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Tropical Storm IRENE (Text)

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TROPICAL STORM IRENE DISCUSSION NUMBER   6
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL092011
1100 PM AST SUN AUG 21 2011
 
IRENE HAS CONTINUED TO BECOME BETTER ORGANIZED OVER THE PAST FEW
HOURS...AS THE CIRCULATION HAS BECOME BETTER DEFINED AND RADIAL
VELOCITIES HAVE INCREASED ON THE SAN JUAN DOPPLER RADAR. THERE HAS
ALSO BEEN A MARKED INCREASE IN DEEP CONVECTION NEAR THE CENTER ON
SATELLITE IMAGERY.  DATA FROM THE AIR FORCE RESERVE HURRICANE
HUNTER AIRCRAFT INDICATE THAT THE CENTRAL PRESSURE HAS FALLEN TO
993 MB.  THE INITIAL INTENSITY IS SET AT 60 KT BASED ON A PEAK
BELIEVABLE SFMR VALUE OF 61 KT AND AN EXPANDING AREA OF 65 TO 70 KT
DOPPLER VELOCITIES AT 5500 TO 6000 FEET ON THE SAN JUAN RADAR.
UPPER-LEVEL OUTFLOW IS WELL ESTABLISHED IN ALL QUADRANTS EXCEPT THE
SOUTHEAST AND ATMOSPHERIC AND OCEANIC CONDITIONS APPEAR FAVORABLE
FOR INTENSIFICATION...SO THE INTENSITY OF IRENE WILL LARGELY BE
DETERMINED BY HOW MUCH THE CYCLONE INTERACTS WITH LAND IN THE NEXT
24 TO 36 HOURS. SOME INTENSIFICATION IS STILL EXPECTED IN THE NEXT
FEW HOURS...AND IRENE COULD BECOME A HURRICANE BEFORE MOVING OVER
PUERTO RICO. THE CYCLONE IS THEN EXPECTED TO MOVE ACROSS THE
NORTHERN PORTION OF HISPANIOLA BETWEEN 24 AND 36 HOURS...AND SOME
WEAKENING IS INDICATED DURING THAT TIME. HOWEVER...IF IRENE MOVES
NORTH OF HISPANIOLA...IT COULD BE STRONGER THAN INDICATED HERE IN
36 TO 48 HOURS...OR WEAKER THAN FORECAST IF IT MOVES OVER MORE OF
THE HIGH TERRAIN OF THAT ISLAND. BEYOND 48 HOURS GRADUAL
STRENGTHENING IS EXPECTED AS IRENE MOVES OVER WARM WATERS NEAR THE
BAHAMAS...BUT THE UNCERTAINTY REMAINS VERY HIGH LATE IN THE PERIOD
DUE TO THE POTENTIAL FOR LAND INTERACTION IN THE SHORT TERM.
 
WHILE THE FORWARD MOTION OF THE CENTER APPEARED TO HAVE SLOWED ON
RADAR IMAGERY AS THE CYCLONE ORGANIZED DURING THE EVENING HOURS...
THE LONG TERM INITIAL MOTION ESTIMATE IS TOWARD 285 DEGREES AT ABOUT
13 KNOTS.  THE SUBTROPICAL RIDGE CURRENTLY SITUATED NORTH OF THE
CYCLONE WILL BREAK DOWN OVER THE NEXT DAY OR SO AS A LONGWAVE
MID-LEVEL TROUGH AMPLIFIES OVER THE WESTERN ATLANTIC.  THE TROUGH
BEGINS TO LIFT OUT AFTER ABOUT 36 HOURS OR SO...BUT LEAVES A
PRONOUNCED WEAKNESS IN THE RIDGE OVER THE SOUTHWESTERN ATLANTIC AND
THE BAHAMAS.  MOST OF THE GUIDANCE...WITH THE EXCEPTION OF THE GFDL
AND UKMET...ARE IN RELATIVELY GOOD AGREEMENT ON TURNING IRENE
NORTHWESTWARD INTO THE BREAK IN THE RIDGE IN 2 OR 3 DAYS. 
HOWEVER...SIGNIFICANT SPREAD REMAINS IN THE SHARPNESS OF THIS TURN
IN THE MODEL SUITE.  FOR THIS CYCLE THE CONSENSUS TRACK MODELS HAVE
SHIFTED BACK A LITTLE TO THE LEFT...WITH THE GFS...ECMWF...AND HWRF
MODELS ON THE EAST SIDE OF THE GUIDANCE ENVELOPE AND THE
AFOREMENTIONED UKMET AND GFDL FARTHER WEST.  THE OFFICIAL FORECAST
HAS BEEN NUDGED TO THE RIGHT...TOWARD BUT STILL TO THE LEFT OF THE
MODEL CONSENSUS...AND LIES CLOSE TO THE FIM MODEL PROVIDED BY THE
HURRICANE FORECAST IMPROVEMENT PROJECT.  

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.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  22/0300Z 17.9N  65.5W   60 KT  70 MPH
 12H  22/1200Z 18.6N  67.7W   60 KT  70 MPH
 24H  23/0000Z 19.4N  70.1W   60 KT  70 MPH...INLAND
 36H  23/1200Z 20.1N  72.1W   50 KT  60 MPH...OVER WATER
 48H  24/0000Z 20.9N  73.9W   55 KT  65 MPH
 72H  25/0000Z 23.5N  77.0W   65 KT  75 MPH
 96H  26/0000Z 26.5N  79.5W   80 KT  90 MPH
120H  27/0000Z 30.0N  81.5W   75 KT  85 MPH...INLAND
 
$$
FORECASTER BRENNAN
 
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