Tropics I WCTV Eyewitness News: Tallahassee, Thomasville, Valdosta

Hurricane Irene Discussion 7

UPDATE: Monday, August 22, 11:00 am

HURRICANE IRENE DISCUSSION NUMBER 8
NWS NATIONAL HURRICANE CENTER MIAMI FL AL092011
1100 AM AST MON AUG 22 2011

DATA FROM A RECONNAISSANCE PLANE...RADAR AND SATELLITE IMAGERY INDICATE THAT IRENE HAS CONTINUED TO GRADUALLY BECOME BETTER ORGANIZED. HOWEVER THE WINDS WINDS HAVE NOT INCREASED SIGNIFICANTLY AND THE MINIMUM CENTRAL PRESSURE WAS AROUND 988 MB IN THE LAST
CENTER DROPSONDE. THE INITIAL INTENSITY HAS BEEN INCREASED TO 70 KNOTS IN THIS ADVISORY. SINCE THE CORE OF THE HURRICANE IS NOT FORECAST TO MOVE DIRECTLY OVER THE HIGH TERRAIN OF HISPANIOLA...AND THE OCEANIC AND ATMOSPHERIC CONDITIONS ARE QUITE FAVORABLE... THE
OFFICIAL FORECAST CALLS FOR INTENSIFICATION...AND MAKES IRENE A CATEGORY 3 HURRICANE AS IT MOVES ACROSS THE CENTRAL BAHAMAS. THIS IS CONSISTENT WITH MOST OF THE DYNAMICAL GUIDANCE AND IN FACT...EVEN GLOBAL MODELS FORECAST A SIGNIFICANT DROP IN THE CENTRAL PRESSURE OVER THE NEXT TWO OR THREE DAYS. IRENE IS BECOMING
A LARGE CYCLONE...AND BASED ON BUOY OBSERVATIONS THE TROPICAL STORM FORCE WINDS EXTEND 160 N MI TO THE NORTHEAST OF THE CENTER.

RADAR AND RECONNAISSANCE FIXES INDICATE THAT THE HURRICANE IS MOVING TOWARD THE WEST-NORTHWEST OR 295 DEGREES AT ABOUT 12 KNOTS. IRENE IS CURRENTLY LOCATED SOUTH OF A NARROW SUBTROPICAL RIDGE WHICH IS FORCING THE CYCLONE ON A WEST-NORTHWEST TRACK. HOWEVER...MOST OF THE GLOBAL MODELS SHOW THE GRADUAL DEVELOPMENT A LARGE UPPER TROUGH ALONG THE UNITED STATES EAST COAST. THIS PATTERN WILL INDUCE A MORE
NORTHWESTWARD AND THEN NORTHWARD TRACK THROUGH THE FORECAST PERIOD WITH A DECREASE IN FORWARD SPEED. BASED ON THE LATEST GUIDANCE... THE OFFICIAL TRACK FORECAST HAS BEEN SHIFTED TO THE EAST...BUT STILL REMAINS SLIGHTLY WEST OF THE MODEL CONSENSUS. ALTHOUGH IT IS TOO EARLY TO BE CERTAIN...THE CURRENT GUIDANCE LESSENS THE THREAT TO SOUTH FLORIDA.

DO 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.

FORECAST POSITIONS AND MAX WINDS

INIT 22/1500Z 19.2N 67.5W 70 KT 80 MPH
12H 23/0000Z 19.9N 69.2W 75 KT 85 MPH
24H 23/1200Z 20.5N 71.4W 80 KT 90 MPH
36H 24/0000Z 21.2N 73.2W 85 KT 100 MPH
48H 24/1200Z 22.0N 75.0W 90 KT 105 MPH
72H 25/1200Z 25.0N 77.5W 100 KT 115 MPH
96H 26/1200Z 28.5N 79.0W 100 KT 115 MPH
120H 27/1200Z 32.5N 80.0W 100 KT 115 MPH

$$
FORECASTER AVILA

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Monday, August 22, 9:00 am

HURRICANE IRENE DISCUSSION NUMBER 7
NWS NATIONAL HURRICANE CENTER MIAMI FL AL092011
500 AM AST MON AUG 22 2011

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.

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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