603 
 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