Tropical Storm HANNA

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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
 
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