TROPICAL STORM ISAAC DISCUSSION NUMBER  13
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL092012
500 AM EDT FRI AUG 24 2012

DATA FROM THE HURRICANE HUNTER AIRCRAFT INDICATE THAT ISAAC REMAINS
POORLY ORGANIZED. WHILE THE 850-MB FLIGHT LEVEL CENTER WAS WELL
SOUTH OF THE PREVIOUSLY ESTIMATED SURFACE CENTER LOCATION…
DROPSONDE DATA SUGGEST THAT THE SURFACE CENTER REMAINS FARTHER
NORTH. FLIGHT LEVEL AND SFMR WINDS SUPPORT MAINTAINING THE
INTENSITY AT 40 KT FOR THIS ADVISORY…WHILE THE AIRCRAFT FOUND A
MINIMUM PRESSURE OF 1000 MB. GIVEN THE CURRENT LACK OF ORGANIZATION
OF THE INNER CORE…ONLY LIMITED STRENGTHENING IS FORECAST BEFORE
THE CYCLONE BEGINS TO INTERACT WITH HISPANIOLA IN THE NEXT 24
HOURS. AFTER THAT TIME LITTLE CHANGE IS FORECAST AS ISAAC INTERACTS
WITH PORTIONS OF CUBA. LATER IN THE PERIOD…ISAAC SHOULD HAVE AN
OPPORTUNITY TO STRENGTHEN OVER THE STRAITS OF FLORIDA AND THE GULF
OF MEXICO…AND THE NHC FORECAST IS CLOSE TO THE LGEM MODEL AT DAYS
4 AND 5. IT SHOULD BE NOTED THAT THERE IS SIGNIFICANT UNCERTAINTY
IN TROPICAL CYCLONE INTENSITY PREDICTIONS AT 3 TO 5 DAYS.

THE LONG TERM INITIAL MOTION ESTIMATE IS A HIGHLY UNCERTAIN
275/13…AS THE INITIAL POSITION ESTIMATE IS BASED ON A BLEND OF
RECENT FIXES AND CONTINUITY. THE TRACK MODEL GUIDANCE CONTINUES TO
BE IN GENERALLY GOOD AGREEMENT ON THE RESUMPTION OF A WEST-
NORTHWESTWARD AND THEN NORTHWESTWARD MOTION IN THE NEXT 24 HOURS AS
THE SUBTROPICAL RIDGE NORTH OF ISAAC WEAKENS. THIS GENERAL MOTION
SHOULD CONTINUE THROUGH DAY 4…WHEN A TURN MORE TOWARD THE NORTH
IS SHOWN AS A SHORTWAVE TROUGH AMPLIFIES OVER THE MISSISSIPPI
VALLEY. LATE IN THE PERIOD THE TRACK MODEL GUIDANCE IS IN BETTER
AGREEMENT ON THIS SCENARIO…AS THE ECMWF HAS TRENDED EASTWARD
TOWARD THE REST OF THE MODELS THIS CYCLE. THE NEW NHC FORECAST HAS
BEEN ADJUSTED ABOUT A DEGREE SOUTHWARD IN THE FIRST 12 HOURS DUE TO
THE INITIAL POSITION AND MOTION…AND IS THEN BLENDED BACK TOWARD
THE PREVIOUS TRACK BY 48 HOURS. AFTER THAT TIME…THE NHC FORECAST
IS ESSENTIALLY AN UPDATE OF THE PREVIOUS ONE…AND IS BETWEEN THE
ECMWF ON THE LEFT AND THE GFS AND TVCA CONSENSUS ON THE RIGHT.

IT IS IMPORTANT NOT TO FOCUS ON THE EXACT TRACK DUE TO UNCERTAINTIES
IN THE INITIAL LOCATION AND THE TRACK FORECAST…AND THE FACT THAT
ISAAC HAS A LARGE AREA OF TROPICAL STORM FORCE WINDS ASSOCIATED
WITH IT.