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| #211992 (Received by flhurricane at: 10:50 AM 31.Aug.2008) TCDAT3 TROPICAL STORM HANNA DISCUSSION NUMBER 14 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL082008 1100 AM EDT SUN AUG 31 2008 HANNA'S OVERALL SATELLITE APPEARANCE REMAINS RATHER RAGGED AND DEEP CONVECTION IS LIMITED NEAR THE LOW-LEVEL CIRCULATION. THE INITIAL INTENSITY IS LOWERED TO 45 KT BASED ON LATEST SATELLITE INTENSITY ESTIMATES AND NEARBY BUOY 41046. HANNA WILL ENCOUNTER A RATHER COMPLICATED ATMOSPHERIC ENVIRONMENT DURING THE NEXT COUPLE OF DAYS MAKING THE INTENSITY FORECAST PROBLEMATIC. WHILE MOST OF THE DYNAMICAL GUIDANCE SUGGEST THAT THE UPPER-LEVEL LOW CURRENTLY OVER HANNA WILL GRADUALLY WEAKEN...THEY HAVE NOT NECESSARILY HAD THE BEST TRACK RECORD FORECASTING THIS FEATURE. EVEN IF THE UPPER-LOW DOES WEAKEN AS SHOWN BY THE GLOBAL MODELS...GUSTAV'S OUTFLOW COULD PRODUCE STRONG NORTHERLY SHEAR OVER HANNA. INDEED WATER VAPOR IMAGERY CURRENTLY SHOWS A DEFORMATION AXIS FORMING NORTH OF HANNA AS A RESULT OF GUSTAV'S OUTFLOW INTERACTING WITH CYCLONIC FLOW AROUND THE UPPER-LEVEL LOW. BOTTOM LINE...THE ATMOSPHERE LOOKS RATHER HOSTILE FOR THE NEXT COUPLE OF DAYS AND SHOULD LIMIT THE POTENTIAL FOR SIGNIFICANT INTENSIFICATION. DYNAMICAL MODELS DO INDICATE SOME IMPROVEMENT IN THE ATMOSPHERIC ENVIRONMENT TOWARD THE END OF THE FORECAST AS A SECOND UPPER-LEVEL LOW OR CYCLONIC SHEAR AXIS FORMS TO THE WEST OF HANNA. HOWEVER...ACCURATELY PREDICTING THE ENVIRONMENT IN WHICH HANNA WILL BE LOCATED AT DAYS 4 AND 5 IS DIFFICULT...AND THE NEW OFFICIAL FORECAST IS LITTLE CHANGED FROM THE PREVIOUS ADVISORY. HANNA IS CURRENTLY SITUATED SOUTH OF 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-LAYERED TROUGH AMPLIFIES SOUTHWARD OVER THE WESTERN ATLANTIC. SUCH AN EVOLUTION IN THE LARGE-SCALE PATTERN IS EXPECTED TO CAUSE THE STEERING CURRENTS TO COLLAPSE RESULTING IN A REDUCTION IN HANNA'S FORWARD SPEED AND POSSIBLY SOME ERRATIC MOTION. BY DAYS 4 AND 5...THE DEEP-LAYERED TROUGH BEGINS TO LIFT NORTHEASTWARD AND THE SUBTROPICAL RIDGE BUILDS BACK WESTWARD IN ITS WAKE. MOST OF THE TRACK MODELS RESPOND BY TURNING HANNA NORTHWEST WITH AN INCREASE IN FORWARD SPEED. HOWEVER...THE UKMET MODEL ACTUALLY SHOWS THE RIDGE AXIS BUILDING SUFFICIENTLY FAR WEST...AND TO THE NORTH OF HANNA...TO RESULT IN MORE WESTWARD TRACK. SINCE THIS IS A NORMALLY GOOD PERFORMING MODEL...IT CANNOT COMPLETELY BE IGNORED AND THE NEW OFFICIAL FORECAST LIES SOUTH OF THE DYNAMICAL MODEL CONSENSUS. GIVEN THE HIGH SPREAD AMONG THE VARIOUS MODELS...THERE IS CONSIDERABLE UNCERTAINTY REGARDING HANNA'S EVENTUAL TRACK AND INTENSITY. FORECAST POSITIONS AND MAX WINDS INITIAL 31/1500Z 23.3N 70.0W 45 KT 12HR VT 01/0000Z 23.4N 71.1W 45 KT 24HR VT 01/1200Z 23.5N 72.1W 45 KT 36HR VT 02/0000Z 23.3N 72.9W 50 KT 48HR VT 02/1200Z 23.0N 73.4W 55 KT 72HR VT 03/1200Z 23.5N 74.5W 60 KT 96HR VT 04/1200Z 26.0N 76.0W 65 KT 120HR VT 05/1200Z 29.5N 79.0W 70 KT $$ FORECASTER RHOME |
