Tropical Storm HANNA

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TROPICAL STORM HANNA DISCUSSION NUMBER  24
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082008
500 PM EDT TUE SEP 02 2008
 
AN AIR FORCE RECONNAISSANCE AIRCRAFT CURRENTLY INVESTIGATING HANNA
CONFIRMS THAT THE SYSTEM HAS WEAKENED. THE PEAK FLIGHT-LEVEL WIND
AT 850 MB WAS 66 KT WHICH CORRESPONDS TO ABOUT 55 KT AT THE
SURFACE. HANNA WILL CONTINUE TO ENCOUNTER STRONG SHEAR FOR ANOTHER
12-24 HOURS WHICH SHOULD LIMIT STRENGTHENING IN THE SHORT TERM. BY
TOMORROW...GLOBAL MODELS FORECAST AN UPPER-LEVEL LOW TO CUT OFF
JUST WEST OF HANNA AND THEN DROP SOUTHWARD DURING THE NEXT FEW
DAYS.  IF THIS EVOLUTION MATERIALIZES...AND ASSUMING HANNA IS ABLE
TO FIGHT OFF THE SHEAR...THE ENVIRONMENT WOULD GRADUALLY BECOME
MORE FAVORABLE FOR STRENGTHENING.  THE OFFICIAL INTENSITY FORECAST
CALLS FOR HANNA TO REGAIN HURRICANE STRENGTH IN A DAY OR SO...IN
LINE WITH NEARLY ALL OF THE AVAILABLE INTENSITY MODELS. 
HOWEVER...THE EVOLUTION OF THE LARGE-SCALE ENVIRONMENT OVER THE
NEXT DAY OR SO IS COMPLEX...WHICH RESULTS IN LOWER THAN NORMAL
CONFIDENCE IN THE INTENSITY FORECAST.
 
THE LAST FEW AIRCRAFT FIXES INDICATE THAT HANNA CONTINUES TO DRIFT
SOUTHEASTWARD WITH AN INITIAL MOTION ESTIMATE OF 150/02. TRACK
MODELS INSIST THAT HANNA WILL START ITS MUCH-ANTICIPATED TURN
TOWARD THE NORTHWEST SOON...AS A LARGE MID-LATITUDE TROUGH OVER THE
WESTERN ATLANTIC LIFTS NORTHWARD AND THE SUBTROPICAL RIDGE BUILDS
BACK IN.  SUCH A TRACK WOULD TAKE THE CYCLONE NORTHWESTWARD THROUGH
THE CENTRAL AND NORTHWESTERN BAHAMAS DURING THE NEXT TWO DAYS. 
HANNA IS THEN EXPECTED TO ACCELERATE AS IT MAKES ITS APPROACH TO
THE SOUTHEASTERN UNITED STATES.  HOWEVER...UNTIL HANNA BEGINS A
MORE DEFINITIVE MOTION...IT WILL BE DIFFICULT TO NARROW THE
POTENTIAL IMPACT AREA.  IN ADDITION...THE ANTICIPATED ANGLE OF
APPROACH TO THE SOUTHEAST UNITED STATES COAST MEANS THAN ONLY A
SMALL DEVIATION LEFT OR RIGHT OF THE TRACK WILL HAVE LARGE
IMPLICATIONS IN BOTH TIME AND LOCATION OF LANDFALL.  IN FACT...THE
WIND SPEED PROBABILITY PRODUCT INCLUDED WITH THIS ADVISORY PACKAGE
SHOWS THAT THE CHANCES OF HURRICANE-FORCE WINDS ARE ABOUT EQUAL AT
EACH SPECIFIC LOCATION ALONG THE COAST FROM FLORIDA THROUGH THE
CAROLINAS.

FORECAST POSITIONS AND MAX WINDS
 
INITIAL      02/2100Z 20.4N  72.7W    55 KT
 12HR VT     03/0600Z 20.9N  73.1W    55 KT
 24HR VT     03/1800Z 22.2N  74.0W    60 KT
 36HR VT     04/0600Z 23.5N  75.5W    65 KT
 48HR VT     04/1800Z 25.4N  77.2W    70 KT
 72HR VT     05/1800Z 30.5N  80.5W    70 KT
 96HR VT     06/1800Z 38.0N  78.0W    30 KT...INLAND
120HR VT     07/1800Z 47.0N  66.0W    25 KT...REMNANT LOW
 
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FORECASTER RHOME/KNABB
 
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