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| #537552 (Received by flhurricane at: 10:59 AM 27.Aug.2012) TCDAT4 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 |
