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| #211857 (Received by flhurricane at: 10:52 PM 30.Aug.2008) TCDAT3 TROPICAL STORM HANNA DISCUSSION NUMBER 12 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL082008 1100 PM EDT SAT AUG 30 2008 ENHANCED INFRARED IMAGERY AND MICROWAVE OVERPASSES DEPICT A CONVECTIVELY CHALLENGED TROPICAL CYCLONE. THE VERTICAL STRUCTURE OF THE HANNA HAS TRANSFORMED INTO A SYSTEM SIMILAR TO A SUBTROPICAL CYCLONE...WITH A SLIGHT TILT TO THE WEST WITH HEIGHT. THE CYCLONE HAS BEEN STRUGGLING TO MAINTAIN DEEP CONVECTION NEAR THE PARTLY EXPOSED CENTER...PARTICULARLY OVER THE WEST AND SOUTH QUADRANTS. THERE APPEARS TO BE SOME MODERATE SUBSIDENCE SPREADING OVER THE AREA PRODUCED BY CONVERGING UPPER-LEVEL WINDS BETWEEN THE UPPER LOW NOW ASSOCIATED WITH HANNA AND GUSTAV'S OUTFLOW PATTERN...WITH THIS DRYER MORE STABLE AIR BEING ENTRAINED INTO THE SYSTEM'S CORE. THE INITIAL INTENSITY IS HELD AT 45 KT BASED ON DVORAK AND HEBERT/POTEAT INTENSITY TECHNIQUES...AS WELL AS A QUIKSCAT PASS THAT SUPPORTED THIS INTENSITY. THE INTENSITY MODELS CONTINUE TO INDICATE AN INCREASE IN NORTHERLY SHEAR AROUND THE 24 HOUR PERIOD...THEN TRANSITIONING TO A MORE SOUTH TO SOUTHWESTERLY COMPONENT AFTERWARD. THE SHIPS AND THE LGEM AGREE WITH GRADUAL STRENGTHENING UNTIL THE STRONG NORTHERLY SHEAR APPEARS...THEN THE MODELS AGREE WITH LITTLE CHANGE THROUGH THE 72 HOUR PERIOD. BEYOND DAY 3...THE DYNAMICAL MODELS INDICATE A HINT OF A MORE FAVORABLE UPPER WIND ENVIRONMENT...WHICH THE AFOREMENTIONED MODELS ALSO DEPICT. THE OFFICIAL FORECAST FOLLOWS THIS PHILOSOPHY AND INDICATES SLIGHT INTENSIFICATION AT THE 120 HOUR PERIOD. THE INITIAL MOTION IS ESTIMATED AT 300/7. DYNAMICAL MODELS SUGGEST THAT AN ANCHORED SUBTROPICAL RIDGE TO THE NORTH OF THE CYCLONE SHOULD INFLUENCE A GENERAL WEST-NORTHWESTWARD MOTION DURING THE NEXT 48 HOURS. AFTERWARD...A DEEP MID-LATITUDE LOW AND ASSOCIATED TROUGH IS EXPECTED TO MOVE OVER THE WESTERN ATLANTIC AND WEAKEN A PORTION OF THE RIDGE LOCATED OVER SOUTHEAST U.S. AND THE BAHAMAS WHILE A HIGH AMPLITUDE BLOCKING HIGH SETS UP OVER THE EASTERN SEABOARD. THIS PARTICULAR PATTERN IS EXPECTED TO CAUSE HANNA TO TEMPORARILY SLOW ITS FORWARD SPEED THROUGH THE 72 HOUR PERIOD WHILE MAINTAINING A WEST-NORTHWESTWARD TRACK. THROUGH THE REMAINDER OF THE FORECAST...A MID-LEVEL RIDGE IS FORECAST TO BUILD NORTHEAST OF HANNA IN RESPONSE TO THE AFOREMENTIONED MID-LATITUDE LOW LIFTING OUT AND MOVING AWAY FROM THE NORTHEAST COAST. THE MODEL SPREAD REMAINS RATHER LARGE...HOWEVER...THE MAJORITY ARE SHOWING A TURN TOWARD THE NORTHWEST OVER OR EAST OF THE BAHAMAS BEYOND 72 HOURS...WHILE THE UKMET IS STILL PREDICTING THE SUBTROPICAL RIDGE TO REMAIN INTACT TO THE NORTH. THE OFFICIAL FORECAST FOLLOWS THE MODEL CONSENSUS AND IS SIMILAR TO THE PREVIOUS ADVISORY BUT INDICATES A MORE NORTHWESTWARD MOTION BEYOND DAY 3. FORECAST POSITIONS AND MAX WINDS INITIAL 31/0300Z 22.9N 67.8W 45 KT 12HR VT 31/1200Z 23.3N 69.0W 45 KT 24HR VT 01/0000Z 23.6N 70.4W 50 KT 36HR VT 01/1200Z 23.6N 71.4W 55 KT 48HR VT 02/0000Z 23.6N 72.2W 55 KT 72HR VT 03/0000Z 23.8N 73.1W 55 KT 96HR VT 04/0000Z 24.5N 74.5W 55 KT 120HR VT 05/0000Z 26.5N 76.5W 60 KT $$ FORECASTER ROBERTS/BEVEN |
