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