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Tropical Storm ISAAC (Text)

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TROPICAL STORM ISAAC DISCUSSION NUMBER  19
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL092012
500 PM EDT SAT AUG 25 2012

THE CENTER OF ISAAC IS CURRENTLY MOVING ALONG THE NORTH COAST OF
EASTERN CUBA.  THE SATELLITE APPEARANCE IS SOMEWHAT DISORGANIZED...
WITH THE MAIN CONVECTIVE MASS 50 N MI OR MORE EAST OF THE LOW-LEVEL
CENTER.  AN AIR FORCE RESERVE HURRICANE HUNTER AIRCRAFT HAS
REPORTED 850 MB FLIGHT-LEVEL WINDS OF 56 KT TO THE NORTHEAST OF THE
CENTER...WITH RELIABLE-LOOKING SFMR SURFACE WIND DATA NEAR 50 KT. 
BASED ON THIS...THE INITIAL INTENSITY REMAINS 50 KT.  THE CENTRAL
PRESSURE OF 997 MB IS BASED ON A COMBINATION OF AIRCRAFT DATA AND
CUBAN SURFACE OBSERVATIONS.

THE INITIAL MOTION IS NOW 310/18.  ISAAC IS CURRENTLY PART OF A
LARGE LOW/MID-LEVEL LOW PRESSURE SYSTEM COVERING THE NORTHWESTERN
CARIBBEAN SEA AND THE ADJACENT LAND AND OCEAN AREAS.  THIS LOW
PRESSURE AREA HAS A SECOND VORTICITY CENTER BETWEEN JAMAICA AND THE
CAYMAN ISLANDS...AND FOR THE NEXT 24-36 HR ISAAC AND THIS CENTER
WILL LIKELY ROTATE AROUND EACH OTHER.  BASED ON THIS EXPECTATION...
THE FORECAST TRACK SHOWS ISAAC MOVING NORTHWESTWARD FOR 12 HR OR
SO...FOLLOWED BY A TURN TOWARD THE WEST-NORTHWEST.  IT IS POSSIBLE
THE STORM COULD MAKE A SHARPER TURN THAN FORECAST...AS THE UKMET IS
SHOWING AN ALMOST DUE WEST MOTION BETWEEN THE FLORIDA KEYS AND CUBA
NEAR THE 36 HR POINT.  THE EARLY PART OF THE FORECAST TRACK IS
SHIFTED A LITTLE TO THE LEFT OF THE PREVIOUS FORECAST...BUT STILL
LIES TO THE RIGHT OF THE CONSENSUS MODELS AND THE BULK OF THE
DYNAMICAL MODELS.  AFTER 36 HR...ISAAC SHOULD BREAK FREE OF OR
ABSORB THE REST OF THE LOW...WITH THE SUBTROPICAL RIDGE TO THE EAST
BECOMING THE DOMINANT STEERING INFLUENCE.  THIS SHOULD ALLOW THE
STORM TO GRADUALLY TURN TOWARD THE NORTHWEST AND NORTH OVER THE
EASTERN AND NORTHERN GULF OF MEXICO...WITH AN EVENTUAL LANDFALL
ALONG THE NORTHERN GULF COAST IN 72-96 HR.  WHERE THIS MIGHT OCCUR
IS STILL RATHER UNCERTAIN...WITH THE TRACK GUIDANCE SPREAD BETWEEN
SOUTHEASTERN LOUISIANA AND THE CENTRAL FLORIDA PANHANDLE.  THIS PART
OF THE TRACK FORECAST LIES IN THE CENTER OF THE GUIDANCE ENVELOPE
NEAR THE CONSENSUS MODELS AND IS SIMILAR TO THE PREVIOUS FORECAST.

A COMBINATION OF LIGHT WESTERLY VERTICAL WIND SHEAR AND LAND
INTERACTION SHOULD AT LEAST SLOW INTENSIFICATION FOR THE NEXT 12
HR.  AFTER THAT...THE SHEAR SHOULD DIMINISH WHILE THE CYCLONE
REACHES OPEN WATER.  THIS SHOULD ALLOW ISAAC TO GRADUALLY STRENGTHEN
INTO A HURRICANE NEAR THE FLORIDA KEYS OR OVER THE SOUTHEASTERN
GULF OF MEXICO.  THE NEW INTENSITY FORECAST IS SIMILAR TO THE
PREVIOUS FORECAST AND IS NEAR OR A LITTLE ABOVE THE INTENSITY
CONSENSUS.  IT SHOULD BE NOTED THAT THE GLOBAL MODELS ARE
FORECASTING A VERY FAVORABLE PATTERN OF UPPER-LEVEL WINDS OVER THE
NORTHERN GULF OF MEXICO BY 72 HR.  SHOULD THIS VERIFY AND SHOULD
ISAAC ESTABLISH AN INNER CORE...THE CYCLONE COULD BE SIGNIFICANTLY
STRONGER THAN CURRENTLY FORECAST.
 
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.

 
FORECAST POSITIONS AND MAX WINDS
 
INIT  25/2100Z 21.3N  76.0W   50 KT  60 MPH
 12H  26/0600Z 22.6N  78.0W   55 KT  65 MPH
 24H  26/1800Z 24.1N  80.6W   60 KT  70 MPH
 36H  27/0600Z 25.2N  82.7W   65 KT  75 MPH
 48H  27/1800Z 26.5N  84.3W   75 KT  85 MPH
 72H  28/1800Z 29.0N  86.0W   85 KT 100 MPH
 96H  29/1800Z 31.5N  86.0W   55 KT  65 MPH...INLAND
120H  30/1800Z 33.5N  85.5W   25 KT  30 MPH...INLAND
 
$$
FORECASTER BEVEN
 
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