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Subject ISAAC now Following Katrina's Path. Steering Currents being Modified to Stall over BP Oil Spill Area.

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Original Message [link to www.nhc.noaa.gov]

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.

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