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Tropical Storm ISAAC (Text)

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TROPICAL STORM ISAAC DISCUSSION NUMBER   3
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL092012
500 PM AST TUE AUG 21 2012
 
AN AIR FORCE RESERVE HURRICANE HUNTER AIRCRAFT INVESTIGATING THE
DEPRESSION REPORTED 44-KT FLIGHT-LEVEL WINDS AT 1000 FT TO THE
NORTHEAST OF THE CENTER...AS WELL AS BIAS-CORRECTED SFMR SURFACE
WINDS ESTIMATES OF 35 KT.  THE MINIMUM CENTRAL PRESSURE WAS NEAR
1005 MB.  BASED ON THESE DATA...THE DEPRESSION IS UPGRADED TO
TROPICAL STORM ISAAC.  DESPITE THE UPGRADE...THE CYCLONE REMAINS
SOMEWHAT DISORGANIZED...WITH THE LOW-LEVEL CENTER PARTLY EXPOSED TO
THE NORTH OF THE MAIN CONVECTIVE MASS.
 
THE LOW-LEVEL CENTER HAS NUDGED NORTHWARD DURING THE AFTERNOON...BUT
THE LONGER TERM MOTION IS 275/15.  ISAAC REMAINS SOUTH OF A
DEEP-LAYER SUBTROPICAL RIDGE THAT IS FORECAST TO REMAIN INTACT
FOR THE NEXT 72 HR...WHICH SHOULD KEEP THE CYCLONE MOVING GENERALLY
WESTWARD AS SHOWN BY THE TIGHTLY CLUSTERED TRACK MODEL GUIDANCE.
AFTER THAT...THE GLOBAL MODELS CONTINUE TO FORECAST A SHORTWAVE
TROUGH TO DIG SOUTHWARD OVER THE SOUTHEASTERN U.S. AND THE GULF OF
MEXICO...WHICH IS EXPECTED TO WEAKEN THE RIDGE ACROSS FLORIDA AND
THE BAHAMAS. THIS SHOULD ALLOW THE CYCLONE TO TURN WEST-
NORTHWESTWARD AND SLOW DOWN OVER THE NORTH-CENTRAL CARIBBEAN SEA AND
THE GREATER ANTILLES.  THE GUIDANCE HAS BECOME LESS CONSISTENT
SINCE THE PREVIOUS ADVISORY.  WHILE THE CENTER OF THE GUIDANCE
ENVELOPE HAS CHANGED LITTLE...THE ECMWF HAS SHIFTED WELL TO THE
LEFT OF ITS PREVIOUS FORECAST AND NOW CALLS FOR A WESTWARD MOTION
AFTER ISAAC PASSES HISPANIOLA.  THE GFS ENSEMBLES HAVE ALSO SHIFTED
WESTWARD...ALTHOUGH THE ENSEMBLE MEAN STILL LIES NEAR THE FORECAST
TRACK.  THE NEW TRACK IS NUDGED A LITTLE TO THE NORTH OF THE
PREVIOUS TRACK BEFORE 72 HR...AND THEN IS NUDGED A LITTLE TO THE
RIGHT THEREAFTER.  THE TRACK FORECAST AFTER 72 HR LIES NEAR THE
LEFT EDGE OF THE GUIDANCE ENVELOPE...WITH ONLY THE ECMWF WELL TO
THE LEFT OF THE CURRENT FORECAST.  USERS ARE CAUTIONED THAT FIVE-DAY
TRACK ERRORS AVERAGE ABOUT 225 N MI.
 
ISAAC CONTINUES TO EXPERIENCE SOME LIGHT NORTHEASTERLY VERTICAL WIND
SHEAR...AND IT MAY NOT YET HAVE FULLY MIXED OUT THE DRY AIR THAT
HAS BEEN AFFECTING IT.  THE SHEAR IS FORECAST TO DIMINISH IN 12-24
HR...WHICH SHOULD ALLOW SOME STRENGTHENING.  THAT BEING SAID...THE
GFDL AND HWRF MODELS HAVE BACKED OFF ON THEIR FORECASTS OF
INTENSIFICATION...AND THE SHIPS AND LGEM MODELS ARE ALSO LESS
BULLISH.  BASED ON THIS...THE NEW INTENSITY FORECAST SHOWS A LITTLE
LESS STRENGTHENING THAN THE PREVIOUS ADVISORY.  THE INTENSITY
FORECAST AFTER 72 HR HAS AGAIN BEEN REDUCED DUE TO THE FORECAST
TRACK SHOWING INCREASED LAND INTERACTION WITH HISPANIOLA AND CUBA.
 
 
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  21/2100Z 15.4N  53.9W   35 KT  40 MPH
 12H  22/0600Z 15.6N  56.4W   40 KT  45 MPH
 24H  22/1800Z 15.9N  59.8W   45 KT  50 MPH
 36H  23/0600Z 16.3N  63.1W   50 KT  60 MPH
 48H  23/1800Z 16.6N  66.3W   65 KT  75 MPH
 72H  24/1800Z 17.0N  71.0W   75 KT  85 MPH
 96H  25/1800Z 19.5N  75.0W   75 KT  85 MPH
120H  26/1800Z 21.5N  78.0W   65 KT  75 MPH...INLAND
 
$$
FORECASTER BEVEN
 
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