Tropical Storm HANNA

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TROPICAL STORM HANNA DISCUSSION NUMBER  16
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082008
1100 PM EDT SUN AUG 31 2008
 
SINCE THE RECONNAISSANCE AIRCRAFT DEPARTED HANNA THIS AFTERNOON...
DEEP CONVECTION HAS BECOME A LITTLE MORE CONCENTRATED JUST
SOUTHEAST OF THE CENTER. SEVERAL MICROWAVE IMAGES FROM BETWEEN 2200
AND 0000 UTC HAVE BEEN VERY HELPFUL IN LOCATING THE CENTER. DVORAK
INTENSITY ESTIMATES HAVE INCREASED AGAIN TO T3.0 OR 45 KT... AND
NOAA BUOY 41046 RECENTLY REPORTED A PEAK ONE-MINUTE WIND SPEED OF
39 KT VERY NEAR THE AREA OF DEEP CONVECTION. THESE DATA SUPPORT
INCREASING THE INTENSITY TO 45 KT.
 
BOTH THE SHIPS MODEL AND UW-CIMSS DIAGNOSE ABOUT 20 KT OF
NORTHWESTERLY SHEAR OVER HANNA AT THIS TIME. THE SHIPS GUIDANCE AND
THE GLOBAL MODELS PREDICT SIMILAR OR SLIGHTLY HIGHER SHEAR VALUES
DURING THE NEXT 2-3 DAYS. GIVEN THIS SHEAR...IT IS TEMPTING TO NOT
FORECAST ANY STRENGTHENING DURING THIS TIME...BUT MOST OF THE
GUIDANCE SHOWS SOME INTENSIFICATION. LATE IN THE FORECAST
PERIOD...THERE ARE SOME INDICATIONS THAT THE UPPER-LEVEL
ENVIRONMENT COULD BECOME SOMEWHAT MORE CONDUCIVE FOR
STRENGTHENING...HOWEVER IT WILL DEPEND ON EXACTLY WHERE HANNA IS
LOCATED. THE LGEM MODEL IS AT THE LOW END OF THE SPECTRUM AND
FORECASTS VERY LITTLE INTENSITY CHANGE DURING THE NEXT 5 DAYS. THE
NEW INTENSITY FORECAST IS VERY SIMILAR TO PREVIOUS ADVISORY THROUGH
36 HOURS...BUT IS A LITTLE MORE CONSERVATIVE THEREAFTER AND USES A
BLEND OF THE SHIPS AND LGEM MODELS AT 3 DAYS AND BEYOND.
 
HANNA HAS BEEN MOVING WESTWARD OR 280/7 KT. THE BUILDING DEEP-LAYER
RIDGE OVER THE EASTERN UNITED STATES IS FORECAST TO BLOCK HANNA'S
FORWARD MOTION VERY SOON...AND THE TRACK GUIDANCE SHOWS VERY SLOW
AND ERRATIC MOTION OVER THE NEXT COUPLE OF DAYS. AFTER 48
HOURS...THE RIDGE BEGINS TO WEAKEN OVER THE U.S. WHILE ANOTHER
RIDGE BUILDS OVER THE ATLANTIC TO THE NORTHEAST OF HANNA. THE
ATLANTIC RIDGE IS EXPECTED TO STEER THE STORM NORTHWESTWARD BY THAT
TIME. THE TRACK GUIDANCE GENERALLY AGREES ON THIS SCENARIO...BUT
THE SPREAD OF THE GUIDANCE REMAINS QUITE LARGE AT DAYS 4 AND 5. THE
UKMET REMAINS THE SOUTHERN OUTLIER WITH A TRACK NEAR CUBA AND
SOUTHERN FLORIDA...WHILE THE GFS...HWRF...AND GFDL ARE ON THE
EASTERN EDGE OF THE GUIDANCE ENVELOPE WITH A TRACK TOWARDS THE
CAROLINAS. THE OFFICIAL FORECAST REMAINS VERY CLOSE TO THE PREVIOUS
ADVISORY AND CLOSE TO THE MODEL CONSENSUS WHICH HAS SHIFTED EVER SO
SLIGHTLY WESTWARD.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      01/0300Z 23.7N  72.2W    45 KT
 12HR VT     01/1200Z 23.5N  72.9W    45 KT
 24HR VT     02/0000Z 23.4N  73.5W    45 KT
 36HR VT     02/1200Z 23.4N  73.7W    50 KT
 48HR VT     03/0000Z 23.7N  74.2W    50 KT
 72HR VT     04/0000Z 25.4N  75.8W    55 KT
 96HR VT     05/0000Z 29.0N  79.0W    60 KT
120HR VT     06/0000Z 33.5N  82.0W    45 KT...INLAND
 
$$
FORECASTER BROWN
 
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