Tropical Storm Gustav: Advisory #13

TROPICAL STORM GUSTAV DISCUSSION NUMBER 13
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL072008
500 AM EDT THU AUG 28 2008

AN AIR FORCE RECONNAISSANCE PLANE HAS FOUND A SURPRISE THIS MORNING.
GUSTAV HAS EITHER REFORMED TO THE SOUTH OR BEEN MOVING MORE TO THE
SOUTH-SOUTHWEST OVERNIGHT. IN ADDITION...THE PRESSURE IS DOWN 10 MB
DURING THE LAST SIX HOURS. THE PLANE HAS NOT YET SAMPLED THE ENTIRE
STORM...BUT IT IS ASSUMED THE WINDS HAVE COME UP SLIGHTLY WITH THE
SIGNIFICANT PRESSURE FALL AND THE INITIAL INTENSITY IS INCREASED TO
45 KT. MID-LEVEL DRY AIR...SOME NORTHEASTERLY SHEAR...AND
INTERACTION WITH JAMAICA MAY LIMIT THE INTENSIFICATION OF THIS
SYSTEM DURING THE NEXT DAY OR SO. THUS THE OFFICIAL INTENSITY
FORECAST IS ONLY SLIGHTLY RAISED FROM EARLIER. THEREAFTER...THE
GLOBAL MODELS ARE FORECASTING AN UPPER ANTICYCLONE TO BE NEAR THE
CENTER OF GUSTAV...WHICH WOULD BE QUITE FAVORABLE FOR
STRENGTHENING. IT IS EXPECTED THAT GUSTAV WILL BE A POWERFUL
HURRICANE AS IT MOVES INTO THE SOUTHERN GULF OF MEXICO ON SUNDAY.
SOUTHWESTERLY VERTICAL WIND SHEAR COULD INCREASE LATE IN THE PERIOD
WHEN THE SYSTEM IS FORECAST TO BE IN THE NORTHERN GULF BUT THERE IS
CONSIDERABLY UNCERTAINTY AT THAT TIME.

THE BEST ESTIMATE OF INITIAL MOTION IS 215/7. THE REFORMATION OF
THE CENTER HAS CAUSED A BIG SOUTHWARD CHANGE IN THE EARLY PART OF
THE FORECAST...AND GUSTAV IS NOW FORECAST TO PASS NEAR THE SOUTH
COAST OF JAMAICA. THE NORTHEAST-SOUTHWEST ORIENTED RIDGE CENTERED
OVER THE FLORIDA PENINSULA THAT HAS BEEN CAUSING THIS SOUTHWESTWARD
MOTION IS EXPECTED TO WEAKEN OVER THE NEXT FEW DAYS. THIS SHOULD
ALLOW GUSTAV TO TURN MORE TO THE WEST AND THEN NORTHWEST AROUND THE
TIME IT ENTERS THE GULF OF MEXICO. A MID-LATITUDE HIGH IS FORECAST
TO BE IN THE OHIO VALLEY IN ABOUT 5 DAYS...WHICH MAY CONTINUE
STEERING THE STORM IN A NORTHWESTWARD FASHION RATHER THAN ALLOW A
MORE NORTHWARD MOTION LIKE THE GFDL SUGGESTS. THE OFFICIAL FORECAST
IS SHIFTED WESTWARD IN THE LONGER-TERM...ON THE EASTERN SIDE OF THE
MODEL GUIDANCE ENVELOPE...BUT THE ECMWF AND GFDL ARE TO THE RIGHT
OF THE NHC FORECAST.

FORECAST POSITIONS AND MAX WINDS

INITIAL 28/0900Z 17.8N 75.6W 45 KT
12HR VT 28/1800Z 17.5N 77.0W 55 KT
24HR VT 29/0600Z 17.8N 78.5W 60 KT
36HR VT 29/1800Z 18.5N 80.3W 65 KT
48HR VT 30/0600Z 19.5N 82.0W 75 KT
72HR VT 31/0600Z 22.0N 85.5W 95 KT
96HR VT 01/0600Z 26.0N 89.0W 100 KT
120HR VT 02/0600Z 28.5N 91.0W 100 KT


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