5am Discussion weakening and rising pressure as expected

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Hurricane ISABEL

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000
WTNT43 KNHC 130843
TCDAT3
HURRICANE ISABEL DISCUSSION NUMBER 29
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
5 AM EDT SAT SEP 13 2003

THE MUCH ANTICIPATED WEAKENING HAS OCCURRED. DATA FROM A
RECONNAISSANCE PLANE INDICATE THAT THE CENTRAL PRESSURE HAS RISEN
TO 935 MB AND FLIGHT-LEVEL PEAK WINDS ARE 139 KNOTS. INITIAL
INTENSITY HAS BEEN LOWERED TO 130 KNOTS AND THIS IS PROBABLY ON THE
HIGH SIDE. THE RECON ALSO CONFIRMED THE PRESENCE OF TWO CONCENTRIC
EYEWALLS OF 25 AND 35 NMI AND A BAND OF MAXIMUM WINDS ASSOCIATED
WITH EACH EYEWALL...RESPECTIVELY. THERE IS NO SKILL IN PREDICTING IF
THE CURRENT EYEWALL CYCLE WILL RESULT IN FURTHER WEAKENING OR IN
ANOTHER ROUND OF INTENSIFICATION. BEST BET IS TO INDICATE SOME
FLUCTUATIONS IN INTENSITY IN THE NEXT DAY OR TWO WITH A WEAKENING
TREND DUE TO COOLER SSTS OR SHEAR THEREAFTER. NEVERTHERELESS...
ISABEL IS EXPECTED TO REMAIN A MAJOR HURRICANE THROUGHOUT THE
FORECAST PERIOD.

ISABEL IS MOVING TOWARD THE WEST OR 280 DEGREES AT 8 KNOTS AROUND
THE PERIPHERY OF A STRONG DEEP-LAYER ANTICYCLONE. THIS PATTERN IS
EXPECTED TO STEER THE HURRICANE TOWARD THE WEST WITH A GRADUAL TURN
TO THE WEST-NORTHWEST AS INDICATED IN THE OFFICIAL FORECAST. IT
APPEARS THAT THE TROUGH WHICH IS CURRENTLY ERODING THE WESTERN
PORTION OF THE RIDGE WILL WEAKEN AND THE HIGH WILL EXPAND WESTWARD.
THIS EXPANSION OF THE RIDGE WILL CONTROL HOW CLOSE ISABEL
APPROACHES THE U.S EAST COAST.

HIGH UNCERTAINTIES CONTINUE AT LONGER RANGE. THE CONSISTENT AND
GOOD PERFORMER UK MODEL HAS THE HURRICANE NEAR THE COAST OF NORTH
CAROLINA BETWEEN 5 AND 6 DAYS. ON THE OTHER HAND...THE RELIABLE
GFDL TURNS ISABEL NORTHWARD IN FIVE DAYS AT A GOOD DISTANCE AWAY
FROM THE U.S EAST COAST. BECAUSE BOTH MODELS HAVE BEEN VERY GOOD SO
FAR...THE OFFICIAL FORECAST IS IN BETWEEN THESE TWO MODELS. AT THIS
TIME...I WOULD NOT EVEN DISCUSS THE GFS WHICH MOVES THE HURRICANE
EASTWARD FOR THE FIRST 12 TO 18 HOURS IN THE LATEST 00Z RUN AND
ISABEL IS ALREADY WEST OF 60 DEGREES WEST. HOWEVER...THE LARGE
SCALE PATTERN PROVIDED BY THE GFS IS QUITE REASONABLE.

NOTE: THERE ARE NO DYNAMICAL MODELS SUGGESTING THAT ISABEL MIGHT
EVER REACH FLORIDA.

FORECASTER AVILA

FORECAST POSITIONS AND MAX WINDS

INITIAL 13/0900Z 22.0N 60.4W 130 KT
12HR VT 13/1800Z 22.2N 61.6W 130 KT
24HR VT 14/0600Z 23.0N 63.7W 130 KT
36HR VT 14/1800Z 23.5N 65.5W 130 KT
48HR VT 15/0600Z 24.5N 67.5W 130 KT
72HR VT 16/0600Z 26.0N 70.0W 120 KT
96HR VT 17/0600Z 28.5N 72.0W 115 KT
120HR VT 18/0600Z 32.0N 74.5W 105 KT

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TBW Extended



EXTENDED (TUE THRU FRI): MADE ONLY MINOR CHANGES TO GRIDS/FCST THIS
PERIOD AS EVENTUAL TRACK OF ISABEL WILL BE THE MAJOR PLAYER. I HAD TO
THROW OUT 00Z GFS AS IT WAS MUCH SLOWER THAN TPC FCST...SO
GENERALLY WENT WITH PREVIOUS RUNS THINKING...AND THIS AGREES WELL
WITH SURROUNDING OFFICE GRIDS. I DID BUMP UP THE CLOUDS A TOUCH AND
DOWN THE TEMPS A TOUCH ON TUESDAY AS UPPER JET PASSING BY IS EXPECTED
TO INCREASE THE HIGH CLOUD COVER A LITTLE...BUT OTHERWISE NICE
SEPTEMBER WEATHER WITH SEASONABLE TEMPS...LOW RAIN CHANCES...AND
LOWER HUMIDITY VALUES THAN USUAL DUE TO SUBSIDENCE ON THE
WEST/SOUTHWEST SIDE OF ISABEL.