726 
 WTNT43 KNHC 310855
 TCDAT3
 TROPICAL STORM HANNA DISCUSSION NUMBER  13
 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082008
 500 AM EDT SUN AUG 31 2008
  
 HANNA HAS BEEN GOING THROUGH SOME STRUCTURAL CHANGES THAT HAVE
 RESEMBLED A SUBTROPICAL CYCLONE AT TIMES AS AN UPPER-LEVEL COLD LOW
 AND DRY AIR HAVE BECOME ENTANGLED WITH THE INNER-CORE CIRCULATION.
 HOWEVER...DURING THE PAST 6 HOURS WATER VAPOR IMAGERY HAS INDICATED
 THAT THE UPPER-LOW HAS BEGUN TO DROP SOUTHWARD TOWARD HISPANIOLA
 ...WHICH MAY ALLOW HANNA TO TRANSITION BACK INTO A FULL WARM CORE
 TROPICAL CYCLONE LATER TODAY. THE INTENSITY WAS INCREASED TO 50 KT
 AND THE WIND RADII WERE EXPANDED SIGNIFICANTLY IN THE NORTHERN
 SEMICIRCLE BASED ON A 30/2231Z QUIKSCAT OVERPASS THAT WAS NOT
 AVAILABLE FOR THE PREVIOUS ADVISORY. SATELLITE ESTIMATES FROM TAFB
 AND SAB ARE ST3.0/45 KT.
  
 THE INITIAL MOTION ESTIMATE AT 290/10. ALL OF THE NHC MODEL GUIDANCE
 NOW INDICATES SOME SOUTHWARD COMPONENT TO HANNA'S TRACK BY 48 HOURS
 AS STRONG NORTHERLY FLOW IMPINGES ON THE CYCLONE BEHIND A DIGGING
 TROUGH OFF THE U.S. EAST COAST COMBINED WITH OUTFLOW FROM HURRICANE
 GUSTAV LOCATED WELL TO THE WEST OF HANNA. THE SHIPS MODEL INDICATES
 AT LEAST 30 KT OF NORTHERLY SHEAR ACROSS HANNA AT 48 HOURS...WHICH
 SHOULD ACT TO DISPLACE ANY DEEP CONVECTION INTO THE SOUTHERN
 SEMICIRCLE AND ALSO HELP TO FORCE THE CYCLONE SOUTHWARD. SHOULD
 HANNA BE STRIPPED OF ALL CONVECTION AROUND THAT TIME PERIOD...THE
 SHALLOW BAM MODEL ALSO INDICATES A SOUTHWARD COMPONENT OF MOTION
 ...INCLUDING A CYCLONIC LOOP. HOWEVER...BY 96 HOURS... NEARLY ALL OF
 THE MODELS FORECAST THE STRONG UPPER-TROUGH TO LIFT OUT TO THE
 NORTHEAST AND BE REPLACED BY A LARGE RIDGE TO THE NORTH AND EAST OF
 HANNA. THE LATTER FEATURE IS EXPECTED TO GRADUALLY INDUCE A
 NORTHWESTWARD COMPONENT OF MOTION BY DAYS 4 AND 5. THE KEY TO THE
 FORECAST TRACK AT THOSE FORECAST TIMES WILL BE JUST HOW FAR SOUTH
 AND WEST HANNA MOVES BEFORE IT BEGINS TO MOVE NORTHWESTWARD. MOST
 OF THE MODEL GUIDANCE KEEPS HANNA OVER OR JUST EAST OF THE
 BAHAMAS...THE EXCEPTION BEING THE UKMET AND ECMWF MODELS...WHICH
 BRING HANNA NEAR THE NORTH-CENTRAL COAST OF CUBA BY DAY 4 AND INTO
 THE FLORIDA STRAITS BY DAY 5. AT THIS TIME...THE SPECIFICS OF THOSE
 TWO TRACKS ARE CONSIDERED TO BE OUTLIERS...BUT THEY HAVE NOT BEEN
 ENTIRELY DISCOUNTED. THE OFFICIAL TRACK IS CLOSE TO THE PREVIOUS
 FORECAST TRACK AND THE NHC MODEL CONSENSUS.
 
 THE INTENSITY FORECAST IS SOMEWHAT TRICKY. WITH MORE THAN 30 KT OF
 SHEAR FORECAST TO AFFECT HANNA BY 48 HOURS...THE CYCLONE COULD
 EASILY BE SHEARED APART. HOWEVER...WHILE THE SHEAR WILL BE
 STRONG...ITS NEGATIVE EFFECTS MAY BE OFFSET BY THE STRONG
 UPPER-LEVEL DIFLUENT PATTERN THAT IS ALSO FORECAST TO EXIST ACROSS
 HANNA AT THAT TIME. BY 72-96 HOURS...ALL OF THE MODELS AGREE ON
 ANTICYCLONIC OUTFLOW REPLACING THE STRONG NORTHERLY SHEAR
 PATTERN...INCLUDING OUTFLOW JETS DEVELOPING TO THE NORTH AND
 SOUTH...WHICH WOULD FAVOR SIGNIFICANT STRENGTHENING OVER 29-30C
 SSTS BENEATH THE CYCLONE. HOWEVER...THE INTENSITY FORECAST HAS BEEN
 HELD BELOW ALL OF THE AVAILABLE MODEL GUIDANCE DUE TO THE
 UNCERTAINTY OF WHAT THE EXACT STRUCTURE OF HANNA WILL BE AFTER THE
 UPCOMING STRONG SHEAR PATTERN ABATES IN AROUND 72 HOURS.
  
 FORECAST POSITIONS AND MAX WINDS
  
 INITIAL      31/0900Z 23.2N  69.0W    50 KT
  12HR VT     31/1800Z 23.6N  70.2W    50 KT
  24HR VT     01/0600Z 23.7N  71.4W    50 KT
  36HR VT     01/1800Z 23.7N  72.5W    50 KT
  48HR VT     02/0600Z 23.3N  73.1W    50 KT
  72HR VT     03/0600Z 23.6N  73.9W    55 KT
  96HR VT     04/0600Z 25.1N  75.4W    60 KT
 120HR VT     05/0600Z 28.0N  78.0W    70 KT
  
 $$
 FORECASTER STEWART