[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 230300
TCDAT4

HURRICANE IRENE DISCUSSION NUMBER  11
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL092011
1100 PM AST MON AUG 22 2011

AFTER DEEPENING QUICKLY EARLIER THIS EVENING...THE INTENSITY OF
IRENE APPEARS TO HAVE LEVELED OFF A BIT OVER THE PAST HOUR OR TWO.
THE CENTRAL PRESSURE HAS DECREASED TO 980 MB ...AND MAXIMUM WINDS
HAVE NOT INCREASED SINCE THE EARLIER MAXIMUM OF 106 KT AT 850 MB.
THEREFORE...THE INTENSITY WILL REMAIN 85 KT FOR THIS ADVISORY.
OVERALL THE STRUCTURE OF THE CYCLONE HAS CONTINUED TO IMPROVE...
WITH A LARGE CDO FORMING NEAR AND NORTH OF THE CENTER AND THE
AIRCRAFT REPORTING A CLOSED EYEWALL.  SOUTHERLY SHEAR OF ABOUT 10
KT IS PRESENT OVER IRENE ACCORDING TO SATELLITE ANALYSIS FROM
UW-CIMSS AND THE SHIPS MODEL...BUT OTHERWISE THE ENVIRONMENT
APPEARS CONDUCIVE FOR CONTINUED STRENGTHENING.  THE NHC INTENSITY
FORECAST IS UNCHANGED FROM THE PREVIOUS SPECIAL ADVISORY AND SHOWS
IRENE BECOMING A MAJOR HURRICANE WITHIN 24 HOURS AND REACHING A
PEAK OF 115 KT IN 72 HOURS...FOLLOWED BY SLOW WEAKENING LATE IN THE
PERIOD.  THE NHC FORECAST IS HIGHER THAN MOST OF THE GUIDANCE
THROUGH 48 HOURS AND IS CLOSE TO A BLEND OF THE GFDL AND HWRF
MODELS AFTER THAT TIME.

AIRCRAFT FIXES INDICATE THAT THE CENTER WAS LOCATED A LITTLE TO THE
RIGHT OF THE PREVIOUS TRACK AND THAT THE FORWARD MOTION HAS SLOWED
A BIT...WITH AN INITIAL MOTION ESTIMATE OF 295/10. OVERALL THE
TRACK FORECAST SCENARIO HAS NOT CHANGED...AS IRENE WILL CONTINUE
MOVING WEST-NORTHWESTWARD FOR THE NEXT DAY OR SO ON THE SOUTHWEST
PERIPHERY OF A WEAKENING MID-LEVEL RIDGE. A MID- TO UPPER-LEVEL
TROUGH LOCATED EAST OF NORTH AMERICA WILL LIFT OUT WITHIN 24
HOURS...LEAVING A PRONOUNCED WEAKNESS IN THE RIDGE OVER THE
BAHAMAS.  THIS SHOULD ALLOW IRENE TO GRADUALLY TURN NORTHWESTWARD
AND NORTH-NORTHWESTWARD BY DAYS 2 AND 3.  AFTER THAT TIME...THE
TRACK OF IRENE APPEARS TO BE SENSITIVE TO THE TIMING AND AMPLITUDE
OF SEVERAL SHORTWAVE TROUGHS MOVING EASTWARD ACROSS THE UNITED
STATES/CANADIAN BORDER. THE MODELS ARE SHOWING SOME RUN-TO-RUN
VARIABILITY IN HOW MUCH THESE SHORTWAVES WILL AMPLIFY AND DEEPEN
THE MEAN TROUGH OVER THE NORTHEASTERN UNITED STATES...WHICH WILL BE
CRITICAL TO HOW SOON IRENE TURNS NORTHWARD OR EVEN EAST OF DUE
NORTH AT DAYS 4 AND 5. OVERALL...THE SPREAD OF THE GUIDANCE SUITE
HAS DECREASED THIS CYCLE...AND THE COMBINATION OF THE SLIGHT
RIGHTWARD ADJUSTMENT OF THE INITIAL POSITION AND A SLIGHT WESTWARD
SHIFT IN THE MODEL CONSENSUS RESULTS IN ONLY MINOR ADJUSTMENTS TO
THE NEW NHC TRACK FORECAST. THE BIGGEST CHANGE IS A SLOWER FORWARD
SPEED AT THE END OF THE PERIOD FOLLOWING THE BULK OF THE GUIDANCE
THAT KEEPS THE CYCLONE OFFSHORE AT DAY 5.

IT IS IMPORTANT TO REMIND USERS NOT TO FOCUS ON THE EXACT FORECAST
TRACK...ESPECIALLY AT DAYS 4 AND 5...SINCE THE MOST RECENT 5-YEAR
AVERAGE ERRORS AT THOSE FORECAST TIMES ARE 200 AND 250 MILES...
RESPECTIVELY.

OF NOTE...SYNOPTIC SURVEILLANCE FLIGHTS FROM THE NOAA G-IV AND AN
AIR FORCE RESERVE C-130 AIRCRAFT PROVIDED NUMEROUS DROPSONDES FOR
ASSIMILATION INTO THE 0000 UTC MODEL CYCLE.  THIS SHOULD IMPROVE
THE ANALYSIS OF THE ENVIRONMENT AROUND AND UPSTREAM OF IRENE.


FORECAST POSITIONS AND MAX WINDS

INIT  23/0300Z 19.9N  69.2W   85 KT 100 MPH
 12H  23/1200Z 20.4N  70.6W   95 KT 110 MPH
 24H  24/0000Z 21.1N  72.4W  105 KT 120 MPH
 36H  24/1200Z 22.2N  74.2W  110 KT 125 MPH
 48H  25/0000Z 23.5N  75.6W  110 KT 125 MPH
 72H  26/0000Z 26.5N  78.0W  115 KT 135 MPH
 96H  27/0000Z 30.0N  79.0W  105 KT 120 MPH
120H  28/0000Z 33.5N  78.5W  100 KT 115 MPH

$$
FORECASTER BRENNAN


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