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000
WTNT41 KNHC 180837
TCDAT1
HURRICANE IGOR DISCUSSION NUMBER  41
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL112010
500 AM AST SAT SEP 18 2010

DATA FROM THE HURRICANE HUNTER AIRCRAFT INDICATE THAT WHILE THE
CENTRAL PRESSURE OF IGOR HAD FALLEN TO 939 MB...THE WINDS AT FLIGHT
LEVEL WERE NOT EFFICIENTLY MIXING DOWN TO THE SURFACE.  THE INITIAL
INTENSITY IS HELD AT 95 KT BASED ON A BLEND OF THE 115 KT FLIGHT-
LEVEL WIND MAXIMUM AND A MAXIMUM WIND OF 77 KT FROM THE SFMR. THE
OUTFLOW IS WELL ESTABLISHED...PARTICULARLY TO THE NORTH OF THE
CENTER...AND THE ENVIRONMENTAL CONDITIONS APPEAR FAVORABLE FOR A
LITTLE MORE STRENGTHENING OVER THE NEXT DAY OR SO BEFORE THE SHEAR
INCREASES.  THE OFFICIAL FORECAST SHOWS A PEAK AT 105 KT IN 24
HOURS AND THEN A SLOW WEAKENING THROUGH 48 HOURS. REGARDLESS OF THE
DETAILS OF THE INTENSITY FORECAST...IGOR WILL BE A LARGE AND
DANGEROUS HURRICANE AS IT APPROACHES BERMUDA ON SUNDAY. WEAKENING
IS EXPECTED TO CONTINUE BEYOND 48 HOURS...AS IGOR MOVES OVER COOLER
WATERS AND THE SHEAR INCREASES FURTHER.  ONCE IGOR COMPLETES
EXTRATROPICAL TRANSITION...IT WILL BE A LARGE AND POWERFUL CYCLONE
OVER THE NORTH ATLANTIC.

AFTER A SHORT TERM WOBBLE TOWARD THE WEST-NORTHWEST...THE LONG-TERM
INITIAL MOTION ESTIMATE IS 310/11.  IGOR IS SITUATED SOUTH OF A
MID-LEVEL RIDGE EXTENDING ALONG 35N...AND THIS RIDGE IS FORECAST BY
ALL OF THE MODELS TO WEAKEN AS A SHORTWAVE TROUGH PROGRESSES EAST
OFF THE U.S. EAST COAST IN THE NEXT DAY OR SO.  IGOR WILL TURN
NORTHWARD INTO THE WEAKNESS ALONG 65W ON SUNDAY AND PASS VERY NEAR
BERMUDA BEFORE TURNING NORTHEASTWARD ON MONDAY AHEAD OF ANOTHER
SHORTWAVE TROUGH MOVING OFF THE COAST OF NORTH AMERICA.  THE GLOBAL
MODELS SHOW THE RIDGE NORTH OF IGOR REMAINING A LITTLE STRONGER
THAN PREVIOUSLY FORECAST...AND AS A RESULT MOST OF THE GUIDANCE HAS
SHIFTED A LITTLE TO THE LEFT AND SLOWER THROUGH 48 HOURS.  THE
OFFICIAL FORECAST HAS BEEN ADJUSTED IN THIS DIRECTION...BUT REMAINS
TO THE RIGHT OF MOST OF THE GUIDANCE AND A LITTLE FASTER THAN
THE CONSENSUS.  BEYOND 48 HOURS...THE NEW NHC FORECAST IS LARGELY
AN UPDATE OF THE PREVIOUS PACKAGE...AND SHOWS IGOR ACCELERATING
TOWARD THE NORTHEAST AS IT BECOMES EXTRATROPICAL.

IGOR HAS A VERY LARGE WIND FIELD...WITH A RADIUS OF MAXIMUM WINDS OF
AROUND 45 NAUTICAL MILES AND HURRICANE FORCE WINDS EXTENDING OUT UP
TO 90 NM FROM THE CENTER.  CONDITIONS WILL BEGIN TO DETERIORATE ON
BERMUDA BY TONIGHT...WELL IN ADVANCE OF THE CENTER.

FORECAST POSITIONS AND MAX WINDS

INITIAL      18/0900Z 25.1N  62.8W    95 KT
 12HR VT     18/1800Z 26.2N  64.1W   100 KT
 24HR VT     19/0600Z 27.8N  65.1W   105 KT
 36HR VT     19/1800Z 29.9N  65.3W   100 KT
 48HR VT     20/0600Z 32.2N  64.7W    95 KT
 72HR VT     21/0600Z 38.5N  58.5W    80 KT
 96HR VT     22/0600Z 47.5N  46.5W    65 KT...POST-TROP/EXTRATROP
120HR VT     23/0600Z 51.5N  39.5W    55 KT...POST-TROP/EXTRATROP

$$
FORECASTER BRENNAN

000
WTNT41 KNHC 190853
TCDAT1
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

$$
FORECASTER BRENNAN