000 WTNT44 KNHC 250319 TCDAT4 TROPICAL STORM ISAAC DISCUSSION NUMBER 16 NWS NATIONAL HURRICANE CENTER MIAMI FL AL092012 1100 PM EDT FRI AUG 24 2012 SATELLITE IMAGERY AND RECONNAISSANCE AIRCRAFT DATA INDICATE THAT THE CIRCULATION AND CONVECTIVE ORGANIZATION OF ISAAC HAS CONTINUED TO IMPROVE SINCE THE PREVIOUS ADVISORY. INFRARED AND MICROWAVE SATELLITE IMAGERY SUGGEST THAT AN EYE IS FORMING...AND THIS IS THE BASIS FOR INCREASING THE INTENSITY TO 60 KT. THE INITIAL MOTION ESTIMATE IS 315/12 BASED ON RECON AND MICROWAVE FIXES. ALTHOUGH THE TRACK FORCAST WAS ADJUSTED SLIGHTLY TO THE RIGHT THROUGH 48 HOURS...NO OTHER SIGNIFICANT CHANGES WERE MADE TO THE PREVIOUS ADVISORY TRACK. RECENT UPPER-AIR DATA AND NOAA G-IV AIRCRAFT DROPSONDE DATA INDICATE THE TROUGH THAT HAS CREATED THE BREAK IN THE SUBTROPICAL RIDGE ACROSS FLORIDA HAS SHIFTED TO A POSITION JUST EAST OF THE FLORIDA PENINSULA. THE 18Z GFS MODEL... WHICH SHIFTED TO THE RIGHT OF ITS 12Z TRACK...CAPTURED THIS FARTHER EAST POSITION...WHEREAS THE 18Z UKMET AND 12Z ECMWF KEPT THE TROUGH ACROSS THE WESTERN FLORIDA PENINSULA VALID AT 00Z THIS EVENING. BASED ON THE GFS VERIFYING BETTER THAN THE OTHER MODELS AT 00Z THIS EVENING...THE NHC TRACK HAS BEEN SHIFTED A LITTLE TO THE RIGHT. AFTER 48 HOURS...THE RIDGE IS EXPECTED TO BUILD BACK WESTWARD ACROSS FLORIDA AND KEEPING ISAAC ON A MOSTLY NORTHWESTWARD HEADING ACROSS THE EASTERN AND NORTH-CENTRAL GULF MEXICO. THE OFFICIAL FORECAST TRACK IS CLOSE TO THE CONSENSUS MODELS TVCA AND TV15 THROUGH 48 HOURS...AND THEN IS A LITTLE TO THE RIGHT OF THOSE MODELS AFTER THAT. WITH THE INNER CORE CONVECTION IMPROVING...THERE IS STILL THE POSSIBILITY THAT ISAAC COULD REACH HURRICANE INTENSITY BEFORE REACHING HAITI TONIGHT. THE INTENSITY FROM 12-24 HOURS WILL BE DETERMINED BY THE AMOUNT OF LAND INTERACTION WITH HAITI AND EASTERN CUBA. AFTER THAT...THE SLIGHT EASTWARD SHIFT IN THE FORECAST TRACK IS ENOUGH TO PLACE ISAAC OVER WATER LONGER THAN WHAT WAS INDICATED IN THE PREVIOUS INTENSITY FORECAST REQUIRED AN INCREASE IN THE INTENSITY FORECAST. THE EASTWARD TRACK SHIFT AND A LARGER INITIAL AND FORECAST WIND FIELD TO THE NORTH ARE WHAT NECESSITATED THE CHANGES IN THE WATCHES AND WARNINGS FOR FLORIDA. IT IS IMPORTANT NOT TO FOCUS ON THE EXACT TRACK DUE TO THE UNCERTAINTIES IN THE FORECAST AND THE FACT THAT ISAAC HAS A LARGE AREA OF TROPICAL-STORM-FORCE WINDS ASSOCIATED WITH IT. FORECAST POSITIONS AND MAX WINDS INIT 25/0300Z 17.7N 72.5W 60 KT 70 MPH 12H 25/1200Z 19.4N 74.1W 50 KT 60 MPH 24H 26/0000Z 21.7N 76.7W 50 KT 60 MPH 36H 26/1200Z 23.4N 79.4W 55 KT 65 MPH 48H 27/0000Z 24.9N 81.6W 60 KT 70 MPH 72H 28/0000Z 27.1N 84.6W 70 KT 80 MPH 96H 29/0000Z 30.0N 86.5W 80 KT 90 MPH 120H 30/0000Z 32.5N 86.5W 40 KT 45 MPH...INLAND $$ FORECASTER STEWART