Hurricane IRENE

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HURRICANE IRENE DISCUSSION NUMBER  18
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL092011
500 PM EDT WED AUG 24 2011
 
THE SATELLITE PRESENTATION OF THE HURRICANE HAS CONTINUED TO IMPROVE
THIS AFTERNOON.  A LARGE AREA OF DEEP CONVECTION WITH VERY COLD
CLOUD TOPS HAS EXPANDED NEAR THE EYEWALL AND NOW SOLIDLY SURROUNDS
THE EYE.  BEFORE DEPARTING IRENE...THE AIR FORCE RESERVE HURRICANE
HUNTER AIRCRAFT MEASURED A PEAK 700 MB FLIGHT-LEVEL WIND OF 116 KT
IN THE NORTHEASTERN EYEWALL AND A DROPSONDE MEASURED A MINIMUM
PRESSURE OF 954 MB.  THESE DATA SUPPORT AN INITIAL INTENSITY OF
105 KT.
 
ENVIRONMENTAL CONDITIONS ARE EXPECTED TO REMAIN FAVORABLE FOR SOME
ADDITIONAL STRENGTHENING DURING THE NEXT DAY OR TWO. THE UW-CIMMS
OBJECTIVE SECONDARY EYEWALL FORMATION TECHNIQUE CONTINUES TO
PREDICT A HIGH CHANCE OF AN EYEWALL REPLACEMENT DURING THE NEXT DAY
OR SO. IN FACT...A RECENT TRMM MICROWAVE PASS SHOWED SOME EVIDENCE
OF A SECONDARY EYEWALL. THESE EYEWALL CYCLES CAN PRODUCE HARD TO
PREDICT FLUCTUATIONS IN INTENSITY. FOR NOW...THE NEW INTENSITY
FORECAST CALL FOR SOME ADDITIONAL STRENGTHENING FOLLOWED BY A
LEVELING OFF OF THE INTENSITY. IN 3-4 DAYS....INCREASING
SOUTHWESTERLY SHEAR AND COOLER SSTS SHOULD INDUCE GRADUAL
WEAKENING...ALTHOUGH IRENE IS EXPECTED TO REMAIN A LARGE AND
POWERFUL HURRICANE...OR EVEN GROW IN SIZE...AS IT NEARS NEW
ENGLAND. 
 
THE HURRICANE HAS BEEN MOVING NORTHWESTWARD OR 310/10 KT. IRENE IS
EXPECTED TO TURN TOWARD THE NORTH-NORTHWEST THEN NORTH THROUGH A
BREAK IN THE SUBTROPICAL RIDGE OVER THE WESTERN ATLANTIC DURING
THE NEXT COUPLE OF DAYS.  AFTER THAT TIME...THE HURRICANE IS
FORECAST TO TURN NORTH-NORTHEASTWARD AROUND THE NORTHWESTERN SIDE
OF THE RIDGE.  BETWEEN 72 AND 96 HOURS...THERE REMAINS UNCERTAINTY
AS TO WHETHER IRENE TURNS BACK TOWARD THE NORTH AHEAD OF A
MID-LATITUDE TROUGH MOVING INTO THE GREAT LAKES REGION AT THAT
TIME.  THE NEW ECMWF SHOWS MUCH MORE AMPLIFICATION OF THE TROUGH...
WHICH HAS RESULTED IN ITS TRACK SHIFTING OVER 100 MILES TO THE
WEST...AND THIS SKILLFUL LONG-RANGE MODEL NOW DEFINES THE WESTERN
EDGE OF THE GUIDANCE ENVELOPE.  OVERALL...THE GUIDANCE ENVELOPE HAS
SHIFTED A LITTLE WESTWARD AT DAYS 4 AND 5. THE NEW TRACK FORECAST
IS CLOSE TO THE PREVIOUS ADVISORY AND IS BETWEEN THE GFS
AND ECMWF MODELS.  THIS IS ALSO CLOSE TO THE TVCA AND EXPERIMENTAL
HURRICANE FORECAST IMPROVEMENT PROJECT...HFIP...CONSENSUS TRACKS.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  24/2100Z 23.1N  74.7W  105 KT 120 MPH
 12H  25/0600Z 24.3N  75.8W  110 KT 125 MPH
 24H  25/1800Z 26.1N  76.8W  115 KT 135 MPH
 36H  26/0600Z 28.2N  77.3W  115 KT 135 MPH
 48H  26/1800Z 30.1N  77.3W  110 KT 125 MPH
 72H  27/1800Z 34.3N  75.8W  100 KT 115 MPH
 96H  28/1800Z 39.5N  73.0W   85 KT 100 MPH
120H  29/1800Z 47.0N  69.0W   55 KT  65 MPH...POST-TROP/EXTRATROP
 
$$
FORECASTER BROWN
 
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