[Tropical] Tropical Cyclone Discussion

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WTNT42 KNHC 290853
TCDAT2
TROPICAL STORM GUSTAV DISCUSSION NUMBER  18
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL072008
500 AM EDT FRI AUG 29 2008

SATELLITE IMAGES SHOW THAT GUSTAV HAS AN EXPANDING CLOUD SHIELD AND
IMPRESSIVE BANDING FEATURES. AN AIR FORCE RECONNAISSANCE PLANE THAT
HAS BEEN SAMPLING THE STORM'S CIRCULATION FOUND THAT THE SIZE OF
GUSTAV HAS SIGNIFICANTLY INCREASED. ALTHOUGH THEY HAVE BEEN UNABLE
TO SAMPLE THE CENTER...PEAK FLIGHT LEVEL WINDS OF 52 KT WERE
OBSERVED...ALONG WITH SIMILAR VALUES FOR THE SFMR. THE INITIAL
INTENSITY REMAINS 55 KT.

THE BEST ESTIMATE OF INITIAL MOTION IS 285/7. THERE IS A COMPLICATED
EVOLUTION TO THE STEERING CURRENTS AROUND GUSTAV DURING THE
FORECAST PERIOD. THE MID-LEVEL RIDGE OVER FLORIDA THAT HAS BEEN
STEERING THE STORM WESTWARD IS FORECAST TO WEAKEN TODAY. THE
BERMUDA HIGH IS THEN EXPECTED TO BUILD IN FROM THE WESTERN ATLANTIC
AND CAUSE GUSTAV TO MOVE MORE TOWARD THE NORTHWEST OVER THE
WEEKEND. ONE BIG QUESTION IS THE EVOLUTION OF A MID- TO UPPER-
LEVEL LOW OVER THE SOUTHEASTERN UNITED STATES. SOME OF THE
MODELS...SUCH AS THE GFS/GFDL/HWRF...ARE SHOWING THIS LOW CREATING
ENOUGH OF A WEAKNESS IN THE BERMUDA HIGH TO ALLOW A MOTION MORE
TOWARD THE NORTH-NORTHWEST AS THE SYSTEM CROSSES CUBA. IT WOULD NOT
BE SURPRISING IF THAT TRACK BRIEFLY OCCURRED...BUT IN THE
LONGER-TERM...A HIGH OVER THE OHIO VALLEY IS EXPECTED TO CAUSE
GUSTAV TO GRADUALLY BEND TO THE LEFT. HOWEVER...THERE IS A LARGE
UNCERTAINTY IN THE TIMING AND SHARPNESS OF THE BEND. THE UKMET AND
GFS HAVE HAD LARGE CHANGES TO THEIR FORECAST FROM THE PREVIOUS
RUNS...AND GIVEN THE INCREASING MODEL SPREAD...WE'VE OPTED TO KEEP
THE FORECAST SIMILAR TO THE PREVIOUS ONE. HOPEFULLY A G-IV MISSION
LATER TODAY WILL HELP RESOLVE SOME OF THESE DIFFERENCES.

AFTER THE STORM MOVES AWAY FROM JAMAICA...SIGNIFICANT
INTENSIFICATION IS POSSIBLE.  VERTICAL WIND SHEAR IS FORECAST TO BE
VERY LIGHT DURING THE NEXT COUPLE OF DAYS...AND COMBINED WITH THE
DEEP WARM CARIBBEAN WATERS...COULD PRODUCE A STRONG HURRICANE VERY
QUICKLY. THE OFFICIAL FORECAST WILL SHOW RAPID INTENSIFICATION
BEFORE IT REACHES WESTERN CUBA AND COULD BE CONSERVATIVE AS SOME
MODELS SHOW CATEGORY FOUR STRENGTH AT THAT TIME. A LARGE TROUGH
CURRENTLY SEEN IN WATER VAPOR IMAGES OVER THE CENTRAL GULF OF
MEXICO IS EXPECTED TO MOVE WESTWARD SLOWLY DURING THE NEXT DAY OR
TWO.  THIS TROUGH MAY IMPART SOME SOUTHWESTERLY SHEAR ON GUSTAV
AS IT ENTERS THE CENTRAL GULF OF MEXICO.   ALTHOUGH THE FORECAST
SHOWS SOME WEAKENING DUE TO THE SHEAR...GUSTAV IS EXPECTED TO BE A
LARGE POWERFUL HURRICANE AS IT APPROACHES THE NORTHERN GULF COAST.


FORECAST POSITIONS AND MAX WINDS

INITIAL      29/0900Z 18.1N  78.1W    55 KT
 12HR VT     29/1800Z 18.6N  79.3W    60 KT
 24HR VT     30/0600Z 19.6N  81.0W    75 KT
 36HR VT     30/1800Z 21.0N  82.8W    90 KT
 48HR VT     31/0600Z 22.7N  84.6W   100 KT
 72HR VT     01/0600Z 26.4N  88.1W   110 KT
 96HR VT     02/0600Z 29.0N  91.0W   100 KT
120HR VT     03/0600Z 30.0N  93.0W    85 KT...INLAND

$$
FORECASTER BLAKE/AVILA



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