000
WTNT44 KNHC 222039
TCDAT4
 
TROPICAL STORM ISAAC DISCUSSION NUMBER   7
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL092012
500 PM AST WED AUG 22 2012

SATELLITE...AIRCRAFT...RADAR...AND SURFACE OBSERVATIONS INDICATE
THAT ISAAC HAS A BROAD CENTRAL AREA OF LIGHT WINDS WITH MULTIPLE
VORTICITY MAXIMA EMBEDDED.  THE LOWEST PRESSURES ARE IN AN AREA
NEAR GUADELOUPE...WHICH IS USED AS THE CURRENT CENTER SINCE IT HAS
SOME CONTINUITY WITH THE CENTER PREVIOUSLY BEING TRACKED. 
HOWEVER...THE MEAN CENTER IS LIKELY TO THE SOUTHEAST OF THE
ADVISORY POSITION.  SOME ADJUSTMENT OF THE CENTER POSITION MAY BE
NECESSARY WHEN THE CENTRAL CORE AGAIN CONSOLIDATES.  THE INITIAL
INTENSITY IS HELD AT 40 KT...ALTHOUGH THE HURRICANE HUNTER AIRCRAFT
DATA SUGGESTS THIS COULD BE GENEROUS.
 
THE INITIAL MOTION IS NOW A SOMEWHAT UNCERTAIN 280/19.  OTHER THAN
THE CENTER POSITION ISSUES...THERE IS NO SIGNIFICANT CHANGE TO THE
FORECAST REASONING DURING THE FIRST 48 HR OF THE TRACK FORECAST. THE
EAST-WEST ORIENTED SUBTROPICAL RIDGE TO THE NORTH OF ISAAC ALONG
30N LATITUDE IS FORECAST BY ALL OF THE GLOBAL MODELS TO REMAIN
INTACT ACROSS FLORIDA AND THE SOUTHEASTERN U.S. DURING THAT TIME. 
THIS EARLY PART OF THE FORECAST TRACK IS SHIFTED A LITTLE TO THE
SOUTH OF THE PREVIOUS FORECAST BASED ON THE OBSERVED POSITION AND
MOTION.  AFTER 48 HR...THE RIDGE IS EXPECTED TO WEAKEN AS A
MID/UPPER-LEVEL TROUGH MOVES ONTO THE EASTERN UNITED STATES.  THE
GLOBAL MODELS HAVE COME INTO BETTER AGREEMENT SINCE THE LAST
ADVISORY...WITH THE NOGAPS AND CANADIAN MODELS SHIFTING TO THE WEST
AND THE ECMWF TO THE NORTH.  HOWEVER...THE ECMWF CONTINUES TO SHOW
A STRONGER RIDGE...AND THUS SHOWS A MORE WESTERLY MOTION THAN THE
OTHER MODELS.  THE 72-120 HR PORTION OF THE FORECAST TRACK IS
SIMILAR TO THE PREVIOUS FORECAST AND LIES NEAR THE CONSENSUS
MODELS.  THERE REMAINS CONSIDERABLE UNCERTAINTY AS TO WHICH PORTIONS
OF FLORIDA...IF ANY...COULD BE AFFECTED BY ISAAC.

WATER VAPOR IMAGERY AND MICROWAVE TOTAL PRECIPITABLE WATER DATA
CONTINUE TO SHOW DRY AIR TO THE NORTHEAST OF ISAAC...AND A LACK OF
CONVECTION IN THIS AREA SUGGESTS SOME OF THIS AIR IS ENTRAINING
INTO THE CYCLONE AND DISRUPTING THE CONVECTIVE STRUCTURE.  GIVEN
THE CURRENT STRUCTURE...THE NEW INTENSITY FORECAST CALLS FOR LITTLE
CHANGE IN STRENGTH FOR THE NEXT 12 HR...FOLLOWED BY INTENSIFICATION
INTO A HURRICANE BY 48 HR SIMILAR TO THE PREVIOUS ADVISORY.  AFTER
48 HR...THE MAIN FACTOR CONTROLLING THE INTENSITY IS THE AMOUNT OF
LAND INTERACTION.  THIS LOW-CONFIDENCE PART OF THE INTENSITY
FORECAST IS SIMILAR TO THE PREVIOUS ADVISORY BUT ABOVE THE INTENSITY
CONSENSUS.

 
FORECAST POSITIONS AND MAX WINDS
 
INIT  22/2100Z 16.0N  61.2W   40 KT  45 MPH
 12H  23/0600Z 16.3N  63.6W   40 KT  45 MPH
 24H  23/1800Z 16.7N  66.7W   50 KT  60 MPH
 36H  24/0600Z 17.2N  69.4W   60 KT  70 MPH
 48H  24/1800Z 17.9N  71.6W   70 KT  80 MPH...INLAND
 72H  25/1800Z 20.5N  75.5W   60 KT  70 MPH...INLAND
 96H  26/1800Z 23.5N  79.5W   60 KT  70 MPH...OVER WATER
120H  27/1800Z 26.5N  82.5W   70 KT  80 MPH
 
$$
FORECASTER BEVEN