000
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