000
WTNT44 KNHC 260900
TCDAT4

TROPICAL STORM ISAAC DISCUSSION NUMBER  21
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL092012
500 AM EDT SUN AUG 26 2012

WHILE THE SATELLITE PRESENTATION OF ISAAC REMAINS SOMEWHAT
UNIMPRESSIVE...WITH DEEP CONVECTION LIMITED TO A RATHER SMALL AREA
NORTH AND EAST OF THE CENTER...AIRCRAFT DATA SUGGEST THAT THE
CYCLONE HAS STRENGTHENED A LITTLE OVERNIGHT. PEAK 850-MB
FLIGHT-LEVEL WINDS OF 69 KT IN THE NORTHEAST QUADRANT ADJUST TO AN
INTENSITY OF 55 KT...WHICH IS THE VALUE FOR THIS ADVISORY. AIRCRAFT
DATA INDICATE THAT THE MINIMUM PRESSURE HAS ALSO DROPPED A LITTLE...
TO 995 MB. ISAAC STILL APPEARS TO BE BATTLING THE EFFECTS OF SOME
LOW- TO MID-LEVEL DRY AIR IN THE WESTERN SIDE OF THE CIRCULATION...
AS SEEN IN UPPER-AIR DATA FROM MIAMI AND KEY WEST. ASIDE FROM
THIS...HOWEVER...CONDITIONS APPEAR FAVORABLE FOR STEADY
INTENSIFICATION AS THE CYCLONE MOVE ACROSS THE FLORIDA STRAITS AND
INTO THE EASTERN GULF OF MEXICO. WATERS ALONG THE FORECAST TRACK ARE
VERY WARM...AND UPPER-LEVEL WINDS ARE FORECAST BY THE GLOBAL MODELS
TO BECOME CONDUCIVE FOR STRENGTHENING. THE NEW NHC FORECAST IS
SIMILAR TO THE PREVIOUS ONE...BUT SHOWS A LITTLE SLOWER RATE OF
STRENGTHENING FOR THE FIRST COUPLE OF DAYS...TRENDING TOWARD THE
LATEST CYCLE OF GUIDANCE.

THE INITIAL MOTION ESTIMATE IS 305/16...A LITTLE FASTER THAN BEFORE.
AS THE UPPER-LOW TO THE WEST OF ISAAC RETREATS WESTWARD AND THE
CLOSED LOW OVER THE MID-ATLANTIC STATES LIFTS NORTHWARD...ISAAC
WILL MOVE GENERALLY WEST-NORTHWESTWARD OR NORTHWESTWARD AROUND THE
WESTERN PERIPHERY OF THE ATLANTIC SUBTROPICAL RIDGE FOR THE NEXT 48
HOURS. BEYOND THAT TIME...THERE IS CONSIDERABLE SPREAD IN THE MODEL
GUIDANCE. MOST OF THE MODELS...WITH THE EXCEPTION OF THE UKMET...NO
LONGER SHOW ISAAC RECURVING AHEAD OF A SHORTWAVE TROUGH MOVING
EASTWARD ACROSS THE EASTERN UNITED STATES. ONE CAMP OF GUIDANCE...
INCLUDING THE GFS...HWRF...AND GFS ENSEMBLE MEAN SHOW A
NORTHWESTWARD TRACK TOWARD THE CENTRAL GULF COAST AS THE SHORTWAVE
BYPASSES ISAAC COMPLETELY. THE ECMWF AND GFDL SHOW HAVE MORE OF A
WEAKNESS IN THE RIDGE AND SHOW ISAAC TURNING NORTHWARD TOWARD THE
EASTERN GULF COAST. THE NEW NHC FORECAST IS FASTER THAN THE
PREVIOUS ONE THROUGH MUCH OF THE PERIOD...BUT IS ALONG THE PREVIOUS
TRACK THROUGH 36 HOURS. AFTER THAT TIME...A SIGNIFICANT WESTWARD
ADJUSTMENT WAS MADE...AND THE NEW NHC TRACK IS CLOSE TO THE TVCA
MULTI-MODEL CONSENSUS AND THE ECMWF...SHOWING A NORTH-NORTHWESTWARD
AND NORTHWARD TURN AT DAYS 3 THROUGH 5. GIVEN THE LARGE SPREAD IN
THE GUIDANCE...CONFIDENCE IN THE LONG-RANGE TRACK FORECAST IS QUITE
LOW...AND IT IS TOO EARLY TO DETERMINE EXACTLY WHERE AND WHEN ISAAC
WILL MAKE LANDFALL ALONG THE GULF COAST. FURTHER ADJUSTMENTS TO THE
TRACK MAY BE NECESSARY LATER TODAY.

THROUGHOUT THE PERIOD...IT IS IMPORTANT NOT TO FOCUS ON THE EXACT
FORECAST TRACK SINCE SIGNIFICANT HAZARDS EXTEND WELL AWAY FROM THE
CENTER.

GIVEN THE NEW NHC FORECAST...A HURRICANE WATCH HAS BEEN ISSUED FOR A
PORTION OF THE NORTHERN GULF COAST.

FORECAST POSITIONS AND MAX WINDS

INIT  26/0900Z 23.1N  79.0W   55 KT  65 MPH
 12H  26/1800Z 24.2N  80.8W   60 KT  70 MPH
 24H  27/0600Z 25.5N  83.1W   65 KT  75 MPH
 36H  27/1800Z 26.9N  84.9W   70 KT  80 MPH
 48H  28/0600Z 28.1N  86.5W   80 KT  90 MPH
 72H  29/0600Z 30.2N  88.0W   90 KT 105 MPH
 96H  30/0600Z 32.0N  88.5W   40 KT  45 MPH...INLAND
120H  31/0600Z 34.0N  88.5W   25 KT  30 MPH...INLAND

$$
FORECASTER BRENNAN