Tropical Storm HANNA

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TROPICAL STORM HANNA DISCUSSION NUMBER  14
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082008
1100 AM EDT SUN AUG 31 2008
 
HANNA'S OVERALL SATELLITE APPEARANCE REMAINS RATHER RAGGED AND DEEP
CONVECTION IS LIMITED NEAR THE LOW-LEVEL CIRCULATION.  THE INITIAL
INTENSITY IS LOWERED TO 45 KT BASED ON LATEST SATELLITE INTENSITY
ESTIMATES AND NEARBY BUOY 41046.  HANNA WILL ENCOUNTER A RATHER
COMPLICATED ATMOSPHERIC ENVIRONMENT DURING THE NEXT COUPLE OF DAYS
MAKING THE INTENSITY FORECAST PROBLEMATIC.  WHILE MOST OF THE
DYNAMICAL GUIDANCE SUGGEST THAT THE UPPER-LEVEL LOW CURRENTLY OVER
HANNA WILL GRADUALLY WEAKEN...THEY HAVE NOT NECESSARILY HAD THE
BEST TRACK RECORD FORECASTING THIS FEATURE.  EVEN IF THE UPPER-LOW
DOES WEAKEN AS SHOWN BY THE GLOBAL MODELS...GUSTAV'S OUTFLOW COULD
PRODUCE STRONG NORTHERLY SHEAR OVER HANNA.  INDEED WATER VAPOR
IMAGERY CURRENTLY SHOWS A DEFORMATION AXIS FORMING NORTH OF HANNA
AS A RESULT OF GUSTAV'S OUTFLOW INTERACTING WITH CYCLONIC FLOW
AROUND THE UPPER-LEVEL LOW.   BOTTOM LINE...THE ATMOSPHERE LOOKS
RATHER HOSTILE FOR THE NEXT COUPLE OF DAYS AND SHOULD LIMIT THE
POTENTIAL FOR SIGNIFICANT INTENSIFICATION.  DYNAMICAL MODELS DO
INDICATE SOME IMPROVEMENT IN THE ATMOSPHERIC ENVIRONMENT TOWARD THE
END OF THE FORECAST AS A SECOND UPPER-LEVEL LOW OR CYCLONIC SHEAR
AXIS FORMS TO THE WEST OF HANNA.  HOWEVER...ACCURATELY PREDICTING
THE ENVIRONMENT IN WHICH HANNA WILL BE LOCATED AT DAYS 4 AND 5 IS
DIFFICULT...AND THE NEW OFFICIAL FORECAST IS LITTLE CHANGED FROM
THE PREVIOUS ADVISORY.  

HANNA IS CURRENTLY SITUATED SOUTH OF A MID-LEVEL RIDGE OVER THE
WESTERN ATLANTIC RESULTING IN A MOTION TOWARD THE WEST OR 280/09. 
GLOBAL MODELS INDICATE THIS RIDGE WILL WEAKEN DURING THE NEXT DAY
OR SO AS A LARGE DEEP-LAYERED TROUGH AMPLIFIES SOUTHWARD OVER THE
WESTERN ATLANTIC.  SUCH AN EVOLUTION IN THE LARGE-SCALE PATTERN IS
EXPECTED TO CAUSE THE STEERING CURRENTS TO COLLAPSE RESULTING IN A
REDUCTION IN HANNA'S FORWARD SPEED AND POSSIBLY SOME ERRATIC
MOTION.  BY DAYS 4 AND 5...THE DEEP-LAYERED TROUGH BEGINS TO LIFT
NORTHEASTWARD AND THE SUBTROPICAL RIDGE BUILDS BACK WESTWARD IN ITS
WAKE.  MOST OF THE TRACK MODELS RESPOND BY TURNING HANNA NORTHWEST
WITH AN INCREASE IN FORWARD SPEED.  HOWEVER...THE UKMET MODEL
ACTUALLY SHOWS THE RIDGE AXIS BUILDING SUFFICIENTLY FAR WEST...AND
TO THE NORTH OF HANNA...TO RESULT IN MORE WESTWARD TRACK.  SINCE
THIS IS A NORMALLY GOOD PERFORMING MODEL...IT CANNOT COMPLETELY BE
IGNORED AND THE NEW OFFICIAL FORECAST LIES SOUTH OF THE DYNAMICAL
MODEL CONSENSUS.  

GIVEN THE HIGH SPREAD AMONG THE VARIOUS MODELS...THERE IS
CONSIDERABLE UNCERTAINTY REGARDING HANNA'S EVENTUAL TRACK AND
INTENSITY. 

FORECAST POSITIONS AND MAX WINDS
 
INITIAL      31/1500Z 23.3N  70.0W    45 KT
 12HR VT     01/0000Z 23.4N  71.1W    45 KT
 24HR VT     01/1200Z 23.5N  72.1W    45 KT
 36HR VT     02/0000Z 23.3N  72.9W    50 KT
 48HR VT     02/1200Z 23.0N  73.4W    55 KT
 72HR VT     03/1200Z 23.5N  74.5W    60 KT
 96HR VT     04/1200Z 26.0N  76.0W    65 KT
120HR VT     05/1200Z 29.5N  79.0W    70 KT
 
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