384 
 WTNT44 KNHC 270859
 TCDAT4
  
 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. 
  
 FORECAST POSITIONS AND MAX WINDS
  
 INIT  27/0900Z 25.2N  84.2W   55 KT  65 MPH
  12H  27/1800Z 26.2N  85.9W   60 KT  70 MPH
  24H  28/0600Z 27.6N  87.8W   70 KT  80 MPH
  36H  28/1800Z 28.8N  89.1W   75 KT  85 MPH
  48H  29/0600Z 29.7N  90.1W   80 KT  90 MPH...INLAND
  72H  30/0600Z 31.2N  91.1W   50 KT  60 MPH...INLAND
  96H  31/0600Z 34.0N  92.0W   30 KT  35 MPH...INLAND
 120H  01/0600Z 38.0N  91.5W   20 KT  25 MPH...INLAND
  
 $$
 FORECASTER BRENNAN