Hurricane ISABEL

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HURRICANE ISABEL DISCUSSION NUMBER  27
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
5 PM EDT FRI SEP 12 2003

REPORTS FROM AIR FORCE RESERVE AND NOAA HURRICANE HUNTER AIRCRAFT
CONFIRM THAT ISABEL IS A CATEGORY 5 HURRICANE.  MAXIMUM OBSERVED
FLIGHT-LEVEL WINDS SO FAR ARE 156 KT AT 700 MB...SUPPORTING SURFACE
WINDS NEAR 140 KT.  THE EYE IS 30 NM WIDE WITH A CLOSED WALL AND A
MINIMUM PRESSURE OF 920 MB. THE INITIAL INTENSITY THUS REMAINS 140
KT...WHICH AGREES WITH SATELLITE INTENSITY ESTIMATES.  IT SHOULD BE
NOTED THAT ISABEL LOOKS A LITTLE LESS ORGANIZED IN SATELLITE
IMAGERY THAN IT DID 24 HR AGO.  THUS IT MIGHT HAVE BEEN A LITTLE
STRONGER YESTERDAY.

THE EYE HAS RECENTLY NUDGED TO THE RIGHT...BUT THE LONGER TERM
MOTION REMAINS 270/8.  THERE IS LITTLE CHANGE IN THE SYNOPTIC
PATTERN OR REASONING FROM THE PREVIOUS FORECAST.  ISABEL IS
CURRENTLY SOUTH OF A DEEP LAYER RIDGE...WITH AN ANTICYCLONE SEEN
IN WATER VAPOR IMAGERY NEAR 29N55W...AND UPPER-LEVEL TROUGHS ALONG
68W AND 42W.  IN THE SHORT TERM...THIS SHOULD PRODUCE A WESTWARD TO
POSSIBLY WEST-NORTHWESTWARD MOTION.  UPSTREAM...A DEEP LAYER
CYCLONIC CIRCULATION IS OVER THE SOUTHEASTERN UNITED
STATES WITH A MID/UPPER-LEVEL RIDGE BUILDING OVER SOUTHEASTERN
CANADA.  LARGE-SCALE GUIDANCE FORECASTS THE SOUTHEASTERN SYSTEM TO
MOVE NORTHWARD...LEAVING A WEAKNESS ON THE SOUTHWEST SIDE OF THE
CANADIAN RIDGE AS IT BUILDS EASTWARD INTO THE ATLANTIC.  NHC TRACK
GUIDANCE AGREES ON A WESTWARD TO WEST-NORTHWESTWARD MOTION THROUGH
72 HR... AND TO THAT TIME THE FORECAST TRACK IS AN UPDATE OF THE
PREVIOUS PACKAGE.  CONSIDERABLE DIVERGENCE OCCURS AFTER 72 HR.  THE
UKMET CALLS FOR A GRADUAL TURN TOWARD THE NORTHWEST BY 120 HR IN
RESPONSE TO THE WEAKNESS.  THE GFS...NOGAPS...GFDL...AND CANADIAN
MODELS ALL CALL FOR A MORE NORTHWARD MOTION.  SINCE ALL MODELS KEEP
SOME DEEP LAYER RIDGING OVER THE NORTHWEST ATLANTIC NORTH AND
NORTHEAST OF ISABEL...SO SHARP OF A TURN CONTINUES TO LOOK A LITTLE
DUBIOUS.  THUS...THE 3-5 DAY OFFICIAL FORECAST WILL LEAN TOWARD THE
UKMET.  IT SHOULD BE NOTED THAT THE UKMET IS CALLING FOR MORE
NORTHWARD MOTION AFTER 96 HR THAN FORECAST EARLIER...AND THE
OFFICIAL FORECAST IS THUS SHIFTED TO THE RIGHT OF THE PREVIOUS
FORECAST AFTER 72 HR.  GIVEN THE DIVERGENCE OF THE MODELS AT 120
HR...IT IS STILL TOO EARLY TO SPECULATE WHICH PARTS...IF ANY...OF
EASTERN COAST OF THE UNITED STATES MAY GET AFFECTED BY ISABEL.

ISABEL IS FORECAST TO REMAIN IN A WARM-WATER LOW-SHEAR ENVIRONMENT
FOR ROUGHLY THE NEXT 72 HR.  DURING THAT TIME...THE INTENSITY
SHOULD BE CONTROLLED MAINLY BY EYEWALL REPLACEMENT CYCLES.  THE
INTENSITY FORECAST WILL THUS CALL FOR A SLOW WEAKENING THROUGH THAT
TIME BASED ON CONTINUITY FROM PREVIOUS FORECASTS AND CLIMATOLOGY. 
AFTER 72 HR...THE GFS AND SHIPS MODELS SUGGEST ISABEL MAY BE
AFFECTED BY SOUTHWESTERLY VERTICAL SHEAR.  BASED ON THIS...THE
INTENSITY FORECAST WILL CALL FOR A SOMEWHAT FASTER WEAKENING AFTER
72 HR.

WIND RADII HAVE BEEN REVISED ON THE BASIS OF AIRCRAFT DATA. IT
SHOULD BE NOTED THAT LARGE-SCALE MODELS ALL FORECAST ISABEL TO GROW
IN SIZE BEYOND 72 HR EVEN IF THE INNER CORE WEAKENS.

FORECASTER BEVEN
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      12/2100Z 21.8N  58.6W   140 KT
 12HR VT     13/0600Z 21.9N  59.9W   140 KT
 24HR VT     13/1800Z 22.4N  62.1W   135 KT
 36HR VT     14/0600Z 23.0N  64.3W   135 KT
 48HR VT     14/1800Z 23.6N  66.3W   130 KT
 72HR VT     15/1800Z 24.5N  69.5W   125 KT
 96HR VT     16/1800Z 26.0N  72.0W   115 KT
120HR VT     17/1800Z 28.5N  75.0W   105 KT
 
 
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