Hurricane IGOR

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HURRICANE IGOR DISCUSSION NUMBER  42
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL112010
1100 AM AST SAT SEP 18 2010

THERE HAS BEEN LITTLE CHANGE IN THE STRUCTURE OF IGOR DURING THE
PAST SEVERAL HOURS.  RECENT MICROWAVE OVERPASSES SUGGEST THE
HURRICANE IS UNDERGOING AN EYEWALL REPLACEMENT CYCLE...WITH THE
20 NM WIDE INNER EYE SEEN IN THE PREVIOUS AIRCRAFT MISSION GRADUALLY
WEAKENING.  SATELLITE INTENSITY ESTIMATES FROM TAFB AND SAB REMAIN
102 KT...AND THE INITIAL INTENSITY REMAINS 95 KT.  AN AIR FORCE
RESERVE HURRICANE HUNTER AIRCRAFT IS CURRENTLY EN ROUTE TO
INVESTIGATE IGOR.

THE INITIAL MOTION IS 310/10.  DURING THE NEXT 36-72 HR...IGOR WILL
ROUND THE WESTERN END OF A DEEP-LAYER RIDGE OVER THE CENTRAL AND
EASTERN ATLANTIC AND ENTER THE WESTERLIES TO THE EAST OF A
SHORTWAVE TROUGH MOVING OFF THE COAST OF EASTERN NORTH AMERICA. 
THE TRACK GUIDANCE IS IN GOOD AGREEMENT WITH THIS SCENARIO AND
HAS CHANGED LITTLE SINCE THE PREVIOUS ADVISORY.  THIS PART OF THE
THE NEW FORECAST TRACK IS THUS AN UPDATE OF THE PREVIOUS ONE...WITH
THE CENTER OF IGOR PASSING NEAR OR OVER BERMUDA IN 36-48 HR.  AFTER
72 HR...THERE IS SOME SPREAD IN THE GUIDANCE AS IGOR UNDERGOES
EXTRATROPICAL TRANSITION.  THE ECMWF TURNS THE CYCLONE SHARPLY TO
THE NORTH...WHILE THE OTHER DYNAMICAL MODELS SHOW A MORE
NORTHEASTWARD TO EASTWARD MOTION.  THIS PART OF THE TRACK IS SHIFTED
TO THE EAST OF THE PREVIOUS FORECAST BASED ON A BLEND OF THE
DYNAMICAL MODELS...BUT LIES TO THE NORTH OF THE CENTER OF THE
GUIDANCE ENVELOPE.

IGOR IS IN AN ENVIRONMENT OF LIGHT SOUTHERLY VERTICAL SHEAR...AND
THIS IS FORECAST TO CONTINUE FOR THE NEXT 24 HR OR SO.  AFTER
THAT...THE HURRICANE IS FORECAST TO ENCOUNTER INCREASING SHEAR.  IT
IS UNCERTAIN IF THE CURRENT EYEWALL REPLACEMENT CYCLE WILL BE
COMPLETE ENOUGH TO ALLOW STRENGTHENING BEFORE THE SHEAR
INCREASES...BUT CURRENT TRENDS SUGGEST IT WILL NOT.  MOST OF THE
INTENSITY GUIDANCE NOW SHOWS LITTLE CHANGE IN STRENGTH OR SLOW
WEAKENING DURING THE FIRST 36 HR...WITH ONLY THE SHIPS MODEL
SHOWING MODEST STRENGTHENING.  THE NEW INTENSITY FORECAST CALLS FOR
IGOR TO MAINTAIN A 95 KT INTENSITY FOR 36 HR OR SO...FOLLOWED BY
WEAKENING DUE TO SHEAR.  IT CANNOT BE RULED OUT THAT IGOR COULD
REGAIN MAJOR HURRICANE STATUS BEFORE REACHING BERMUDA...BUT IT
APPEARS LESS LIKELY THAN PREVIOUSLY THOUGHT.  EXTRATROPICAL
TRANSITION SHOULD BE UNDERWAY BY 72 HR AND BE COMPLETE BY 96 HR. 
IGOR IS EXPECTED TO REMAIN A LARGE AND POWERFUL CYCLONE AFTER
EXTRATROPICAL TRANSITION.

IGOR CONTINUES TO HAVE A VERY LARGE WIND FIELD...AND CONDITIONS WILL
BEGIN TO DETERIORATE ON BERMUDA BY TONIGHT WELL IN ADVANCE OF THE
CENTER.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      18/1500Z 26.0N  63.6W    95 KT
 12HR VT     19/0000Z 27.1N  64.6W    95 KT
 24HR VT     19/1200Z 28.9N  65.3W    95 KT
 36HR VT     20/0000Z 31.1N  65.2W    95 KT
 48HR VT     20/1200Z 33.9N  64.2W    90 KT
 72HR VT     21/1200Z 40.5N  56.0W    75 KT
 96HR VT     22/1200Z 47.5N  44.0W    65 KT...POST-TROP/EXTRATROP
120HR VT     23/1200Z 50.0N  36.5W    50 KT...POST-TROP/EXTRATROP
 
$$
FORECASTER BEVEN
 
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