Tropical Storm HANNA

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TROPICAL STORM HANNA DISCUSSION NUMBER   3
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082008
500 PM EDT THU AUG 28 2008
 
AFTER LOOKING QUITE IMPRESSIVE IN SATELLITE IMAGERY THIS
MORNING...VERTICAL SHEAR HAS TAKEN ITS TOLL ON HANNA AND THE
LOW-LEVEL CIRCULATION HAS BECOME COMPLETELY EXPOSED.  WHILE IT IS
TEMPTING TO LOWER THE INTENSITY BASED ON THIS TREND...IT IS BEST TO
KEEP HANNA AS A MINIMAL TROPICAL STORM FOR THIS ADVISORY.  THE LACK
OF ORGANIZATION NOTWITHSTANDING...THE OVERALL FORECAST PHILOSOPHY
REMAINS UNCHANGED. DYNAMICAL MODELS INSISTS THAT THE UPPER-LEVEL
LOW CURRENTLY IMPARTING WESTERLY SHEAR OVER HANNA WILL BEGIN TO MOVE
WESTWARD ALLOWING A MORE FAVORABLE ENVIRONMENT FOR STRENGTHENING TO
DEVELOP OVER THE CYCLONE.  SIMILARLY...THE SHIPS AND LGEM MODELS
ARE BASING THEIR FORECAST ON A LOW SHEAR ENVIRONMENT.  ANALYSIS OF
WATER VAPOR IMAGERY INDICATES THAT THE UPPER-LEVEL LOW HAS NOT
BUDGED ALL DAY SO IT WOULD APPEAR THAT THE SHEAR WILL NOT RELAX IN
THE SHORT-TERM.  THIS IS THE BASIS FOR THE NEW OFFICIAL FORECAST
WHICH IS BELOW THE INTENSITY MODEL CONSENSUS.  IF THE UPPER-LOW
DOES EVENTUALLY MOVE OUT OF HANNA'S WAY...ENVIRONMENTAL CONDITIONS
WOULD FAVOR STRENGTHENING. BY DAY 5...UPPER-LEVEL OUTFLOW FROM
GUSTAV COULD IMPART NORTHERLY SHEAR OVER HANNA POSSIBLY AFFECTING
THE INTENSITY.
 
HANNA CONTINUES MOVING WEST-NORTHWESTWARD...OR 300/10.  HOWEVER...A
MOTION TOWARD THE NORTHWEST IS FORECAST OVER THE NEXT 2-3 DAYS AS
HANNA IS EXPECTED TO STRENGTHEN AND BECOMES A DEEPER SYSTEM.
THEREAFTER...THE TRACK FORECAST IS RIDDLED WITH UNCERTAINTY AS IT
IS UNCLEAR HOW HANNA WILL INTERACT WITH A MID-LATITUDE TROUGH
FORECAST TO MOVE OFF THE U.S. EAST COAST. MOST OF THE DYNAMICAL
MODELS SHOW THE TROUGH BYPASSING HANNA TO THE NORTH AND LEAVING IT
BEHIND IN AN AREA OF WEAK STEERING. SUCH AN EVOLUTION WOULD RESULT
IN ERRATIC OR LITTLE OVERALL MOTION. ACCORDINGLY...THE OFFICIAL
FORECAST SHOWS A SLOW WESTWARD MOTION AT DAYS 4 AND 5.  IT SHOULD BE
ADDED THAT THERE IS LARGER THAN NORMAL UNCERTAINTY ASSOCIATED WITH
BOTH THE TRACK AND INTENSITY FORECASTS OF HANNA.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      28/2100Z 20.7N  60.1W    35 KT
 12HR VT     29/0600Z 21.7N  61.4W    35 KT
 24HR VT     29/1800Z 23.4N  63.4W    40 KT
 36HR VT     30/0600Z 24.9N  65.3W    50 KT
 48HR VT     30/1800Z 25.9N  67.0W    60 KT
 72HR VT     31/1800Z 27.0N  69.0W    70 KT
 96HR VT     01/1800Z 27.5N  70.5W    70 KT
120HR VT     02/1800Z 27.5N  71.5W    65 KT
 
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FORECASTER RHOME
 
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