[Tropical] Tropical Cyclone Discussion

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WTNT43 KNHC 041450
TCDAT3
TROPICAL STORM HANNA DISCUSSION NUMBER  31
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082008
1100 AM EDT THU SEP 04 2008

VERY DRY AIR ASSOCIATED WITH THE UPPER-LEVEL LOW OVER THE
NORTHWESTERN BAHAMAS IS CHOKING OFF CONVECTION NEAR THE CORE OF
HANNA...AND THE CYCLONE HAS A VERY SUBTROPICAL APPEARANCE.
CONVECTION HAS DIMINISHED SUBSTANTIALLY OVER THE PAST 6-12 HOURS
AND THE HIGHEST FLIGHT-LEVEL AND SFMR WINDS HAVE COME DOWN AS WELL.
BASED ON SOME 52 KT SFMR WINDS AROUND 11Z...THE INITIAL INTENSITY
IS LOWERED TO 55 KT.

CONDITIONS ARE NOT CONDUCIVE TO STRENGTHENING RIGHT NOW...WITH
MODERATE SOUTHERLY SHEAR AND POOR THERMODYNAMICS.  THE GFS SUGGESTS
THAT DRY AIR IS LIKELY TO CONTINUE TO BE AN ISSUE UP TO LANDFALL.
HOWEVER...AS HANNA AND THE UPPER LOW BEGIN TO SEPARATE SOME OF THE
GLOBAL MODELS...INCLUDING THE GFS...SUGGEST THAT THE SHEAR WILL
LESSEN SOMEWHAT.  THIS WOULD ALLOW FOR SOME INTENSIFICATION...AND
SO THE OFFICIAL FORECAST CONTINUES TO SHOW HANNA ATTAINING
HURRICANE STRENGTH.  THIS IS IN GOOD AGREEMENT WITH THE
HWRF...ALTHOUGH THE GFDL AND SHIPS GUIDANCE IS LOWER.

THE INITIAL MOTION IS 315/10.  THE FORECAST SCENARIO IS UNCHANGED
FROM PREVIOUS ADVISORIES.  MID-LEVEL HIGH PRESSURE IS BUILDING OVER
THE WESTERN ATLANTIC AS A DEEP LAYER LOW MOVES AWAY FROM NEW
ENGLAND.  HANNA IS EXPECTED TO RECURVE AROUND THE PERIPHERY OF THE
HIGH AHEAD OF A MID-LATITUDE SHORT WAVE TROUGH IN THE WESTERLIES.
MODEL GUIDANCE REMAINS IN FAIRLY GOOD AGREEMENT AND LITTLE CHANGE
TO THE FORECAST TRACK IS REQUIRED.  THE OFFICIAL FORECAST IS
SLIGHTLY TO THE RIGHT OF THE MODEL CONSENSUS BECAUSE THE BETTER
PERFORMING MODELS THIS YEAR...NAMELY THE GFS AND GFDL...ARE ON THE
RIGHT SIDE OF THE GUIDANCE ENVELOPE.  BECAUSE OF THE LARGE...
SPRAWLING NATURE OF THE CIRCULATION...THE EXACT TRAJECTORY OF THE
CENTER IS RELATIVELY UNIMPORTANT.


FORECAST POSITIONS AND MAX WINDS

INITIAL      04/1500Z 24.5N  73.5W    55 KT
 12HR VT     05/0000Z 26.2N  75.2W    55 KT
 24HR VT     05/1200Z 28.7N  77.2W    60 KT
 36HR VT     06/0000Z 31.7N  78.2W    65 KT
 48HR VT     06/1200Z 35.4N  77.4W    55 KT
 72HR VT     07/1200Z 43.0N  69.5W    40 KT...INLAND
 96HR VT     08/1200Z 50.0N  54.0W    40 KT...EXTRATROPICAL
120HR VT     09/1200Z 51.5N  37.5W    45 KT...EXTRATROPICAL

$$
FORECASTER FRANKLIN

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