Tropical Storm GUSTAV

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TROPICAL STORM GUSTAV DISCUSSION NUMBER  17
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL072008
1100 PM EDT THU AUG 28 2008
 
THE CENTER OF GUSTAV HAS BEEN MOVING NEAR OR OVER THE SOUTH COAST OF
JAMAICA DURING THE EVENING.  SURFACE OBSERVATIONS FROM THE KINGSTON
AREA SHOWED WINDS OF TROPICAL STORM FORCE...AND THE LOWEST OBSERVED
PRESSURE WAS 990 MB.  GUSTAV IS PRODUCING A LARGE AREA OF COLD
CONVECTIVE TOPS NEAR AND EAST OF THE CENTER...WITH SATELLITE
INTENSITY ESTIMATES OF 65 KT FROM TAFB AND 55 KT FROM SAB.  THE
INITIAL INTENSITY REMAINS 60 KT.

THE INITIAL MOTION IS A SOMEWHAT UNCERTAIN 270/6...AFTER A SOUTH OF
WEST MOTION FOR A FEW HOURS.  GUSTAV IS ON THE SOUTH SIDE OF A
MID-LEVEL RIDGE OVER FLORIDA AND THE ADJACENT WESTERN ATLANTIC. 
THE DYNAMICAL GUIDANCE FORECASTS GUSTAV TO TURN NORTHWESTWARD ON THE
SOUTHWEST SIDE OF THE RIDGE DURING THE NEXT 48 HR...THEN CONTINUE IN
A GENERAL NORTHWESTWARD DIRECTION THROUGH 96 HR.  THE GUIDANCE IS
RATHER TIGHTLY CLUSTERED DURING THIS TIME PERIOD.  AFTER 96
HR...THERE IS SOME SPREAD...AS THE DYNAMICAL MODELS DO NOT AGREE ON
WHETHER A RIDGE WILL BUILD TO THE NORTH AND WEST OF GUSTAV.  THE
GFDL MOVES THE STORM QUICKLY INLAND...WHILE THE LATEST GFS RUN
SHOWS IT SLOWING DOWN OVER SOUTHERN LOUISIANA.  THE NOGAPS CALLS
FOR A LEFT TURN AFTER 96 HR TOWARD THE TEXAS COAST.  THE NEW
FORECAST TRACK IS AN UPDATE OF THE PREVIOUS PACKAGE THAT IS DOWN
THE MIDDLE OF THE GUIDANCE ENVELOPE.  THE TRACK IS SHIFTED ONLY
SLIGHTLY TO THE LEFT AFTER 72 HR....WITH RELIABLE MODEL FORECASTS
REMAINING ON EITHER SIDE OF THE TRACK.  SINCE TRACK FORECASTS ARE
ALWAYS SUBJECT TO LARGE ERRORS AT 3-5 DAYS...IT IS SIMPLY
IMPOSSIBLE AT THIS TIME TO DETERMINE EXACTLY WHERE AND WHEN GUSTAV
WILL MAKE FINAL LANDFALL.

THE INTENSITY FORECAST IS PROBLEMATIC.  GUSTAV IS SHOWING GOOD
OUTFLOW IN ALL DIRECTIONS AT THIS TIME...BUT THERE IS EVIDENCE OF
NORTHERLY VERTICAL SHEAR UNDERCUTTING THE OUTFLOW.  THE LARGE-SCALE
MODELS SUGGEST THIS SHOULD SUBSIDE IN ABOUT 24 HOURS...WHICH WOULD
ALLOW SIGNIFICANT AND POSSIBLY RAPID INTENSIFICATION OVER THE WARM
WATERS OF THE NORTHWESTERN CARIBBEAN.  A COMPLICATING FACTOR IS THE
UPPER-LEVEL TROUGH CURRENTLY SEEN IN WATER VAPOR IMAGERY OVER THE
GULF OF MEXICO.  THE LARGE-SCALE MODELS FORECAST THIS FEATURE TO
RETREAT WESTWARD AS GUSTAV APPROACHES...WITH HIGH PRESSURE BUILDING
OVER THE SOUTHEASTERN GULF.  HOWEVER...THE MODELS DO NOT AGREE ON
HOW MUCH THE TROUGH WILL MOVE...WITH THE UKMET IN PARTICULAR
SHOWING THE POSSIBILITY OF SHEAR AS GUSTAV APPROACHES THE NORTHERN
GULF COAST.  THE SHIPS MODEL CALLS FOR A PEAK INTENSITY OF 99
KT...THE LGEM MODEL 94 KT...THE GFDL 111 KT...AND THE HWRF 137 KT. 
THE LATTER IS DEFINITELY NOT OUT OF THE QUESTION.  THE INTENSITY
FORECAST IS INCREASED OVER THE PREVIOUS FORECAST IN BEST AGREEMENT
WITH THE GFDL.  HOWEVER...IT WOULD BE NO SURPRISE IF RAPID
INTENSIFICATION OCCURRED AND GUSTAV BECAME A CATEGORY 4 OR 5
HURRICANE BY 72 HR.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      29/0300Z 17.8N  77.3W    60 KT
 12HR VT     29/1200Z 18.3N  78.6W    60 KT
 24HR VT     30/0000Z 19.1N  80.4W    75 KT
 36HR VT     30/1200Z 20.4N  82.3W    90 KT
 48HR VT     31/0000Z 21.9N  84.2W   100 KT...NEAR COAST OF CUBA
 72HR VT     01/0000Z 25.5N  87.5W   110 KT
 96HR VT     02/0000Z 28.5N  90.5W   100 KT
120HR VT     03/0000Z 30.0N  92.5W    90 KT...INLAND
 
$$
FORECASTER BEVEN
 
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