[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 251455
TCDAT4

HURRICANE IRENE DISCUSSION NUMBER  21
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL092011
1100 AM EDT THU AUG 25 2011

IT HAS BEEN DIFFICULT TO DETERMINE WHETHER THE HURRICANE HAS GONE
THROUGH AN EYEWALL REPLACEMENT. ALTHOUGH THE RADIUS OF MAXIMUM WIND
IS LARGER TODAY THAN YESTERDAY...MICROWAVE AND AIRCRAFT DATA HAVE
NOT BEEN VERY CONCLUSIVE. RECENT DATA DO NOT SHOW ANY EVIDENCE OF A
SECONDARY EYEWALL AT THIS TIME. THE EYE IS A LITTLE MORE EVIDENT IN
THE LAST FEW VISIBLE SATELLITE IMAGES...BUT IS NOT AS CLEAR AS IT
WAS YESTERDAY MORNING. THE AIR FORCE AIRCRAFT HAS MEASURED A PEAK
FLIGHT-LEVEL WIND OF 112 KT AND SFMR WINDS OF 90 KT...WHICH ROUGHLY
SUPPORT THE 100-KT INITIAL INTENSITY.

CENTER FIXES FROM THE AIRCRAFT OVERNIGHT AND THIS MORNING INDICATE
THAT THE EXPECTED NORTH-NORTHWESTWARD TURN HAS OCCURRED. A
NORTHWARD TURN IS EXPECTED TONIGHT OR EARLY FRIDAY AS IRENE MOVES
THROUGH A BREAK IN THE SUBTROPICAL RIDGE. A MID/UPPER-LEVEL TROUGH
MOVING THROUGH THE GREAT LAKES REGION IS EXPECTED TO THEN TURN
IRENE TOWARD THE NORTH-NORTHEAST IN A COUPLE OF DAYS. THIS TROUGH
IS FORECAST TO BYPASS IRENE...LEAVING THE HURRICANE IN DEEP
SOUTHERLY FLOW BETWEEN A TROUGH OVER THE CENTRAL UNITED STATES AND
A BUILDING RIDGE OVER THE ATLANTIC. THIS PATTERN WILL LIKELY KEEP
THE HURRICANE VERY NEAR THE EAST COAST OF THE UNITED STATES. THE
GUIDANCE ENVELOPE HAS SHIFTED A LITTLE WESTWARD ON THIS CYCLE...AND
THE NEW OFFICIAL TRACK HAS BEEN NUDGED WEST AS WELL. THE NEW
FORECAST LIES BETWEEN THE GFS AND ECMWF...AND IS A LITTLE RIGHT OF
THE TVCA CONSENSUS. SINCE IRENE IS SUCH A LARGE TROPICAL
CYCLONE...SIGNIFICANT IMPACTS ARE LIKELY ALONG THE UNITED STATES
EAST COAST REGARDLESS OF THE EXACT TRACK IT TAKES.

THE ENVIRONMENT REMAINS CONDUCIVE FOR SOME ADDITIONAL STRENGTHENING
DURING THE NEXT DAY OR TWO.  THE MAIN INFLUENCING FACTOR DURING THE
SHORT TERM WILL LIKELY BE EYEWALL CYCLES WHICH ARE DIFFICULT TO
PREDICT.  AFTER 48 HOURS...SOUTHWESTERLY SHEAR IS FORECAST TO
INCREASE WHICH WILL LIKELY START WEAKENING PROCESS.  HOWEVER...
SINCE IRENE HAS SUCH A LARGE AND INTENSE CIRCULATION...IT
WILL PROBABLY BE RATHER SLOW TO WEAKEN.

FORECAST POSITIONS AND MAX WINDS

INIT  25/1500Z 25.9N  76.8W  100 KT 115 MPH
 12H  26/0000Z 27.5N  77.5W  105 KT 120 MPH
 24H  26/1200Z 29.5N  77.8W  110 KT 125 MPH
 36H  27/0000Z 31.4N  77.5W  105 KT 120 MPH
 48H  27/1200Z 33.3N  77.1W  100 KT 115 MPH
 72H  28/1200Z 37.8N  75.3W   85 KT 100 MPH
 96H  29/1200Z 45.0N  71.5W   55 KT  65 MPH...INLAND
120H  30/1200Z 54.0N  64.5W   45 KT  50 MPH...POST-TROP/EXTRATROP

$$
FORECASTER BROWN



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