122 
 WTNT44 KNHC 220249
 TCDAT4
  
 TROPICAL STORM ISAAC DISCUSSION NUMBER   4
 NWS NATIONAL HURRICANE CENTER MIAMI FL       AL092012
 1100 PM AST TUE AUG 21 2012
  
 THERE HAS BEEN NO AERIAL RECONNAISSANCE INTO ISAAC RECENTLY...BUT
 THE CLOUD PATTERN OF THE STORM HAS NOT BECOME SIGNIFICANTLY BETTER
 ORGANIZED OVER THE PAST FEW HOURS.  THE INTENSITY IS HELD AT 35 KT
 WHICH IS IN AGREEMENT WITH A DVORAK INTENSITY ESTIMATE FROM TAFB.
 ANOTHER HURRICANE HUNTER AIRCRAFT IS SCHEDULED TO INVESTIGATE ISAAC
 AT AROUND 0600 UTC.
  
 THERE IS SOME SPREAD IN CENTER FIXES...BUT BASED ON GEOSTATIONARY
 AND MICROWAVE IMAGERY THE INITIAL MOTION REMAINS ABOUT THE
 SAME...275/16.  A WELL-DEFINED MID-TROPOSPHERIC RIDGE TO THE NORTH
 OF THE TROPICAL CYCLONE SHOULD MAINTAIN THE WESTWARD TRACK FOR THE
 NEXT COUPLE OF DAYS.  LATER IN THE FORECAST PERIOD...GLOBAL MODELS
 DEPICT ISAAC NEARING A BREAK IN THE RIDGE NEAR FLORIDA.
 CONSEQUENTLY THE FORWARD SPEED IS LIKELY TO SLOW...AND THE HEADING
 TO GRADUALLY TURN TOWARD THE RIGHT...IN THE LATTER HALF OF THE
 FORECAST PERIOD.  ALTHOUGH THE GFS HAS SHIFTED JUST A BIT TO THE
 NORTH OF ITS PREVIOUS FORECAST...OVERALL THE TRACK GUIDANCE HAS NOT
 CHANGED MUCH SINCE THE PREVIOUS CYCLE.  THE OFFICIAL FORECAST IS
 VERY CLOSE TO THE LATEST HWRF TRACK AND NOT FAR FROM THE DYNAMICAL
 CONSENSUS.
  
 WATER VAPOR IMAGERY SUGGESTS THAT EAST-NORTHEASTERLY SHEAR IS
 CURRENTLY AFFECTING ISAAC...BUT THE DYNAMICAL GUIDANCE PREDICTS A
 DECREASE IN SHEAR WITH UPPER-LEVEL ANTICYCLONIC FLOW OVER THE
 TROPICAL CYCLONE DURING THE NEXT SEVERAL DAYS.  THEREFORE
 STRENGTHENING IS ANTICIPATED...AND ISAAC COULD BECOME A HURRICANE
 WITHIN THE NEXT COUPLE OF DAYS.  LATER ON...THE INTENSITY FORECAST
 IS COMPLICATED BY HOW MUCH THE CORE OF THE SYSTEM INTERACTS WITH
 THE MOUNTAINOUS LAND MASSES OF HISPANIOLA AND EASTERN CUBA.  THE
 OFFICIAL WIND SPEED FORECAST IS FAIRLY CLOSE TO THE STATISTICAL-
 DYNAMICAL GUIDANCE FOR THE FIRST 48 HOURS AND THEN TRIES TO TAKE
 INTO ACCOUNT THE LAND INTERACTION.  NEEDLESS TO SAY...THERE IS
 CONSIDERABLE UNCERTAINTY IN THE INTENSITY OF ISAAC IN THE 3 TO 5
 DAY TIME FRAME.
  
 FORECAST POSITIONS AND MAX WINDS
  
 INIT  22/0300Z 15.6N  55.6W   35 KT  40 MPH
  12H  22/1200Z 15.8N  58.1W   40 KT  45 MPH
  24H  23/0000Z 16.2N  61.2W   45 KT  50 MPH
  36H  23/1200Z 16.7N  64.3W   55 KT  65 MPH
  48H  24/0000Z 17.1N  67.1W   65 KT  75 MPH
  72H  25/0000Z 18.3N  72.0W   65 KT  75 MPH...INLAND
  96H  26/0000Z 20.5N  76.0W   70 KT  80 MPH...INLAND
 120H  27/0000Z 23.0N  79.0W   70 KT  80 MPH...OVER WATER
  
 $$
 FORECASTER PASCH