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| #212078 (Received by flhurricane at: 4:41 PM 31.Aug.2008) TCDAT3 TROPICAL STORM HANNA DISCUSSION NUMBER 15 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL082008 500 PM EDT SUN AUG 31 2008 AN AIR FORCE RECONNAISSANCE AIRCRAFT INVESTIGATING HANNA DURING THE LAST SEVERAL HOURS FOUND A PEAK FLIGHT LEVEL WIND OF 48 KT WHICH REDUCES TO APPROXIMATELY 40 KT AT THE SURFACE. WHILE THERE WERE A FEW HIGHER VALUES FROM THE SFMR...THESE WERE LIKELY RAIN INFLATED. THE OVERALL INTENSITY FORECAST REASONING IS ESSENTIALLY UNCHANGED. HANNA IS CURRENTLY EXPERIENCING STRONG NORTHERLY SHEAR ASSOCIATED WITH AN UPPER-LEVEL LOW TO ITS SOUTHEAST. GLOBAL MODELS DO FORECAST THIS UPPER-LEVEL LOW TO WEAKEN OVER THE NEXT DAY OR SO. HOWEVER...STRONG NORTHERLY FLOW ASSOCIATED WITH GUSTAV'S UPPER-LEVEL OUTFLOW SHOULD KEEP NORTHERLY SHEAR OVER HANNA THROUGH DAY 3. BY DAYS 4 AND 5...DYNAMICAL MODELS FORECAST A SOMEWHAT IMPROVED ENVIRONMENT AS A SECOND UPPER-LEVEL LOW OR CYCLONIC SHEAR AXIS FORMS TO THE WEST OF HANNA. THIS WOULD PUT HANNA IN A MORE FAVORABLE ENVIRONMENT FOR STRENGTHENING. HOWEVER...ACCURATELY PREDICTING HANNA'S PROXIMITY TO LARGE-SCALE ATMOSPHERIC FEATURES AT DAYS 4 AND 5 IS DIFFICULT...TO SAY THE LEAST. GIVEN THE UNCERTAINTY...THE OFFICIAL FORECAST SHOWS SLOW STRENGTHENING...BUT IS BELOW THE INTENSITY MODEL CONSENSUS. HANNA CONTINUES TO BE STEERED BY A MID-LEVEL RIDGE OVER THE WESTERN ATLANTIC...RESULTING IN A MOTION TOWARD THE WEST OR 280/09. GLOBAL MODELS INDICATE THIS RIDGE WILL WEAKEN DURING THE NEXT DAY OR SO AS A LARGE DEEP-LAYER TROUGH AMPLIFIES SOUTHWARD OVER THE WESTERN ATLANTIC. SUCH AN EVOLUTION IN THE LARGE-SCALE PATTERN SHOULD CAUSE THE STEERING CURRENTS TO COLLAPSE RESULTING IN A DECREASE IN HANNA'S FORWARD SPEED. IN FACT...SOME OF THE MODELS SHOW HANNA MEANDERING OVER OR JUST NORTHEAST OF THE CENTRAL BAHAMAS. BY DAY 3...THE DEEP-LAYER TROUGH IS FORECAST TO LIFT NORTHEASTWARD ALLOWING THE SUBTROPICAL RIDGE TO BUILD BACK WESTWARD IN ITS WAKE. THERE REMAINS CONSIDERABLE SPREAD AMONG THE DYNAMICAL MODELS AT THE EXTENDED RANGE. THE GFS...GFDL...AND THE HWRF MODELS LIE ALONG THE RIGHT SIDE OF THE MODEL ENVELOPE AND SHOW HANNA LIFTING QUICKLY NORTHWESTWARD AROUND THE WESTERN PERIPHERY OF THE RIDGE. CONVERSELY...THE UKMET MODEL SHOWS THE RIDGE NOSING WESTWARD TO THE NORTH OF HANNA...PRODUCING A TRACK ALONG THE LEFT...OR SOUTHWEST...SIDE OF THE MODEL ENVELOPE. SINCE THERE IS NO CLEAR REASON TO FAVOR ONE SCENARIO OVER THE OTHER AT THIS TIME...I'VE ELECTED TO LEAVE THE OFFICIAL FORECAST ESSENTIALLY UNCHANGED BEYOND 72 HOURS. FORECAST POSITIONS AND MAX WINDS INITIAL 31/2100Z 23.5N 71.4W 40 KT 12HR VT 01/0600Z 23.5N 72.3W 40 KT 24HR VT 01/1800Z 23.3N 73.1W 45 KT 36HR VT 02/0600Z 23.3N 73.7W 50 KT 48HR VT 02/1800Z 23.5N 74.1W 55 KT 72HR VT 03/1800Z 24.5N 75.0W 60 KT 96HR VT 04/1800Z 27.5N 77.5W 65 KT 120HR VT 05/1800Z 31.5N 81.0W 65 KT $$ FORECASTER RHOME |
