Irene becomes first hurricane, still generally headed to South Florida

Posted on 22 August 2011 by LeslieM

IRENE HAS BECOME MUCH BETTER ORGANIZED DURING THE PAST 6 HOURS DESPITE ITS INTERACTION WITH THE MOUNTAINOUS TERRAIN OF PUERTO
RICO...AND A RAGGED EYE HAS BECOME APPARENT IN IMAGERY FROM THE FAA TERMINAL DOPPLER WEATHER RADAR. ALSO...LARGE PATCHES OF DOPPLER
VELOCITY VALUES AS HIGH AS 75-82 KT AT 1000-1500 FT AND 85-91 KT AT 2000-3000 FT HAVE BEEN INDICATED NORTHWEST AND NORTHEAST OF THE
CENTER OVER WATER FOR THE PAST 2 HOURS...WHICH EQUATES TO AT LEAST 65-KT SURFACE WINDS. SOME DOPPLER VELOCITIES AS HIGH AS 72 KT HAVE
ALSO BEEN DETECTED AT 500-600 FT OVER WATER. AS A RESULT OF THESE DATA...IRENE HAS BEEN UPGRADED TO THE FIRST HURRICANE OF THE 2011
ATLANTIC HURRICANE SEASON.

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TRENDING THROUGH ALL OF THE WOBBLES IN THE TRACK OVER THE PAST 6 HOURS YIELDS AN AVERAGE MOTION OF ABOUT 290/10 KT. UPPER-AIR DATA
AT 00Z INDICATE 500 MB HEIGHTS HAVE NOT CHANGED AT BERMUDA IN THE PAST 24 HOURS...WHICH SUGGESTS THAT THE SUBTROPICAL RIDGE TO THE
NORTH OF IRENE REMAINS QUITE STRONG. THE RIDGE IS EXPECTED TO CHANGE LITTLE FOR THE NEXT 48 HOURS OR SO...WHICH WOULD ACT TO KEEP
IRENE MOVING IN A GENERAL WEST-NORTHWESTWARD DIRECTION OFF THE NORTH COAST OF PUERTO RICO THIS MORNING AND JUST SKIRTING THE
NORTHERN COAST OF HISPANIOLA LATER TODAY AND ON TUESDAY. AFTER THAT...THE GLOBAL MODELS DIVERGE ON THE SPECIFICS OF HOW LARGE A
BREAK OR WEAKNESS IN THE SUBTROPICAL RIDGE DEVELOPS OVER FLORIDA AND THE SOUTHEASTERN U.S. AT 72 HOUR AND BEYOND...WHICH WILL ALLOW
IRENE TO MOVE NORTHWESTWARD. THE ECMWF...NOGAPS...AND GFDN MODELS ARE THE RIGHTMOST OF THE NHC MODEL GUIDANCE SUITE AND KEEP IRENE
WELL EAST OF FLORIDA...WHEREAS THE UKMET AND THE GFDL MODELS ARE THE LEFTMOST OF THE MODELS AND TAKE IRENE NEAR THE WEST COAST OF
FLORIDA. THE GFS AND THE REST OF THE NHC MODEL GUIDANCE IS TIGHTLY PACKED BETWEEN THESE TWO EXTREMES AND TAKE IRENE THROUGH THE
BAHAMAS AND JUST OFF THE FLORIDA EAST COAST. WHAT IS NOTEWORTHY IS THAT ALL OF THE GLOBAL AND REGIONAL MODELS AGREE THAT THE OUTFLOW
OF IRENE WILL CONVERGE WITH WESTERLY TO NORTHWESTERLY UPPER-LEVEL MID-LATITUDE FLOW OVER THE MID-ATLANTIC STATES BY 96 HOURS AND
BEYOND. THIS UPPER-LEVEL MASS CONFLUENCE COULD MAINTAIN THE MID- TO LOW-LEVEL RIDGE A LITTLE BIT STRONGER AND LONGER THAN WHAT NOGAPS
AND THE ECMWF ARE FORECASTING...WHICH WOULD IN TURN KEEP IRENE A LITTLE CLOSER TO THE FLORIDA EAST COAST. AS A RESULT...THE OFFICIAL
FORECAST TRACK WAS ONLY SHIFTED SLIGHTLY TO THE EAST OF THE PREVIOUS TRACK AND IS NEAR THE CONSENSUS MODELS TVCN AND TVCA.

IT NOW APPEARS THAT IRENE WILL NOT INTERACT WITH HISPANIOLA AS MUCH OR AS LONG AS PREVIOUSLY EXPECTED. THIS WILL ALSO RESULT IN MORE
STRENGTHENING THAN PREVIOUSLY EXPECTED. IRENE IS FORECAST BY ALL OF THE MODELS TO HAVE A VERY IMPRESSIVE OUTFLOW PATTERN THROUGHOUT THE
FORECAST PERIOD...INCLUDING A LARGE UPPER-LEVEL TROUGH/LOW ABOUT 1200 NMI EAST OF THE CYCLONE ACTING AS A MASS SINK. WITH IRENE ALSO
EXPECTED TO BE OVER SSTS NEAR 30C AFTER 48 HOURS...SIGNIFICANT STRENGTHENING COULD OCCUR. HOWEVER...THE OFFICIAL INTENSITY WILL
REMAIN ON THE CONSERVATIVE SIDE DUE TO THE UNCERTAINTY IN HOW MUCH IRENE WILL INTERACT WITH THE MOUNTAINOUS TERRAIN OF HISPANIOLA FOR
THE NEXT 24-36 HOURS. THE OFFICIAL INTENSITY FORECAST IS A BLEND OF THE SHIPS/LGEM MODELS AND THE HWRF/GFDL MODELS. HOWEVER...GIVEN THE
IMPRESSIVE UPPER-LEVEL FLOW PATTERN EXPECTED ACROSS IRENE...IT WOULD NOT SURPRISE ME IF THIS CYCLONE BECAME A MAJOR HURRICANE AT SOME TIME DURING ITS LIFETIME LIKE THE GFDL AND HWRF MODELS ARE FORECASTING. 
IT IS IMPORTANT NOT TO FOCUS ON THE EXACT FORECAST TRACK... ESPECIALLY AT DAYS 4 TO 5...SINCE THE MOST RECENT 5-YEAR AVERAGE
ERRORS AT THOSE FORECAST TIMES ARE 200 AND 250 MILES...RESPECTIVELY.

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