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| #537687 (Received by flhurricane at: 4:59 PM 27.Aug.2012) TCDAT4 TROPICAL STORM ISAAC DISCUSSION NUMBER 27 NWS NATIONAL HURRICANE CENTER MIAMI FL AL092012 400 PM CDT MON AUG 27 2012 THE MUCH ANTICIPATED INTENSIFICATION PROCESS APPEARS TO HAVE FINALLY STARTED BASED ON REPORTS FROM AN AIR FORCE RESERVE RECONNAISSANCE AIRCRAFT AND FROM NEARBY NOAA BUOY 42003. THE CENTRAL PRESSURE HAS DECREASED BY AT LEAST 7 MB SINCE THE PREVIOUS ADVISORY. THE HIGHEST FLIGHT-LEVEL WINDS OBSERVED HAVE BEEN 74 KT IN THE SOUTHWEST QUADRANT AND THE HIGHEST BIAS-ADJUSTED SFMR WINDS ARE NEAR 58 KT. THESE DATA SUPPORT AN INTENSITY OF 60 KT. ISAAC WOBBLED WESTWARD THIS AFTERNOON...BUT A REPRESENTATIVE INITIAL MOTION ESTIMATE IS 305/10. THE LATEST 12Z NHC MODEL GUIDANCE IS IN BETTER AGREEMENT ON A WEST-NORTHWEST TO NORTHWEST MOTION TOWARD THE SOUTHEAST LOUISIANA AND MISSISSIPPI COASTAL AREAS OVER THE NEXT 36-48 HOURS. AFTER 48 HOURS...HOWEVER...THE MODEL GUIDANCE REMAINS DIVERGENT ON THE HANDLING OF A SHORTWAVE TROUGH FORECAST TO DIG SOUTHEASTWARD FROM THE UPPER MIDWEST AND INTO THE MID-ATLANTIC REGION...FOLLOWED BY A RIDGE BUILDING IN BEHIND THE TROUGH. THE GFS AND ECMWF...OUR MOST RELIABLE MODELS... NOW HAVE VERY SIMILAR SOLUTIONS THROUGH DAY 5...AND THE OFFICIAL FORECAST TRACK IS BETWEEN THOSE TWO MODELS. SATELLITE DATA INDICATE ISAAC HAS BECOME BETTER ORGANIZED OVER THE PAST 6 HOURS WITH THE DEVELOPMENT OF A CENTRAL DENSE OVERCAST CLOUD PATTERN OVER THE CENTER ALONG WITH A DEVELOPING EYE IN RECON DATA. SATELLITE DATA T-NUMBERS HAVE INCREASED TO T4.5/77 KT FROM TAFB AND SAB. RECON WIND DATA INDICATE THE RADIUS OF MAXIMUM WINDS HAS ALSO PULLED INWARD TO ABOUT 30 N MI. ALTHOUGH ADDITIONAL CONVECTION HAS BEEN DEVELOPING IN THE NORTHEASTERN QUADRANT...DRY MID-LEVEL AIR STILL LURKS JUST TO THE EAST AND SOUTHEAST OF THE CDO FEATURE... WHICH COULD BE ENTRAINED INTO THE INNER CORE AT ANY TIME AND DISRUPT THE DEVELOPMENT PROCESS. DUE TO THE PROXIMITY OF DRY AIR...THE INTENSITY FORECAST INDICATES ONLY MODEST DEVELOPMENT FOR THE NEXT 24 HOURS. HOWEVER...STRENGTHENING IS EXPECTED TO CONTINUE RIGHT UP UNTIL LANDFALL OCCURS DUE TO THE VERY WARM WATERS THE CYCLONE WILL BE PASSING OVER AND THE VERY FAVORABLE UPPER-LEVEL OUTFLOW REGIME THAT ISAAC IS FORECAST TO DEVELOP BY 24 HOURS. THE INTENSITY FORECAST IS JUST SLIGHTLY HIGHER THAN THE MODEL GUIDANCE. BECAUSE SIGNIFICANT HAZARDS EXTEND WELL AWAY FROM THE CENTER...IT IS IMPORTANT NOT TO FOCUS ON THE EXACT FORECAST TRACK. THE SLOW FORWARD SPED OF AROUND 5 KT AFTER LANDFALL WILL RESULT IN A HEAVY RAIN AND FLOOD SITUATION FOR PORTIONS OF THE CENTRAL GULF COAST AND LOWER MISSISSIPPI VALLEY REGIONS. FORECAST POSITIONS AND MAX WINDS INIT 27/2100Z 26.4N 86.2W 60 KT 70 MPH 12H 28/0600Z 27.4N 87.5W 70 KT 80 MPH 24H 28/1800Z 28.6N 89.0W 80 KT 90 MPH 36H 29/0600Z 29.5N 90.0W 85 KT 100 MPH...NEAR THE COAST 48H 29/1800Z 30.3N 90.7W 75 KT 85 MPH...INLAND 72H 30/1800Z 32.2N 91.8W 35 KT 40 MPH...INLAND 96H 31/1800Z 35.9N 92.1W 25 KT 30 MPH...INLAND 120H 01/1800Z 38.7N 89.3W 20 KT 25 MPH...INLAND $$ FORECASTER STEWART |
