Tropical Storm HANNA

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TROPICAL STORM HANNA DISCUSSION NUMBER  11
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082008
500 PM EDT SAT AUG 30 2008
 
THE CLOUD PATTERN OF HANNA HAS CHANGED QUITE A BIT DURING THE DAY
TODAY.  THE TROPICAL STORM HAS BECOME INVOLVED WITH THE UPPER-LEVEL
LOW THAT WAS LOCATED TO ITS WEST...AND HANNA NOW HAS SOME
RESEMBLANCE OF A SUB-TROPICAL CYCLONE.  EVEN THOUGH THE CENTER IS
LOCATED WELL SOUTHWEST OF THE THUNDERSTORM ACTIVITY...BANDING OVER
THE NORTHEASTERN SEMICIRCLE STILL YIELD INTENSITY ESTIMATES OF T3.0
OR 45 KT FROM BOTH AGENCIES.
 
HANNA HAS BEEN MOVING BETWEEN THE WEST AND WEST-NORTHWEST AT
ABOUT 7 KT TODAY.  A SIMILAR MOTION IS FORECAST BY THE NUMERICAL
MODEL GUIDANCE FOR THE NEXT 24-36 HOURS...SO VERY LITTLE CHANGE WAS
NEEDED TO THE PREVIOUS FORECAST DURING THAT TIME.  THEREAFTER...THE
MODEL SPREAD HAS BECOME EXTREMELY LARGE AS A STRONG HIGH PRESSURE
RIDGE BUILDS OVER THE EASTERN UNITED STATES.  THE TRACK GUIDANCE NOW
PREDICTS A SLOW AND ERRATIC MOTION OF HANNA DURING THE 48 TO ABOUT
96 HOUR TIME FRAME.  THE GFDL AND HWRF BOTH PREDICT HANNA TO MAKE A
CYCLONIC LOOP NEAR THE SOUTHEASTERN BAHAMAS.  MEANWHILE...THE ECMWF
AND UKMET SHOW A SOUTHWESTWARD MOTION...WITH THE LATTER MODEL
FORECASTING HANNA TO BE OVER EASTERN CUBA.  BEYOND 96 HOURS...
NEARLY ALL OF THE GUIDANCE...EXCEPT THE UKMET...NOW FORECAST A
WEST-NORTHWESTWARD OR NORTHWESTWARD MOTION TO RESUME AS THE RIDGE
OVER THE EASTERN UNITED STATES WEAKENS.  WITH ALL OF THAT SAID...
THERE IS VERY LOW CONFIDENCE IN THE TRACK FORECAST.  THE NEW TRACK
SLOWS DOWN THE STORM CONSIDERABLY...AND NOW PREDICTS A SLOW
WEST-NORTHWESTWARD MOTION LATE IN THE PERIOD.  THE NHC TRACK IS A
LITTLE SOUTH OF THE CONSENSUS...ESPECIALLY AT DAY 5...BUT IS IN
GENERAL AGREEMENT WITH THE 12Z ECMWF.
 
GIVEN ALL THE UNCERTAINTIES IN THE TRACK FORECAST...THE INTENSITY
FORECAST SEEMS JUST AS DIFFICULT.  THE DYNAMICAL MODELS AND SHIPS
GUIDANCE SUGGEST THAT WEST TO NORTHWESTERLY SHEAR WILL CONTINUE
DURING THE NEXT 24-36 HOURS...FOLLOWED BY AN INCREASE IN NORTHERLY
SHEAR.  GIVEN THE UNCERTAINTIES AS TO HOW HANNA WILL RESPOND TO THE
SHEAR...THE INTENSITY FORECAST ALLOWS FOR SOME STRENGTHENING IN THE
SHORT-TERM...BUT LEVELS OFF THE INTENSITY BETWEEN 36-96 HOURS. 
THERE ARE SOME INDICATIONS THAT THE UPPER-LEVEL ENVIRONMENT COULD
BECOME MORE CONDUCIVE VERY LATE IN THE PERIOD...SO SLIGHT
STRENGTHENING IS INDICATE AT DAY 5.  THE NHC FORECAST REMAINS BELOW
ALL OF THE RELIABLE INTENSITY GUIDANCE.
 
THE 12-FT SEA RADII WERE ADJUSTED OUTWARD OVER THE NORTHWESTERN
SEMICIRCLE BASED ON REPORTS FROM NOAA BUOY 41046.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      30/2100Z 22.4N  67.2W    45 KT
 12HR VT     31/0600Z 22.8N  68.6W    45 KT
 24HR VT     31/1800Z 23.3N  70.2W    50 KT
 36HR VT     01/0600Z 23.5N  71.4W    55 KT
 48HR VT     01/1800Z 23.5N  72.1W    55 KT
 72HR VT     02/1800Z 23.5N  72.8W    55 KT
 96HR VT     03/1800Z 24.0N  74.0W    55 KT
120HR VT     04/1800Z 25.0N  76.5W    60 KT
 
$$
FORECASTER BROWN
 
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