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Hurricane IGOR (Text)

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HURRICANE IGOR DISCUSSION NUMBER  45
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL112010
500 AM AST SUN SEP 19 2010

DATA FROM THE HURRICANE HUNTER AIRCRAFT AND SATELLITE IMAGERY
INDICATE THAT IGOR HAS CONTINUED TO SLOWLY WEAKEN.  THE AIRCRAFT
DID NOT REPORT AN EYEWALL...AND SOME DRY AIR ALSO APPEARS TO BE
WORKING ITS WAY INTO THE CIRCULATION...PARTICULARLY OVER THE
WESTERN SEMICIRCLE.  THE INITIAL INTENSITY IS LOWERED TO 75 KT
BASED ON A BLEND OF THE FLIGHT LEVEL MAXIMUM WIND OF 91 KT AND A
MAXIMUM SFMR READING OF 68 KT.  WHILE THE STRONGEST WINDS HAVE
DECREASED...THE SFMR DATA SHOWS THAT THE WIND FIELD OF IGOR
CONTINUES TO BROADEN...AND THE 50-KT AND 64-KT WIND RADII HAVE BEEN
INCREASED.  LITTLE CHANGE IN INTENSITY IS EXPECTED FOR THE NEXT
COUPLE OF DAYS...AND IGOR WILL BE A LARGE HURRICANE AS IT PASSES BY
BERMUDA IN THE NEXT 24 HOURS AND SUBJECTS THAT ISLAND TO A LONG
PERIOD OF STRONG WINDS...WITH TROPICAL STORM CONDITIONS LIKELY
LASTING AT LEAST 24 HOURS.  BY 72 HOURS...IGOR WILL TRANSITION TO A
POWERFUL AND LARGE EXTRATROPICAL CYCLONE AND THEN SLOWLY WEAKEN
THROUGH THE END OF THE PERIOD. 
 
THE INITIAL MOTION ESTIMATE IS 340/10.  THE LATEST AIRCRAFT FIXES
WERE LOCATED A LITTLE TO THE WEST OF THE APPARENT CENTER IN
GEOSTATIONARY IMAGERY...AND ALSO A LITTLE TO THE LEFT OF THE
PREVIOUS FORECAST TRACK.  THE OVERALL TRACK FORECAST REASONING
REMAINS UNCHANGED...AS IGOR WILL TURN NORTHWARD SOON AS IT MOVES
AROUND THE WESTERN PERIPHERY OF THE SUBTROPICAL RIDGE...AND THEN
ACCELERATE TOWARD THE NORTHEAST ON MONDAY AS IT RECURVES AHEAD
OF A MID-LATITUDE TROUGH MOVING OFF THE COAST OF NORTH AMERICA. 
THROUGH 36 HOURS THE OFFICIAL FORECAST HAS BEEN ADJUSTED A LITTLE
TO THE LEFT OF THE PREVIOUS ONE TO ACCOUNT FOR THE INITIAL POSITION
AND LIES ALONG THE EASTERN EDGE OF THE TIGHTLY CLUSTERED GUIDANCE
ENVELOPE.  THE SPREAD IN THE GUIDANCE INCREASES BEYOND 48 HOURS DUE
TO VARIATIONS IN THE DETAILS OF HOW IGOR INTERACTS WITH THE 
MID-LATITUDE FLOW DURING AND AFTER EXTRATROPICAL TRANSITION.  THE
OFFICIAL FORECAST HAS BEEN ADJUSTED NORTHWARD AT DAYS 3 AND 4 AND
IS IN BEST AGREEMENT WITH THE GFS MODEL.  AT DAY 5 THE OFFICIAL
FORECAST SHOWS POST-TROPICAL IGOR SLOWING AND TURNING MORE NORTHWARD
...HOWEVER CONFIDENCE IS LOW IN THIS SCENARIO AS THE SPREAD IN THE
GUIDANCE IS VERY LARGE. 
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      19/0900Z 28.9N  65.3W    75 KT
 12HR VT     19/1800Z 30.5N  65.5W    75 KT
 24HR VT     20/0600Z 33.4N  64.9W    75 KT
 36HR VT     20/1800Z 37.0N  62.5W    75 KT
 48HR VT     21/0600Z 41.0N  57.5W    75 KT
 72HR VT     22/0600Z 49.0N  47.0W    70 KT...POST-TROP/EXTRATROP
 96HR VT     23/0600Z 53.0N  38.0W    45 KT...POST-TROP/EXTRATROP
120HR VT     24/0600Z 56.5N  36.5W    35 KT...POST-TROP/EXTRATROP
 
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FORECASTER BRENNAN
 
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