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Tropical Storm ISAAC


ZCZC MIATCDAT4 ALL
TTAA00 KNHC DDHHMM
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
 
NNNN


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Page last modified: Thursday, 28-Sep-2006 20:35:02 UTC