553 
 WTNT44 KNHC 282030
 TCDAT4
 TROPICAL STORM ISAAC DISCUSSION NUMBER   5
 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL092006
 500 PM EDT THU SEP 28 2006
  
 DURING THE DAY... THE STRUCTURE OF ISAAC HAS BEEN CHANGING. THE
 LARGER RADIUS OF MAXIMUM WINDS APPEARS TO HAVE DECREASED WITH
 SATELLITE IMAGES SHOWING A MORE COMPACT SYSTEM. AFTER STRUGGLING
 MOST OF THE DAY... MODEST CONVECTION HAS BEEN FORMING IN A CURVED
 BAND AROUND THE CENTER. THE CONVECTION HAS NOT BEEN AS DEEP AS
 TYPICALLY SEEN IN A TROPICAL CYCLONE... POSSIBLY DUE TO THE STORM
 MOVING OVER SLIGHTLY COOLER WATER THAT WAS STIRRED UP BY TROPICAL
 CYCLONES GORDON AND HELENE. MORE IMPORTANTLY... DRY AIR IS NOTED ON
 WATER VAPOR PICTURES NEAR THE CORE OF THE CYCLONE... WHICH ALSO MAY
 ACCOUNT FOR THE DESSICATED SATELLITE APPEARANCE. A FURTHER ANALYSIS
 OF THE 0845 UTC QUIKSCAT DATA SUGGESTS THAT WINDS OF 40-45 KT WERE
 PRESENT AT THAT TIME. THE INITIAL INTENSITY IS THEREFORE BUMPED UP
 TO 40 KT TO MATCH THIS ESTIMATE ALONG WITH THE IMPROVED CONVECTIVE
 STRUCTURE.
  
 THE INTENSITY FORECAST IS A BIT TRICKY DUE TO THE SOMEWHAT HYBRID
 NATURE OF ISAAC. FOR THE NEXT DAY OR SO... THE COLD WAKES OF GORDON
 AND HELENE COULD HINDER CONVECTIVE DEVELOPMENT. IN ADDITION...
 SOUTHERLY SHEAR FROM AN UPPER LOW DROPPING TO THE SOUTH OF THE
 ISAAC... ALONG WITH NEARBY DRY AIR IN THE MIDDLE TO UPPER LEVELS...
 SHOULD ALSO HELP TO INHIBIT DEVELOPMENT. HOWEVER BY LATE
 TOMORROW... VERTICAL WIND SHEAR IS FORECAST TO BECOME LIGHTER AS
 ISSAC BECOMES MORE UNDER THE INFLUENCE OF UPPER RIDGING TO THE
 NORTH. THE SHIPS MODEL ALSO DIAGNOSES AN INCREASE IN MIDDLE-LEVEL
 HUMIDITY BEYOND 36 HOURS AND THE STORM IS FORECAST TO MOVE OVER
 SLIGHTLY WARMER WATERS THAT WERE MOSTLY UNTOUCHED BY OTHER TROPICAL
 CYCLONES. THE INTENSITY FORECAST IS REDUCED SOMEWHAT FOR THE FIRST
 24 HOURS... BUT THEN BLENDED WITH THE PREVIOUS FORECAST IN THE
 LATER-TERM.  THIS SOLUTION IS IN REASONABLE AGREEMENT WITH A BLEND
 OF THE SHIPS-GFDL-FSSE (FLORIDA STATE SUPERENSEMBLE) GUIDANCE.
  
 DUE TO THE CYCLONE'S INTERACTION WITH AN UPPER LOW... THE MOTION OF
 ISAAC HAS SHIFTED EVEN FARTHER TO THE RIGHT... AND IS NOW 330/7. AS
 THE UPPER LOW MOVES SOUTHWARD... THE CIRCULATION AROUND THE UPPER
 LOW SHOULD HELP TO PULL THE TROPICAL CYCLONE BACK TO THE LEFT.
 THEREAFTER... MIDDLE LEVEL RIDGING OVER THE CENTRAL ATLANTIC SHOULD
 CAUSE A GENERAL NORTHWESTWARD MOTION UNTIL ABOUT 48 HOURS. A RATHER
 LARGE TROUGH IS FORECAST OVER THE WESTERN ATLANTIC BEYOND THAT
 TIME... WHICH SHOULD STEER ISAAC NORTHWARD THEN ACCELERATE IT
 NORTHEASTWARD. THE FORECAST TRACK HAS BEEN SHIFTED A LITTLE TO THE
 RIGHT EARLY ON TO ACCOUNT FOR SHORT-TERM TRENDS BUT IS SIMILAR TO
 THE PREVIOUS FORECAST IN THE EXTENDED PERIODS. THE OFFICAL FORECAST
 REMAINS JUST TO THE RIGHT OF THE MODEL CONSENSUS.  
   
  
 FORECAST POSITIONS AND MAX WINDS
  
 INITIAL      28/2100Z 28.9N  54.9W    40 KT
  12HR VT     29/0600Z 29.7N  55.6W    40 KT
  24HR VT     29/1800Z 30.5N  57.0W    45 KT
  36HR VT     30/0600Z 31.4N  58.1W    50 KT
  48HR VT     30/1800Z 32.8N  59.2W    50 KT
  72HR VT     01/1800Z 36.5N  60.0W    50 KT...BECOMING EXTRATROPICAL
  96HR VT     02/1800Z 42.0N  56.0W    50 KT...EXTRATROPICAL
 120HR VT     03/1800Z 47.0N  47.0W    50 KT...EXTRATROPICAL
  
 $$
 FORECASTER BLAKE/STEWART