Tropical Storm HANNA

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TROPICAL STORM HANNA DISCUSSION NUMBER  15
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082008
500 PM EDT SUN AUG 31 2008
 
AN AIR FORCE RECONNAISSANCE AIRCRAFT INVESTIGATING HANNA DURING THE
LAST SEVERAL HOURS FOUND A PEAK FLIGHT LEVEL WIND OF 48 KT WHICH
REDUCES TO APPROXIMATELY 40 KT AT THE SURFACE.  WHILE THERE WERE A
FEW HIGHER VALUES FROM THE SFMR...THESE WERE LIKELY RAIN INFLATED.
THE OVERALL INTENSITY FORECAST REASONING IS ESSENTIALLY UNCHANGED.
HANNA IS CURRENTLY EXPERIENCING STRONG NORTHERLY SHEAR ASSOCIATED
WITH AN UPPER-LEVEL LOW TO ITS SOUTHEAST.  GLOBAL MODELS DO
FORECAST THIS UPPER-LEVEL LOW TO WEAKEN OVER THE NEXT DAY OR SO.
HOWEVER...STRONG NORTHERLY FLOW ASSOCIATED WITH GUSTAV'S
UPPER-LEVEL OUTFLOW SHOULD KEEP NORTHERLY SHEAR OVER HANNA THROUGH
DAY 3.  BY DAYS 4 AND 5...DYNAMICAL MODELS FORECAST A SOMEWHAT
IMPROVED ENVIRONMENT AS A SECOND UPPER-LEVEL LOW OR CYCLONIC SHEAR
AXIS FORMS TO THE WEST OF HANNA.  THIS WOULD PUT HANNA IN A MORE
FAVORABLE ENVIRONMENT FOR STRENGTHENING.  HOWEVER...ACCURATELY
PREDICTING HANNA'S PROXIMITY TO LARGE-SCALE ATMOSPHERIC FEATURES AT
DAYS 4 AND 5 IS DIFFICULT...TO SAY THE LEAST.  GIVEN THE
UNCERTAINTY...THE OFFICIAL FORECAST SHOWS SLOW STRENGTHENING...BUT
IS BELOW THE INTENSITY MODEL CONSENSUS.
 
HANNA CONTINUES TO BE STEERED BY 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-LAYER TROUGH AMPLIFIES SOUTHWARD OVER THE WESTERN
ATLANTIC. SUCH AN EVOLUTION IN THE LARGE-SCALE PATTERN SHOULD CAUSE
THE STEERING CURRENTS TO COLLAPSE RESULTING IN A DECREASE IN
HANNA'S FORWARD SPEED. IN FACT...SOME OF THE MODELS SHOW HANNA
MEANDERING OVER OR JUST NORTHEAST OF THE CENTRAL BAHAMAS. BY DAY
3...THE DEEP-LAYER TROUGH IS FORECAST TO LIFT NORTHEASTWARD
ALLOWING THE SUBTROPICAL RIDGE TO BUILD BACK WESTWARD IN ITS WAKE.
THERE REMAINS CONSIDERABLE SPREAD AMONG THE DYNAMICAL MODELS AT THE
EXTENDED RANGE. THE GFS...GFDL...AND THE HWRF MODELS LIE ALONG THE
RIGHT SIDE OF THE MODEL ENVELOPE AND SHOW HANNA LIFTING QUICKLY
NORTHWESTWARD AROUND THE WESTERN PERIPHERY OF THE RIDGE.
CONVERSELY...THE UKMET MODEL SHOWS THE RIDGE NOSING WESTWARD TO THE
NORTH OF HANNA...PRODUCING A TRACK ALONG THE LEFT...OR
SOUTHWEST...SIDE OF THE MODEL ENVELOPE. SINCE THERE IS NO CLEAR
REASON TO FAVOR ONE SCENARIO OVER THE OTHER AT THIS TIME...I'VE
ELECTED TO LEAVE THE OFFICIAL FORECAST ESSENTIALLY UNCHANGED BEYOND
72 HOURS.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      31/2100Z 23.5N  71.4W    40 KT
 12HR VT     01/0600Z 23.5N  72.3W    40 KT
 24HR VT     01/1800Z 23.3N  73.1W    45 KT
 36HR VT     02/0600Z 23.3N  73.7W    50 KT
 48HR VT     02/1800Z 23.5N  74.1W    55 KT
 72HR VT     03/1800Z 24.5N  75.0W    60 KT
 96HR VT     04/1800Z 27.5N  77.5W    65 KT
120HR VT     05/1800Z 31.5N  81.0W    65 KT
 
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