Tropical Storm ISAAC

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TROPICAL STORM ISAAC DISCUSSION NUMBER   4
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL092006
1100 AM EDT THU SEP 28 2006
 
ALTHOUGH THE CURRENT SATELLITE APPEARANCE IS A LITTLE SKELETAL... A
QUIKSCAT PASS NEAR 0845 UTC PROVIDES STRONG EVIDENCE OF AT LEAST 35
KT WINDS IN THE SYSTEM. THUS THE NINTH TROPICAL STORM OF THE
YEAR... ISAAC... IS BORN. THE STORM IS ALSO EXHIBITING SOME
SUBTROPICAL CHARACTERISTICS WITH A RADIUS OF MAXIMUM WINDS LARGER
THAN AVERAGE NEAR 75 NM... AND ALSO AN UPPER LOW IS NEARLY ON TOP
OF THE CYCLONE. FURTHER INTENSIFICATION IS POSSIBLE FOR THE NEXT
COUPLE OF DAYS AS VERTICAL WIND SHEAR REMAINS RELATIVELY LIGHT WITH
UPPER RIDGING NEAR THE STORM. ALTHOUGH SSTS ARE HOVERING NEAR 26C
IN THE PATH OF ISAAC... COLDER-THAN-AVERAGE UPPER-LEVEL
TEMPERATURES ARE ALSO FORECAST... WHICH WOULD ALLOW FOR MORE
INSTABILITY AND MORE CONVECTION. THE INTENSITY FORECAST IS A LITTLE
HIGHER THAN SHIPS AND NOT TOO DISSIMILAR FROM THE FSU
SUPERENSEMBLE.  
 
THE INITIAL MOTION IS 320/7... A LITTLE TO THE RIGHT AND SLOWER THAN
EARLIER. A GENERAL NORTHWESTWARD MOTION SHOULD CONTINUE FOR THE
NEXT 36 HOURS OR SO AS THE STORM IS STEERED BY MIDDLE-LEVEL RIDGING
OVER THE CENTRAL ATLANTIC. THE RIDGE SHOULD STRENGTHEN SOMEWHAT AS
WELL DURING THIS TIME AND COULD PUSH THE SYSTEM A LITTLE MORE TO
THE LEFT THAN THE CURRENT MOTION. THEREAFTER... AN ENORMOUS TROUGH
CURRENTLY OVER THE EASTERN UNITED STATES SHOULD MOVE INTO THE
WESTERN ATLANTIC AND STEER THE STORM AWAY FROM BERMUDA AND NORTH
AMERICA. HOWEVER... MODEL GUIDANCE IS GENERALLY SUGGESTING THAT THE
UPPER RIDGING WILL HOLD A LITTLE LONGER THAN PREVIOUSLY EXPECTED...
AND THE TRACK HAS BEEN SHIFTED WESTWARD IN AGREEMENT WITH NHC
GUIDANCE. THE OFFICIAL FORECAST IS ON THE EASTERN SIDE OF THE MODEL
ENVELOPE... BETWEEN THE UKMET AND THE FSU SUPERENSEMBLE. ISAAC
SHOULD BECOME EXTRATROPICAL IN 3-4 DAYS DUE TO IT PASSING NORTH OF
THE GULF STREAM AND INCREASING VERTICAL WIND SHEAR.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      28/1500Z 28.2N  54.7W    35 KT
 12HR VT     29/0000Z 28.9N  55.4W    40 KT
 24HR VT     29/1200Z 29.9N  56.6W    45 KT
 36HR VT     30/0000Z 30.9N  57.8W    50 KT
 48HR VT     30/1200Z 32.2N  58.8W    50 KT
 72HR VT     01/1200Z 35.5N  59.5W    50 KT
 96HR VT     02/1200Z 41.5N  56.0W    50 KT...EXTRATROPICAL
120HR VT     03/1200Z 47.5N  46.5W    50 KT...EXTRATROPICAL
 
$$
FORECASTER BLAKE/STEWART
 
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