Tropical Storm HANNA

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TROPICAL STORM HANNA DISCUSSION NUMBER  27
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082008
1100 AM EDT WED SEP 03 2008
 
THERE HAVE BEEN SOME STRUCTURAL CHANGES WITH HANNA TODAY AS IT
INTERACTS WITH THE UPPER-LEVEL LOW MOVING THROUGH THE NORTHWESTERN
BAHAMAS.  THE WIND FIELD HAS EXPANDED IN THE NORTHERN SEMICIRCLE
AND THE LOW-LEVEL CIRCULATION CENTER HAS BECOME LESS WELL DEFINED. 
VISIBLE IMAGERY SUGGESTS MULTIPLE LOW-LEVEL SWIRLS COMING OUT OF
THE DEEP CONVECTION TO THE NORTH OF A MEAN CENTER...AND THE
SYSTEM'S CONVECTIVE BANDS ARE WELL REMOVED FROM THE CORE.  THE
RECONNAISSANCE MISSION THAT ENDED AROUND 12Z REPORTED SOME SFMR
WINDS OF UP TO 45 KT ABOUT 70 NM FROM THE CENTER...AND THE INITIAL
INTENSITY ESTIMATE REMAINS 50 KT.

BECAUSE THE CENTER IS NOT WELL DEFINED...THE INTIAL MOTION IS HARD
TO GAUGE.  MY BEST ESTIMATE IS 360/5...AND THIS COULD BE THE START
OF THE EXPECTED NORTHWESTWARD TURN.  THE STEERING FLOW SHOULD
BECOME BETTER DEFINED OVER THE NEXT 24-48 HOURS AS THE UPPER LOW
MOVES WEST-SOUTHWESTWARD AND MID-LEVEL RIDGING BUILDS IN BEHIND THE
LARGE DEEP-LAYER LOW OFF THE UPPER U.S. EAST COAST.  MODEL GUIDANCE
RESPONDS TO THIS PATTERN WITH A NORTHWESTWARD ACCELERATION WITH A
RECURVATURE AROUND THE MID-LEVEL RIDGE IN ABOUT THREE DAYS.  THE
GUIDANCE HAS BEEN TOO FAST WITH THIS SCENARIO OVER THE PAST COUPLE
OF DAYS...AND GIVEN THE POOR CORE STRUCTURE OF HANNA I AM INCLINED
TO THINK THAT SUBSEQUENT INTERACTION WITH THE UPPER STEERING MAY BE
A LITTLE OVERDONE...AT LEAST INITIALLY.  THE OFFICIAL FORECAST IS
IN BEST AGREEMENT WITH THE ECMWF AND UKMET IN TERMS OF SPEED...WITH
THE GFS/HWRF/GFDL SHOWING A MORE VERTICAL SYSTEM MOVING MORE
RAPIDLY.  THE GUIDANCE HAS ALSO BEEN TRENDING EASTWARD OVER THE
PAST FEW CYCLES...AND I HAVE SHIFTED THE TRACK SLIGHTLY IN THAT
DIRECTION.

WESTERLY SHEAR AND DRY AIR ASSOCIATED WITH THE UPPER LOW WILL
INHIBIT DEVELOPMENT IN THE SHORT TERM BUT SHOULD DIMINISH OVER THE
NEXT COUPLE OF DAYS...TO BE REPLACED WITH MODERATE SOUTHEASTERLY
SHEAR.  THIS PATTERN WOULD PERMIT SOME RE-INTENSIFICATION...BUT
HANNA WILL NEED TO RECREATE AN INNER CORE IN ORDER TO INTENSIFY
SUBSTANTIALLY.  THE AVAILABLE GUIDANCE STILL BRINGS HANNA TO
MINIMAL HURRICANE INTENSITY AND THE OFFICIAL FORECAST IS JUST ABOVE
THIS GUIDANCE.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      03/1500Z 20.6N  71.8W    50 KT
 12HR VT     04/0000Z 21.5N  72.0W    50 KT
 24HR VT     04/1200Z 23.4N  73.3W    55 KT
 36HR VT     05/0000Z 25.2N  75.2W    60 KT
 48HR VT     05/1200Z 27.3N  77.1W    65 KT
 72HR VT     06/1200Z 33.0N  79.0W    70 KT
 96HR VT     07/1200Z 42.0N  71.0W    35 KT...INLAND EXTRATROPICAL
120HR VT     08/1200Z 47.0N  60.0W    35 KT...EXTRATROPICAL
 
$$
FORECASTER FRANKLIN
 
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