Tropical Storm HANNA

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TROPICAL STORM HANNA DISCUSSION NUMBER  25
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082008
1100 PM EDT TUE SEP 02 2008
 
THE LAST COUPLE OF FIXES FROM THE RECONNAISSANCE AIRCRAFT 
INDICATED THAT HANNA WAS MOVING SLOWLY EASTWARD.  IT SEEMS THAT
HANNA IS MAKING A SMALL COUNTER-CLOCKWISE LOOP BETWEEN HISPANIOLA
AND THE SOUTHEASTERN BAHAMAS.  I HAVE TO GIVE SOME CREDIT TO THE
GFDL AND HWRF MODELS WHICH PREDICTED THIS LOOP A FEW DAYS AGO...
HOWEVER THEY INDICATED THAT HANNA WOULD BE A LITTLE FURTHER NORTH
WHEN IT OCCURRED.  SINCE THE AIRCRAFT DEPARTED HANNA THE CLOUD
PATTERN HAS CONTINUED TO DEGRADE...AND IT IS POSSIBLE THAT HANNA
HAS WEAKENED EVEN MORE.  HOWEVER...WE WILL KEEP THE INITIAL
INTENSITY AT 55 KT SINCE ANOTHER AIRCRAFT IS SCHEDULED TO
INVESTIGATE THE STORM AROUND 0600 UTC.
   
STRONG NORTHWESTERLY UPPER-LEVEL WINDS CONTINUE TO SHEAR HANNA.
BUT...THE GLOBAL MODELS FORECAST AN UPPER-LEVEL LOW TO CUT-OFF TO
THE NORTHWEST OF HANNA VERY SOON...AND THERE IS SOME EVIDENCE IN
WATER VAPOR IMAGERY THAT THIS IS BEGINNING TO OCCUR.  AS THE
UPPER-LOW CUTS-OFF IT IS FORECAST TO DROP SOUTHWARD WHICH SHOULD
TURN THE UPPER-FLOW OVER THE STORM FROM THE SOUTH AND SOUTHEAST...
WHICH WOULD BE MORE FAVORABLE FOR STRENGTHENING DURING THE NEXT
COUPLE OF DAYS.  THE NEW INTENSITY FORECAST IS SIMILAR TO THE
PREVIOUS ADVISORY AND IS CLOSE TO THE CONSENSUS OF THE GUIDANCE.  AT
96 HOURS...HANNA WAS MAINTAINED AS A TROPICAL STORM...BECAUSE IT IS
PREDICTED TO BE FAIRLY CLOSE TO THE COAST OF THE NORTHEAST UNITED
STATES AND THERE COULD STILL BE TROPICAL STORM FORCE WINDS OVER
WATER AT THAT TIME.

THE TRACK MODELS SUGGEST THAT HANNA WILL BEGIN MOVING NORTHWESTWARD
VERY SOON AS THE LARGE UPPER-LEVEL TROUGH OFF THE NORTHEAST UNITED
STATES COAST LIFTS NORTHWARD...ALLOWING THE SUBTROPICAL RIDGE OVER
THE ATLANTIC TO BUILD WESTWARD.  THIS PATTERN IS FORECAST TO STEER
HANNA ON A GENERAL NORTHWESTWARD MOTION WITH SOME INCREASE IN
FORWARD SPEED OVER THE NEXT COUPLE OF DAYS.  SOME OF THE TRACK
MODELS HAVE SHIFTED A LITTLE EASTWARD...BUT NOT ENOUGH TO WARRANT A
CHANGE TO THE OFFICIAL FORECAST.  THE NEW TRACK HAS BEEN ADJUSTED
EASTWARD DURING THE FIRST 24-36 HOURS DUE TO THE MORE EASTWARD
INITIAL LOCATION...BUT THEREAFTER IT REMAINS VERY CLOSE TO THE
PREVIOUS ADVISORY.  AS A REMINDER...ONE SHOULD NOT FOCUS ON THE
EXACT TRACK AS SMALL DEVIATIONS LEFT OR RIGHT WOULD HAVE LARGE
IMPLICATIONS ON BOTH THE TIME AND LOCATION OF LANDFALL.  A G-IV
MISSION WAS FLOWN THIS EVENING AND THE DATA GATHERED BY THIS
AIRCRAFT SHOULD GET ASSIMILATED INTO THE 0000 UTC MODEL GUIDANCE. 
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      03/0300Z 20.5N  72.4W    55 KT
 12HR VT     03/1200Z 21.3N  72.3W    55 KT
 24HR VT     04/0000Z 22.8N  73.6W    60 KT
 36HR VT     04/1200Z 24.4N  75.5W    65 KT
 48HR VT     05/0000Z 26.4N  77.4W    70 KT
 72HR VT     06/0000Z 32.5N  80.5W    75 KT...INLAND
 96HR VT     07/0000Z 41.0N  75.5W    35 KT...INLAND
120HR VT     08/0000Z 50.0N  60.0W    35 KT...EXTRATROPICAL
 
$$
FORECASTER BROWN/AVILA
 
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