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| #211251 (Received by flhurricane at: 4:47 PM 28.Aug.2008) TCDAT3 TROPICAL STORM HANNA DISCUSSION NUMBER 3 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL082008 500 PM EDT THU AUG 28 2008 AFTER LOOKING QUITE IMPRESSIVE IN SATELLITE IMAGERY THIS MORNING...VERTICAL SHEAR HAS TAKEN ITS TOLL ON HANNA AND THE LOW-LEVEL CIRCULATION HAS BECOME COMPLETELY EXPOSED. WHILE IT IS TEMPTING TO LOWER THE INTENSITY BASED ON THIS TREND...IT IS BEST TO KEEP HANNA AS A MINIMAL TROPICAL STORM FOR THIS ADVISORY. THE LACK OF ORGANIZATION NOTWITHSTANDING...THE OVERALL FORECAST PHILOSOPHY REMAINS UNCHANGED. DYNAMICAL MODELS INSISTS THAT THE UPPER-LEVEL LOW CURRENTLY IMPARTING WESTERLY SHEAR OVER HANNA WILL BEGIN TO MOVE WESTWARD ALLOWING A MORE FAVORABLE ENVIRONMENT FOR STRENGTHENING TO DEVELOP OVER THE CYCLONE. SIMILARLY...THE SHIPS AND LGEM MODELS ARE BASING THEIR FORECAST ON A LOW SHEAR ENVIRONMENT. ANALYSIS OF WATER VAPOR IMAGERY INDICATES THAT THE UPPER-LEVEL LOW HAS NOT BUDGED ALL DAY SO IT WOULD APPEAR THAT THE SHEAR WILL NOT RELAX IN THE SHORT-TERM. THIS IS THE BASIS FOR THE NEW OFFICIAL FORECAST WHICH IS BELOW THE INTENSITY MODEL CONSENSUS. IF THE UPPER-LOW DOES EVENTUALLY MOVE OUT OF HANNA'S WAY...ENVIRONMENTAL CONDITIONS WOULD FAVOR STRENGTHENING. BY DAY 5...UPPER-LEVEL OUTFLOW FROM GUSTAV COULD IMPART NORTHERLY SHEAR OVER HANNA POSSIBLY AFFECTING THE INTENSITY. HANNA CONTINUES MOVING WEST-NORTHWESTWARD...OR 300/10. HOWEVER...A MOTION TOWARD THE NORTHWEST IS FORECAST OVER THE NEXT 2-3 DAYS AS HANNA IS EXPECTED TO STRENGTHEN AND BECOMES A DEEPER SYSTEM. THEREAFTER...THE TRACK FORECAST IS RIDDLED WITH UNCERTAINTY AS IT IS UNCLEAR HOW HANNA WILL INTERACT WITH A MID-LATITUDE TROUGH FORECAST TO MOVE OFF THE U.S. EAST COAST. MOST OF THE DYNAMICAL MODELS SHOW THE TROUGH BYPASSING HANNA TO THE NORTH AND LEAVING IT BEHIND IN AN AREA OF WEAK STEERING. SUCH AN EVOLUTION WOULD RESULT IN ERRATIC OR LITTLE OVERALL MOTION. ACCORDINGLY...THE OFFICIAL FORECAST SHOWS A SLOW WESTWARD MOTION AT DAYS 4 AND 5. IT SHOULD BE ADDED THAT THERE IS LARGER THAN NORMAL UNCERTAINTY ASSOCIATED WITH BOTH THE TRACK AND INTENSITY FORECASTS OF HANNA. FORECAST POSITIONS AND MAX WINDS INITIAL 28/2100Z 20.7N 60.1W 35 KT 12HR VT 29/0600Z 21.7N 61.4W 35 KT 24HR VT 29/1800Z 23.4N 63.4W 40 KT 36HR VT 30/0600Z 24.9N 65.3W 50 KT 48HR VT 30/1800Z 25.9N 67.0W 60 KT 72HR VT 31/1800Z 27.0N 69.0W 70 KT 96HR VT 01/1800Z 27.5N 70.5W 70 KT 120HR VT 02/1800Z 27.5N 71.5W 65 KT $$ FORECASTER RHOME |
