8:00 AM EDT Mon Aug 27 Location: 25.7°N 84.7°W Moving: WNW at 14 mph

Min pressure: 988 mb Max sustained: 65 mph

TROPICAL STORM ISAAC DISCUSSION NUMBER  25

NWS NATIONAL HURRICANE CENTER MIAMI FL       AL092012

500 AM EDT MON AUG 27 2012

THE LAST AIRCRAFT FIX BEFORE 06Z SUGGESTED THAT THE CENTER OF ISAAC HAD REFORMED A LITTLE CLOSER TO THE DEEP CONVECTION…AND THAT THE PRESSURE HAD FALLEN TO 990 MB. HOWEVER…NO STRONGER WINDS WERE OBSERVED BY THE AIRCRAFT…AND THE INITIAL INTENSITY IS HELD AT 55

KT FOR THIS ADVISORY. THE CONTINUED LACK OF INNER CORE STRUCTURE… THE LARGE CIRCULATION…AND SOUTHEASTERLY SHEAR NEAR THE OUTFLOW LAYER DUE TO AN UPPER-LEVEL LOW SOUTHWEST OF ISSAC SHOULD RESULT IN ONLY GRADUAL STRENGTHENING FOR THE FIRST 12 HOURS OR SO. AFTER THAT TIME…STEADIER STRENGTHENING IS EXPECTED AS THE CIRCULATION CONSOLIDATES AND AS THE UPPER LOW MOVES AWAY FROM ISAAC. THE NEW NHC FORECAST IS A LITTLE LOWER THAN THE PREVIOUS ONE…AND IS CLOSE TO OR A BIT ABOVE THE INTENSITY CONSENSUS UNTIL LANDFALL IN ABOUT 48 HOURS.

RECENT AIRCRAFT FIXES SHOW THE CENTER IS NOW LOCATED CLOSER TO THE DEEP CONVECTION. THE INITIAL MOTION ESTIMATE IS 300/12…AS ISAAC IS SITUATED ON THE SOUTHWESTERN PERIPHERY OF THE ATLANTIC SUBTROPICAL RIDGE. A DECREASE IN FORWARD SPEED AND A TURN TOWARD THE NORTHWEST ARE EXPECTED IN THE NEXT DAY OR SO…AS ISAAC MOVES INTO A BREAK IN THE RIDGE OVER THE NORTHERN GULF OF MEXICO INDUCED BY A TROUGH MOVING THROUGH THE EASTERN UNITED STATES. THE TRACK MODEL GUIDANCE IS PRETTY TIGHTLY CLUSTERED FOR THE FIRST 36 HOURS OR SO…AND THROUGH THAT TIME THE NEW NHC FORECAST IS A LITTLE TO THE RIGHT OF THE PREVIOUS ONE DUE TO THE INITIAL POSITION AND MOTION. WHILE THE MODEL SPREAD INCREASES AT 48 HOURS AND BEYOND… THE ECMWF AND UKMET HAVE SHIFTED WESTWARD THIS CYCLE AND NOW TAKE ISAAC TOWARD THE MISSISSIPPI COAST. THE GFS…GFS ENSEMBLE MEAN… GFDL …AND HWRF CONTINUE TO SHOW LESS OF A NORTHWARD TURN AND BRING THE CYCLONE INTO SOUTHEASTERN LOUISIANA. THE NEW NHC TRACK FORECAST HAS BEEN ADJUSTED WESTWARD BY ABOUT A DEGREE AT 72 HOURS AND BEYOND AND LIES BETWEEN THE ECMWF AND THE GFS ENSEMBLE MEAN. BECAUSE OF THE LARGE SPREAD IN THE GUIDANCE…THE UNCERTAINTY IN THE TRACK FORECAST CONTINUES TO BE GREATER THAN USUAL AT 48 HOURS AND BEYOND.

BECAUSE OF THIS UNCERTAINTY…AND THE FACT THAT SIGNIFICANT HAZARDS EXTEND WELL AWAY FROM THE CENTER…IT IS IMPORTANT NOT TO FOCUS ON THE EXACT FORECAST TRACK.

GIVEN THE NEW TRACK AND INTENSITY FORECAST…NEW WATCHES AND WARNINGS HAVE BEEN ISSUED FOR PORTIONS OF THE LOUISIANA COAST. THE INITIAL 34-KT WIND RADII HAVE BEEN EXPANDED IN THE SOUTHEAST SEMICIRCLE GIVEN CONTINUING OBSERVATIONS OF TROPICAL STORM CONDITIONS IN THE FLORIDA KEYS.

FORECAST POSITIONS AND MAX WINDS

INIT  27/0900Z 25.2N  84.2W   55 KT  65 MPH

12H  27/1800Z 26.2N  85.9W   60 KT  70 MPH

24H  28/0600Z 27.6N  87.8W   70 KT  80 MPH

36H  28/1800Z 28.8N  89.1W   75 KT  85 MPH

48H  29/0600Z 29.7N  90.1W   80 KT  90 MPH…INLAND

72H  30/0600Z 31.2N  91.1W   50 KT  60 MPH…INLAND

96H  31/0600Z 34.0N  92.0W   30 KT  35 MPH…INLAND

120H  01/0600Z 38.0N  91.5W   20 KT  25 MPH…INLAND