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| #211766 (Received by flhurricane at: 4:55 PM 30.Aug.2008) TCDAT3 TROPICAL STORM HANNA DISCUSSION NUMBER 11 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL082008 500 PM EDT SAT AUG 30 2008 THE CLOUD PATTERN OF HANNA HAS CHANGED QUITE A BIT DURING THE DAY TODAY. THE TROPICAL STORM HAS BECOME INVOLVED WITH THE UPPER-LEVEL LOW THAT WAS LOCATED TO ITS WEST...AND HANNA NOW HAS SOME RESEMBLANCE OF A SUB-TROPICAL CYCLONE. EVEN THOUGH THE CENTER IS LOCATED WELL SOUTHWEST OF THE THUNDERSTORM ACTIVITY...BANDING OVER THE NORTHEASTERN SEMICIRCLE STILL YIELD INTENSITY ESTIMATES OF T3.0 OR 45 KT FROM BOTH AGENCIES. HANNA HAS BEEN MOVING BETWEEN THE WEST AND WEST-NORTHWEST AT ABOUT 7 KT TODAY. A SIMILAR MOTION IS FORECAST BY THE NUMERICAL MODEL GUIDANCE FOR THE NEXT 24-36 HOURS...SO VERY LITTLE CHANGE WAS NEEDED TO THE PREVIOUS FORECAST DURING THAT TIME. THEREAFTER...THE MODEL SPREAD HAS BECOME EXTREMELY LARGE AS A STRONG HIGH PRESSURE RIDGE BUILDS OVER THE EASTERN UNITED STATES. THE TRACK GUIDANCE NOW PREDICTS A SLOW AND ERRATIC MOTION OF HANNA DURING THE 48 TO ABOUT 96 HOUR TIME FRAME. THE GFDL AND HWRF BOTH PREDICT HANNA TO MAKE A CYCLONIC LOOP NEAR THE SOUTHEASTERN BAHAMAS. MEANWHILE...THE ECMWF AND UKMET SHOW A SOUTHWESTWARD MOTION...WITH THE LATTER MODEL FORECASTING HANNA TO BE OVER EASTERN CUBA. BEYOND 96 HOURS... NEARLY ALL OF THE GUIDANCE...EXCEPT THE UKMET...NOW FORECAST A WEST-NORTHWESTWARD OR NORTHWESTWARD MOTION TO RESUME AS THE RIDGE OVER THE EASTERN UNITED STATES WEAKENS. WITH ALL OF THAT SAID... THERE IS VERY LOW CONFIDENCE IN THE TRACK FORECAST. THE NEW TRACK SLOWS DOWN THE STORM CONSIDERABLY...AND NOW PREDICTS A SLOW WEST-NORTHWESTWARD MOTION LATE IN THE PERIOD. THE NHC TRACK IS A LITTLE SOUTH OF THE CONSENSUS...ESPECIALLY AT DAY 5...BUT IS IN GENERAL AGREEMENT WITH THE 12Z ECMWF. GIVEN ALL THE UNCERTAINTIES IN THE TRACK FORECAST...THE INTENSITY FORECAST SEEMS JUST AS DIFFICULT. THE DYNAMICAL MODELS AND SHIPS GUIDANCE SUGGEST THAT WEST TO NORTHWESTERLY SHEAR WILL CONTINUE DURING THE NEXT 24-36 HOURS...FOLLOWED BY AN INCREASE IN NORTHERLY SHEAR. GIVEN THE UNCERTAINTIES AS TO HOW HANNA WILL RESPOND TO THE SHEAR...THE INTENSITY FORECAST ALLOWS FOR SOME STRENGTHENING IN THE SHORT-TERM...BUT LEVELS OFF THE INTENSITY BETWEEN 36-96 HOURS. THERE ARE SOME INDICATIONS THAT THE UPPER-LEVEL ENVIRONMENT COULD BECOME MORE CONDUCIVE VERY LATE IN THE PERIOD...SO SLIGHT STRENGTHENING IS INDICATE AT DAY 5. THE NHC FORECAST REMAINS BELOW ALL OF THE RELIABLE INTENSITY GUIDANCE. THE 12-FT SEA RADII WERE ADJUSTED OUTWARD OVER THE NORTHWESTERN SEMICIRCLE BASED ON REPORTS FROM NOAA BUOY 41046. FORECAST POSITIONS AND MAX WINDS INITIAL 30/2100Z 22.4N 67.2W 45 KT 12HR VT 31/0600Z 22.8N 68.6W 45 KT 24HR VT 31/1800Z 23.3N 70.2W 50 KT 36HR VT 01/0600Z 23.5N 71.4W 55 KT 48HR VT 01/1800Z 23.5N 72.1W 55 KT 72HR VT 02/1800Z 23.5N 72.8W 55 KT 96HR VT 03/1800Z 24.0N 74.0W 55 KT 120HR VT 04/1800Z 25.0N 76.5W 60 KT $$ FORECASTER BROWN |
