Tropical Storm HANNA

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TROPICAL STORM HANNA DISCUSSION NUMBER  12
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082008
1100 PM EDT SAT AUG 30 2008
 
ENHANCED INFRARED IMAGERY AND MICROWAVE OVERPASSES DEPICT A
CONVECTIVELY CHALLENGED TROPICAL CYCLONE.  THE VERTICAL STRUCTURE
OF THE HANNA HAS TRANSFORMED INTO A SYSTEM SIMILAR TO A SUBTROPICAL
CYCLONE...WITH A SLIGHT TILT TO THE WEST WITH HEIGHT.  THE CYCLONE
HAS BEEN STRUGGLING TO MAINTAIN DEEP CONVECTION NEAR THE PARTLY
EXPOSED CENTER...PARTICULARLY OVER THE WEST AND SOUTH QUADRANTS. 
THERE APPEARS TO BE SOME MODERATE SUBSIDENCE SPREADING OVER THE
AREA PRODUCED BY CONVERGING UPPER-LEVEL WINDS BETWEEN THE UPPER LOW
NOW ASSOCIATED WITH HANNA AND GUSTAV'S OUTFLOW PATTERN...WITH THIS
DRYER MORE STABLE AIR BEING ENTRAINED INTO THE SYSTEM'S CORE.  THE
INITIAL INTENSITY IS HELD AT 45 KT BASED ON DVORAK AND
HEBERT/POTEAT INTENSITY TECHNIQUES...AS WELL AS A QUIKSCAT PASS
THAT SUPPORTED THIS INTENSITY.

THE INTENSITY MODELS CONTINUE TO INDICATE AN INCREASE IN NORTHERLY
SHEAR AROUND THE 24 HOUR PERIOD...THEN TRANSITIONING TO A MORE
SOUTH TO SOUTHWESTERLY COMPONENT AFTERWARD.  THE SHIPS AND THE LGEM
AGREE WITH GRADUAL STRENGTHENING UNTIL THE STRONG NORTHERLY SHEAR
APPEARS...THEN THE MODELS AGREE WITH LITTLE CHANGE THROUGH THE 72
HOUR PERIOD.  BEYOND DAY 3...THE DYNAMICAL MODELS INDICATE A HINT
OF A MORE FAVORABLE UPPER WIND ENVIRONMENT...WHICH THE
AFOREMENTIONED MODELS ALSO DEPICT.  THE OFFICIAL FORECAST FOLLOWS
THIS PHILOSOPHY AND INDICATES SLIGHT INTENSIFICATION AT THE 120
HOUR PERIOD.

THE INITIAL MOTION IS ESTIMATED AT 300/7.  DYNAMICAL MODELS SUGGEST
THAT AN ANCHORED SUBTROPICAL RIDGE TO THE NORTH OF THE CYCLONE
SHOULD INFLUENCE A GENERAL WEST-NORTHWESTWARD MOTION DURING THE
NEXT 48 HOURS.  AFTERWARD...A DEEP MID-LATITUDE LOW AND ASSOCIATED
TROUGH IS EXPECTED TO MOVE OVER THE WESTERN ATLANTIC AND WEAKEN A
PORTION OF THE RIDGE LOCATED OVER SOUTHEAST U.S. AND THE BAHAMAS
WHILE A HIGH AMPLITUDE BLOCKING HIGH SETS UP OVER THE EASTERN
SEABOARD.  THIS PARTICULAR PATTERN IS EXPECTED TO CAUSE HANNA TO
TEMPORARILY SLOW ITS FORWARD SPEED THROUGH THE 72 HOUR PERIOD WHILE
MAINTAINING A WEST-NORTHWESTWARD TRACK.  THROUGH THE REMAINDER OF
THE FORECAST...A MID-LEVEL RIDGE IS FORECAST TO BUILD NORTHEAST OF
HANNA IN RESPONSE TO THE AFOREMENTIONED MID-LATITUDE LOW LIFTING OUT
AND MOVING AWAY FROM THE NORTHEAST COAST.  THE MODEL SPREAD REMAINS
RATHER LARGE...HOWEVER...THE MAJORITY ARE SHOWING A TURN TOWARD THE
NORTHWEST OVER OR EAST OF THE BAHAMAS BEYOND 72 HOURS...WHILE THE
UKMET IS STILL PREDICTING THE SUBTROPICAL RIDGE TO REMAIN INTACT
TO THE NORTH.  THE OFFICIAL FORECAST FOLLOWS THE MODEL CONSENSUS
AND IS SIMILAR TO THE PREVIOUS ADVISORY BUT INDICATES A MORE
NORTHWESTWARD MOTION BEYOND DAY 3.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      31/0300Z 22.9N  67.8W    45 KT
 12HR VT     31/1200Z 23.3N  69.0W    45 KT
 24HR VT     01/0000Z 23.6N  70.4W    50 KT
 36HR VT     01/1200Z 23.6N  71.4W    55 KT
 48HR VT     02/0000Z 23.6N  72.2W    55 KT
 72HR VT     03/0000Z 23.8N  73.1W    55 KT
 96HR VT     04/0000Z 24.5N  74.5W    55 KT
120HR VT     05/0000Z 26.5N  76.5W    60 KT
 
$$
FORECASTER ROBERTS/BEVEN
 
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