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| #449942 (Received by flhurricane at: 10:46 AM 14.Aug.2011) TCDAT2 TROPICAL DEPRESSION SEVEN DISCUSSION NUMBER 3 NWS NATIONAL HURRICANE CENTER MIAMI FL AL072011 1100 AM AST SUN AUG 14 2011 DURING THE PAST 6 HOURS...CONVECTION HAS BEGUN TO REDEVELOP OVER AND SOUTHEAST OF THE LOW-LEVEL CENTER OF THE DEPRESSION IN RESPONSE TO DECREASING VERTICAL WIND SHEAR AND INCREASING MID-LEVEL MOISTURE. ALTHOUGH DATA T-NUMBERS HAVE DECREASED TO T1.5 AND T1.0 FROM TAFB AND SAB...RESPECTIVELY...THE INITIAL INTENSITY REMAINS AT 30 KT BASED ON SATELLITE CURRENT INTENSITY ESTIMATES OF T2.0/30 KT AND THE IMPROVED CONVECTIVE ORGANIZATION SINCE THE 12Z SATELLITE FIXES. WATER VAPOR IMAGERY INDICATES THE DEPRESSION IS MOVING INTO A COL REGION ON THE NORTH SIDE OF AN UPPER-LEVEL LOW THAT IS LOCATED ABOUT 150 NMI SOUTHWEST OF THE CYCLONE. AS A RESULT...THE PREVIOUS WESTERLY VERTICAL SHEAR REGIME IS BEING REPLACED BY A MORE FAVORABLE SOUTHEASTERLY LOW-SHEAR ENVIRONMENT...WHICH SHOULD FAVOR THE CONTINUED DEVELOPMENT OF DEEP CONVECTION FOR THE NEXT 12-24 HOURS. THE CYCLONE IS EXPECTED TO REMAIN WITHIN A FAVORABLE UPPER-LEVEL FLOW ENVIRONMENT FOR THE NEXT 48-60 HOURS...WHICH WOULD NORMALLY FAVOR SIGNIFICANT INTENSIFICATION. HOWEVER...THERMODYNAMIC CONDITIONS ARE FORECAST TO BE LESS FAVORABLE AS THE CYCLONE MOVES OVER SUB-26C SSTS BY 48 HOURS. THESE OUT-OF-PHASE CONDITIONS ARE EXPECTED TO RESULT IN ONLY MODEST STRENGTHENING. BY DAY 4...THE POST-TROPICAL CYCLONE IS FORECAST TO BE ABSORBED BY A FRONT. THE OFFICIAL INTENSITY FORECAST IS SIMILAR TO THE PREVIOUS ADVISORY AND CLOSELY FOLLOWS A BLEND OF THE SHIPS AND LGEM INTENSITY MODELS. THE DEPRESSION HAS SLOWED AND MADE AN UNEXPECTED JOG TO THE WEST... AND THE INITIAL MOTION ESTIMATE IS NOW 285/09. THE TRACK FORECAST IS LESS STRAIGHTFORWARD THAN THE INTENSITY FORECAST. THE RECENT WESTWARD JOG IS PROBABLY DUE IN PART TO THE SYSTEM BECOMING MORE VERTICALLY SHALLOW AFTER THE DEEP CONVECTION WEAKENED EARLIER THIS MORNING. NOW THAT CONVECTION HAS REDEVELOPED...THE CYCLONE SHOULD TURN NORTHWESTWARD WITHIN THE NEXT 12 HOURS AS IT COMES UNDER THE INFLUENCE OF DEEP-LAYER SOUTHEASTERLY STEERING FLOW. BEYOND 12 HOURS...THE TRACK FORECAST BECOMES MORE COMPLICATED. THE 12Z UPPER-AIR DATA FROM BERMUDA THIS MORNING INDICATES THE MID-LEVEL FLOW FROM 700-400 MB HAS BACKED AROUND TO THE SOUTH-SOUTHWEST AND HEIGHTS HAVE FALLEN 40-60 METERS WITHIN THIS LAYER. THIS SUGGESTS THAT THE SUBTROPICAL RIDGE TO THE NORTH OF THE DEPRESSION HAS WEAKENED...WHICH SHOULD ALLOW THE SYSTEM TO TURN NORTHWESTWARD OR EVEN NORTHWARD. HOWEVER...WATER VAPOR IMAGERY INDICATES THAT A SHORTWAVE TROUGH APPROACHING BERMUDA RAPIDLY FROM THE WEST IS THE LIKELY CAUSE OF THE EROSION OF THE RIDGE. WITH THE MAIN UPPER-LEVEL TROUGH STILL OVER THE U.S. OHIO AND TENNESSEE VALLEYS...THE RIDGE COULD TEMPORARILY BUILD BACK WESTWARD BETWEEN BERMUDA AND THE DEPRESSION...RESULTING IN A STAIR-STEP TYPE OF MOTION TO THE NORTHWEST DURING THE NEXT 24 HOURS OR SO. THE MAIN UPPER-LEVEL TROUGH IS EXPECTED TO LIFT THE CYCLONE QUICKLY TO THE NORTHEAST BY 36-48 HOURS AND TAKE IT ACROSS MUCH COLDER WATER IN 60-72 HOURS...WHICH WILL INDUCE EXTRATROPICAL TRANSITION. THE OFFICIAL FORECAST TRACK HAS BEEN NUDGED A LITTLE TO THE WEST OF THE PREVIOUS FORECAST TRACK THROUGH 24 HOURS BASED ON THE MORE WESTWARD INITIAL POSITION...AND LIES JUST TO THE LEFT OF THE GLOBAL MODEL CONSENSUS...BUT WELL TO THE EAST OF THE TIGHTLY PACKED BAM MODELS. AS A RESULT OF THE WESTWARD SHIFT IN THE FORECAST TRACK...WHICH BRINGS THE CYCLONE CLOSER TO BERMUDA...THE BERMUDA WEATHER SERVICE HAS ISSUED A TROPICAL STORM WARNING FOR BERMUDA. FORECAST POSITIONS AND MAX WINDS INIT 14/1500Z 28.2N 63.2W 30 KT 35 MPH 12H 15/0000Z 29.6N 63.7W 35 KT 40 MPH 24H 15/1200Z 31.8N 63.8W 40 KT 45 MPH 36H 16/0000Z 34.9N 62.4W 45 KT 50 MPH 48H 16/1200Z 38.6N 58.9W 50 KT 60 MPH 72H 17/1200Z 47.0N 49.0W 45 KT 50 MPH...POST-TROP/EXTRATROP 96H 18/1200Z...DISSIPATED $$ FORECASTER STEWART |
