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| #152982 (Received by flhurricane at: 4:52 AM 29.Sep.2007) TCDAT4 TROPICAL STORM MELISSA DISCUSSION NUMBER 4 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL142007 500 AM EDT SAT SEP 29 2007 DEEP CONVECTION HAS BEEN ON THE INCREASE DURING THE PAST SEVERAL HOURS. 0319Z MICROWAVE IMAGERY INDICATED THAT THE CIRCULATION CENTER WAS UNDER THE WESTERN PORTION OF THE CONVECTIVE CANOPY...BUT SUBSEQUENT GOES IMAGES HINT THAT THE CONVECTION IS WRAPPING MORE AROUND THE CENTER. SUBJECTIVE DVORAK INTENSITY ESTIMATES AT 06Z WERE A CONSENSUS 30 KT...BUT OBJECTIVE ESTIMATES ARE HIGHER AND PROVIDE THE BASIS FOR UPGRADING THE SYSTEM TO A TROPICAL STORM WITH 35 KT MAXIMUM WINDS. IT IS CLEAR THAT THE DEPRESSION IS BARELY MOVING...BUT THE EXACT INITIAL MOTION IS STILL UNCERTAIN...ESTIMATED TO BE 270/3. THE CYCLONE IS EMBEDDED WITHIN VERY WEAK STEERING CURRENTS...LACKING A SUBTROPICAL RIDGE TO ITS NORTH DUE TO A DEEP LAYER LOW PRESSURE SYSTEM OVER THE NORTHEASTERN ATLANTIC. AS THAT LOW PROCEEDS EASTWARD...ALL OF THE MODELS DEPICT SOME RIDGING TO GRADUALLY BUILD BACK IN OVER THE TROPICAL EASTERN ATLANTIC...BUT THEY DO NOT AGREE ON HOW THE TRACK OF THE DEPRESSION WILL RESPOND. THE HWRF PROVIDES THE NORTHERNMOST SOLUTION...TAKING THE CYCLONE IMMEDIATELY NORTHWESTWARD...WHILE THE ECMWF AND GFS ENSEMBLE MEAN ARE THE SOUTHERN EXTREMES. IN GENERAL...THE MODELS SEEM TO HAVE SHIFTED AGAIN TO THE LEFT...AND SO HAS THE OFFICIAL FORECAST...WHICH CALLS FOR THE CYCLONE TO MOVE IN A GENERAL WEST-NORTHWESTWARD DIRECTION THROUGHOUT THE FIVE-DAY PERIOD. THE DEPRESSION CURRENTLY RESIDES IN A RELATIVELY WEAK-SHEAR ENVIRONMENT JUST TO THE SOUTH OF A BELT OF STRONG UPPER-LEVEL WESTERLIES. THE SHEAR WILL PROBABLY REMAIN WEAK ENOUGH TO ALLOW FOR A LITTLE STRENGTHENING DURING THE NEXT 24-36 HOURS...BUT THEN IT WILL BE ON THE INCREASE...ESPECIALLY BEYOND 48 HOURS WHEN SIGNIFICANT WEAKENING IS EXPECTED. THE OFFICIAL FORECAST PEAKS AT 40 KT...SLIGHTLY HIGHER THAN ALL OF THE OBJECTIVE GUIDANCE...BUT CALLS FOR THE DEPRESSION TO DISSIPATE IN LESS THAN FIVE DAYS IN THE HOSTILE ENVIRONMENT THAT LIES AHEAD LATER IN THE FORECAST PERIOD. FORECAST POSITIONS AND MAX WINDS INITIAL 29/0900Z 14.1N 27.4W 35 KT 12HR VT 29/1800Z 14.4N 28.7W 35 KT 24HR VT 30/0600Z 14.8N 30.5W 40 KT 36HR VT 30/1800Z 15.5N 32.7W 40 KT 48HR VT 01/0600Z 16.2N 34.8W 35 KT 72HR VT 02/0600Z 17.5N 38.5W 30 KT 96HR VT 03/0600Z 19.0N 42.0W 25 KT...DISSIPATING 120HR VT 04/0600Z 21.0N 45.0W 20 KT...REMNANT LOW $$ FORECASTER KNABB |
