[Tropical] Tropical Cyclone Discussion

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WTNT43 KNHC 290259
TCDAT3
TROPICAL STORM HANNA DISCUSSION NUMBER   4
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082008
1100 PM EDT THU AUG 28 2008

AFTER WESTERLY MID- TO UPPER-LEVEL SHEAR PRODUCED AN EXPOSED
LOW-LEVEL CIRCULATION CENTER...CONVECTION HAS REDEVELOPED WITH A
VENGEANCE OVER OR NEAR THE CENTER CONTAINING CLOUD TOPS AS COLD AS
-83C. HOWEVER...THE CONVECTIVE DEVELOPMENT APPEARS TO BE A BURSTING
PATTERN...WHICH IS USUALLY SHORT-LIVED. THE ADVISORY INTENSITY IS
45 KT BASED ON A DVORAK T-NUMBER OF T3.0/45 KT FROM TAFB AND THE
CONTINUED CONVECTIVE DEVELOPMENT SINCE THE 00Z SATELLITE FIX TIME.

THE INITIAL MOTION ESTIMATE IS 290/06. HANNA HAD BEEN MOVING DUE
WEST AT ABOUT 10 KT...BUT THE 24-HOUR MOTION OF THE PRESSURE
ENVELOPE SURROUNDING THE CYCLONE CENTER HAS ONLY BEEN MOVING AT
AROUND 5 KT. THE PRIMARY STEERING MECHANISMS FOR THE NEXT 3 DAYS
WILL BE THE LARGE SUBTROPICAL HIGH TO THE NORTH OF HANNA AND THE
MID- TO UPPER-LEVEL LOW LOCATED ABOUT 275 NMI WEST OF HANNA. ALL OF
THE MODELS HAVE BEEN WEAKENING AND MOVING THE UPPER-LOW TO THE
SOUTHWEST TOO QUICKLY...WHICH IS LIKELY DUE TO THE FAST OUTFLOW
FROM GUSTAV GETTING ENTRAINED INTO THE LOW AND KEEPING IT STRONGER
THAN THE MODEL FORECASTS. HOWEVER...OUTFLOW FROM GUSTAV IS NOW
BEING SHUNTED SOUTHEASTWARD INTO THE EASTERN CARIBBEAN...SO A
WEAKENING TREND AND SOUTHWESTWARD MOTION BY THE UPPER-LOW IS
EXPECTED TO ENSUE WITHIN THE NEXT 24 HOURS. HANNA IS FORECAST TO
MOVE AROUND THE UPPER-LOW THROUGH 72-96 HOURS BEFORE IT ENCOUNTERS
STRONG MID- TO UPPER-LEVEL NORTHERLY WINDS BEHIND A POWERFUL
MID-TROPOSPHERIC TROUGH...WHICH IS FORECAST TO MOVE OFF THE U.S.
EAST COAST AT DAYS 4 AND 5. THAT DEVELOPING SCENARIO IS INDICATED
BY ALMOST ALL OF THE MODELS TO VARYING DEGREES...BUT ENOUGH TO
INDUCE SOME SOUTHWARD COMPONENT TO HANNA'S MOTION. MOST OF THE
MODEL GUIDANCE HAS HAD A FAST AND RIGHT-OF-TRACK BIAS THE PAST 36
HOURS...SO THE FORECAST TRACK FOR THIS ADVISORY WAS ADJUSTED TO THE
LEFT OF THE PREVIOUS TRACK AND THE NHC MODEL CONSENSUS.

MODERATE WESTERLY SHEAR HAS BEEN REPLACED BY LIGHT SOUTHERLY SHEAR
DURING THE PAST 6 HOURS NOW THAT THE UPPER-LOW HAS SHOWN SIGNS OF
WEAKENING AND MOVING FARTHER AWAY FROM HANNA. AS THE LOW CONTINUES
TO WEAKEN AND MOVES TO A POSITION TO THE SOUTHWEST OF HANNA...
UPPER-LEVEL OUTFLOW SHOULD IMPROVE TO THE NORTHWEST AIDING A STEADY
INTENSIFICATION. ALTHOUGH NOT INDICATED AT ANY OF THE FORECAST
TIMES... ATMOSPHERIC AND OCEANIC CONDITIONS ARE EXPECTED TO BECOME
QUITE FAVORABLE FOR SIGNIFICANT STRENGTHENING BETWEEN 72 AND 96
HOURS...AND HANNA COULD BECOME A MAJOR HURRICANE AS INDICATED BY
THE GFDL MODEL. THE OFFICIAL FORECAST IS A BLEND OF THE SHIPS AND
GFDL MODELS...AND IS SIMILAR TO THE LGEM MODEL.


FORECAST POSITIONS AND MAX WINDS

INITIAL      29/0300Z 21.1N  60.6W    45 KT
 12HR VT     29/1200Z 21.8N  62.0W    50 KT
 24HR VT     30/0000Z 23.2N  64.2W    55 KT
 36HR VT     30/1200Z 24.6N  66.3W    60 KT
 48HR VT     31/0000Z 25.7N  68.0W    70 KT
 72HR VT     01/0000Z 26.6N  70.0W    80 KT
 96HR VT     02/0000Z 26.5N  71.2W    80 KT
120HR VT     03/0000Z 26.0N  71.6W    75 KT

$$
FORECASTER STEWART/BEVEN

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