TROPICAL STORM ISAAC DISCUSSION NUMBER  18
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL092012
1100 AM EDT SAT AUG 25 2012

ISAAC HAS BEEN SOMEWHAT DISRUPTED BY ITS PASSAGE ACROSS HAITI.  THE
LAST REPORT FROM AN AIR FORCE RESERVE HURRICANE HUNTER AIRCRAFT
SHOWED THAT THE CENTER PRESSURE HAD RISEN TO 998 MB…AND THAT THE
STRONGEST FLIGHT-LEVEL WINDS WERE 60-70 N MI FROM THE CENTER.  THESE
WINDS SUPPORT AN INITIAL INTENSITY OF 50 KT.  THE CONVECTIVE
PATTERN IS DISORGANIZED…WITH LARGE CONVECTIVE BANDS SOUTH OF
HISPANIOLA AND LITTLE CONVECTION CURRENTLY NEAR THE CENTER.  IT
SHOULD BE NOTED THAT ISAAC IS AT THE EASTERN END OF A LARGE AND
ELONGATED LOW PRESSURE SYSTEM OVER THE NORTHWESTERN CARIBBEAN
SEA…WHICH IS CREATING A LARGE AREA OF STRONG EASTERLY LOW-LEVEL
WINDS WELL TO THE NORTHWEST OF THE CENTER.

THE LOW-LEVEL CENTER HAS JOGGED WESTWARD DURING THE PAST FEW
HOURS…AND THE INITIAL MOTION IS A SOMEWHAT UNCERTAIN 315/15.  IN
THE SHORT-TERM…A MID/UPPER-LEVEL TROUGH OVER THE EASTERN UNITED
STATES IS LIFTING OUT TO THE NORTHEAST AS A UPPER-LEVEL LOW MOVES
SOUTHWARD ACROSS THE CENTRAL GULF OF MEXICO.  THIS EVOLUTION…
COMBINED WITH THE LOW-LEVEL RIDGE NORTH OF ISAAC…SHOULD STEER THE
CYCLONE GENERALLY NORTHWESTWARD NEAR THE NORTH COAST OF CUBA AND THE
FLORIDA KEYS DURING THE NEXT 48 HR AS INDICATED BY THE TIGHTLY-
CLUSTERED MODEL GUIDANCE.  AFTER THAT…THE CYCLONE IS EXPECTED TO
MOVE AROUND THE WESTERN END OF THE SUBTROPICAL RIDGE AND TURN MORE
NORTHWARD OVER THE EASTERN GULF OF MEXICO.  WHILE THE GUIDANCE
GENERALLY AGREES WITH THIS SCENARIO…THERE IS A SPREAD IN
POTENTIAL LANDFALL LOCATIONS BETWEEN THE EASTERN FLORIDA PANHANDLE
AND SOUTHEASTERN LOUISIANA…WHICH LOWERS CONFIDENCE IN THE FINAL
LANDFALL POINT.  THE NEW FORECAST TRACK FOLLOWS THE CENTER OF THE
GUIDANCE ENVELOPE AND THE CONSENSUS MODELS IN CALLING FOR LANDFALL
IN THE WESTERN FLORIDA PANHANDLE BETWEEN 72-96 HR.

LITTLE CHANGE IN STRENGTH IS EXPECTED TODAY DUE TO A COMBINATION OF
LAND INTERACTION…SOME WESTERLY SHEAR…AND THE CURRENT LACK OF
CONVECTION.  THE SHEAR IS FORECAST TO DECREASE AS THE CENTER MOVES
AWAY FROM LAND…WOULD SHOULD ALLOW FOR STRENGTHENING AFTER 12 HR.
THE NEW INTENSITY FORECAST IS SIMILAR TO THE PREVIOUS FORECAST
THROUGH 48 HR…THEN CALLS FOR A LITTLE MORE STRENGTHENING THAN THE
PREVIOUS FORECAST OVER THE NORTHERN GULF.  THE CYCLONE SHOULD
WEAKEN TO A DEPRESSION AFTER LANDFALL ON THE NORTHERN GULF COAST.

THE NEW NHC FORECAST NECESSITATES THE ISSUANCE OF ADDITIONAL
TROPICAL STORM WARNINGS AND WATCHES FOR PORTIONS OF THE FLORIDA
PENINSULA.

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