Tropical Storm ISAAC

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TROPICAL STORM ISAAC DISCUSSION NUMBER  26
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL092012
1000 AM CDT MON AUG 27 2012
 
DATA FROM THE TAMPA AND KEY WEST DOPPLER RADARS...AND FROM AIR FORCE
RESERVE AND NOAA RECONNAISSANCE AIRCRAFT DATA INDICATE THE INNER
CORE STRUCTURE AND INTENSITY OF ISAAC HAS CHANGED LITTLE SINCE THE
PREVIOUS ADVISORY. A RAGGED EYE FEATURE HAS OCCASIONALLY APPEARED
IN THESE DATA...BUT IT HAS BEEN UNABLE TO PERSIST. THE CENTRAL
PRESSURE HAS DECREASED TO AROUND 987-988 MB...BUT THIS HAS NOT
RESULTED IN ANY INCREASE IN THE WIND SPEEDS. THE PREVIOUS INTENSITY
OF 55 KT IS BEING MAINTAINED FOR THIS ADVISORY BASED ON THE RECON
WIND DATA.

THE INITIAL MOTION ESTIMATE IS 305/12. NO SIGNIFICANT CHANGE WAS
MADE TO THE PREVIOUS FORECAST TRACK. THE LATEST 12Z UPPER-AIR DATA
INDICATE 20-40 METER HEIGHT RISES AT THE 500 MB AND 400 MB LEVELS
ACROSS THE SOUTHEASTERN AND SOUTH-CENTRAL U.S....WHICH SUGGESTS
THAT THE SUBTROPICAL RIDGE TO THE NORTH OF ISAAC HAS STRENGTHENED
SLIGHTLY. THIS WOULD SUPPORT A NORTHWESTWARD TO WEST-NORTHWESTWARD
MOTION UNTIL LANDFALL IN ABOUT 36-48 HOURS...AND THIS MOTION IS
SUPPORTED BY THE NHC MODEL CONSENSUS. AFTER 48 HOURS...HOWEVER...
THE MODEL GUIDANCE IS WIDELY DIVERGENT WITH A SPREAD OF MORE THAN
500 N MI BETWEEN THE GFS ON THE LEFT AND THE ECMWF ON THE RIGHT.
THE FORECAST TRACK AT DAYS 3-5 IS ALSO SIMILAR TO THE PREVIOUS
ADVISORY AND LIES CLOSE TO THE MODEL CONSENSUS...BUT THIS IS A LOW
CONFIDENCE FORECAST AT THOSE TIME PERIODS.

THE LACK OF STRENGTHENING APPEARS TO TO BE DUE TO THE
LARGER-THAN-AVERAGE WIND RADII...WHICH HAVE YET TO CONTRACT
INWARD...ALONG WITH THE PERIODIC ENTRAINMENT OF DRY MID-LEVEL AIR
FROM THE EASTERN SEMICIRCLE INTO THE INNER-CORE REGION. UNTIL THE
DRY AIR MIXES OUT...EROSION OF THE INNER-CORE CONVECTION WILL
LIKELY CONTINUE UNTIL ISAAC APPROACHES THE NORTH-CENTRAL GULF COAST
IN ABOUT 24 HOURS. AT THAT TIME...INCREASED FRICTIONAL EFFECTS FROM
THE GULF COAST LAND MASS IS EXPECTED TO CONTRIBUTE TO AN INCREASE
IN CONVECTION IN THE NORTHEAST QUADRANT...WHICH COULD ENABLE THE
STRENGTHENING PROCESS. THE OFFICIAL INTENSITY FORECAST IS SIMILAR
TO THE PREVIOUS ADVISORY AND IS SLIGHTLY ABOVE THE INTENSITY
CONSENSUS MODEL IV15.
 
BECAUSE SIGNIFICANT HAZARDS EXTEND WELL AWAY FROM THE CENTER...IT IS
IMPORTANT NOT TO FOCUS ON THE EXACT FORECAST TRACK.
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  27/1500Z 26.1N  85.3W   55 KT  65 MPH
 12H  28/0000Z 27.1N  86.8W   65 KT  75 MPH
 24H  28/1200Z 28.3N  88.4W   70 KT  80 MPH
 36H  29/0000Z 29.3N  89.6W   80 KT  90 MPH
 48H  29/1200Z 30.2N  90.5W   70 KT  80 MPH...INLAND
 72H  30/1200Z 31.9N  91.7W   40 KT  45 MPH...INLAND
 96H  31/1200Z 35.2N  92.3W   25 KT  30 MPH...INLAND
120H  01/1200Z 39.3N  91.3W   20 KT  25 MPH...INLAND
 
$$
FORECASTER STEWART
 
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