EXCEPT FOR THE GFS AND GFS-ENSEMBLE MODELS...SUPPORTS THE GENERAL WESTWARD MOTION SCENARIO. THE MAIN REASON THAT THE GFS MOVES DEBBY EASTWARD ACROSS THE NORTHEASTERN GULF IS DUE TO IT HAVING A WEAKER AND MORE VERTICALLY SHALLOW STORM DEPICTED IN THE MODEL. GIVEN THE EXPECTED FAVORABLE UPPER-LEVEL FLOW PATTERN WITH WEAKER VERTICAL SHEAR FORECAST TO DEVELOP AFTER 24 HOURS...A DEEPER AND MORE ROBUST WESTWARD MOVING TROPICAL CYCLONE AS DEPICTED IN THE ECMWF...UKMET... NOGAPS MODELS SEEMS A MORE REASONABLE SCENARIO. THE OFFICIAL FORECAST TRACK IS JUST AN UPDATE OF THE PREVIOUS ADVISORY TRACK. MODERATE SOUTHWESTERLY VERTICAL WIND SHEAR IS EXPECTED TO GRADUALLY GIVE WAY TO SOUTHERLY AND EVENTUALLY SOUTHEASTERLY WEAKER WIND SHEAR BY 36-48 HOURS AS AN UPPER-LEVEL LOW NORTHWEST OF THE CYCLONE MOVES SOUTHWESTWARD...WHICH ALLOWS DEBBY TO MOVE UNDERNEATH A 200 MB RIDGE AXIS. THE OFFICIAL INTENSITY FORECAST LEANS TOWARD THE ECMWF MODEL...WHICH IS FORECASTING A HURRICANE CENTRAL PRESSURE BY 96 HOURS...AND IS ABOVE THE GFS-BASED SHIPS AND LGEM MODELS SINCE THAT OUTPUT LIKELY DOES NOT REPRESENT THE ENVIRONMENTAL CONDITIONS THAT DEBBY WILL ENCOUNTER. THE OFFICIAL INTENSITY FORECAST IS A LITTLE HIGHER THAN THE PREVIOUS ADVISORY...AND BRINGS DEBBY TO HURRICANE STRENGTH BY 72 HOURS. FORECAST POSITIONS AND MAX WINDS INIT 24/0300Z 26.3N 87.5W 45 KT 50 MPH 12H 24/1200Z 26.7N 87.8W 50 KT 60 MPH 24H 25/0000Z 27.0N 88.1W 50 KT 60 MPH 36H 25/1200Z 27.1N 88.8W 55 KT 65 MPH 48H 26/0000Z 27.2N 89.8W 60 KT 70 MPH 72H 27/0000Z 27.2N 91.8W 65 KT 75 MPH 96H 28/0000Z 27.2N 93.7W 70 KT 80 MPH 120H 29/0000Z 27.2N 95.2W 70 KT 80 MPH $$ FORECASTER STEWART |
