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 737 
 WTNT44 KNHC 220259
 TCDAT4
  
 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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