[Tropical] Tropical Cyclone Discussion

[Nashville EMWIN]

WTNT44 KNHC 090231
TCDAT4
HURRICANE IVAN DISCUSSION NUMBER  27
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
11 PM EDT WED SEP 08 2004

IVAN WENT THROUGH A PERIOD OF RAPID DEEPENING THIS AFTERNOON...WITH
THE CENTRAL PRESSURE FALLING FROM 951 TO 938 MB IN A PERIOD OF
ABOUT 5.5 HOURS.  THE MAXIMUM 700 MB FLIGHT LEVEL-WINDS WERE 131
KT...AND A NORTHEAST EYEWALL DROPSONDE SUGGESTED SURFACE WINDS NEAR
120 KT.  BASED ON THE DROPSONDE AND THE PRESSURE...THE MAXIMUM
INTENSITY IS INCREASED TO 125 KT...WHICH IS IN GOOD AGREEMENT WITH
127 KT SATELLITE INTENSITY ESTIMATES FROM TAFB...SAB...AND AFWA.
THE SATELLITE SIGNATURE OF THE HURRICANE HAS NOT CHANGED
SIGNIFICANTLY SINCE THE HURRICANE HUNTER LEFT...SO IT IS UNKNOWN IF
THE DEEPENING HAS CONTINUED.  THE INITIAL INTENSITY COULD BE A
LITTLE CONSERVATIVE.

IVAN HAS NUDGED A LITTLE TO THE LEFT DURING THE EVENING AND THE
INITIAL MOTION IS NOW 290/15.  THERE IS LITTLE CHANGE TO THE
FORECAST PHILOSOPHY DURING THE FIRST 48 HR...AS IVAN SHOULD
CONTINUE WEST-NORTHWESTWARD TOWARD JAMAICA ON THE SOUTH SIDE OF THE
SUBTROPICAL RIDGE.  THINGS GETS MORE UNCERTAIN AFTER 48 HR...AS THE
RIDGE STARTS TO WEAKEN DUE TO A DIGGING MID/UPPER-LEVEL TROUGH OVER
THE EASTERN UNITED STATES AND AN UPPER-LEVEL LOW MOVING WESTWARD
UNDER THE RIDGE.  THIS COMBINATION SHOULD ALLOW IVAN TO TURN
NORTHWESTWARD AND EVENTUALLY NORTHWARD...WITH THE USUAL QUESTIONS
OF WHERE AND WHEN.  THE GFS...GFDL...NOGAPS...AND FSU SUPERENSEMBLE
HAVE ALL SHIFTED TO THE RIGHT...WITH THE GFS RECURVING IVAN THROUGH
THE BAHAMAS...THE GFDL MOVING IT INTO SOUTHEASTERN FLORIDA...THE
NOGAPS MOVING IT UP THE LENGTH OF THE FLORIDA PENINSULA...AND THE
SUPERENSEMBLE PASSING JUST WEST OF KEY WEST.  GIVEN THE
INCONSISTENCY OF THE MODELS...AND THE LEFTWARD NUDGE THAT OCCURRED
THIS EVENING...THE FORECAST TRACK WILL NOT SHIFT AS FAR EAST AS THE
GUIDANCE AT THIS TIME.  THE NEW TRACK IS SHIFTED A LITTLE WEST OF
THE PREVIOUS TRACK FOR THE FIRST 24-36 HR AND THEN SOMEWHAT TO THE
RIGHT AFTER 72 HR.

THE INTENSITY FORECAST CALLS FOR IVAN TO REACH 135 KT IN 12 HR ON
THE PREMISE THAT THE AFTERNOON STRENGTHENING WILL CONTINUE.  AFTER
THAT...VARIOUS ANALYSES INDICATE THAT IVAN SHOULD PASS OVER COOLER
SEA SURFACE TEMPERATURES AND REDUCED MAXIMUM POTENTIAL INTENSITY
FOR ABOUT 24 HR.  THIS COMBINED WITH AN EXPECTED CONCENTRIC EYEWALL
CYCLE SHOULD CAUSE SOME WEAKENING.  WARMER WATER AWAITS OVER THE
NORTHWESTERN CARIBBEAN AND THE STRAITS OF FLORIDA...WHICH WOULD
ALLOW SOME STRENGTHENING WHEN IVAN IS NOT HITTING LAND OR
UNDERGOING CONCENTRIC EYEWALL CYCLES.  AFTER 96 HR...SOME WESTERLY
SHEAR MAY DEVELOP AND THE INTENSITY FORECAST REFLECTS A SLIGHT
WEAKENING AT 120 HR.  WHILE THE INTENSITY UPS AND DOWNS MAY BE MORE
PRONOUNCED THAN FORECAST HERE...IVAN IS EXPECTED TO REMAIN A MAJOR
HURRICANE THROUGH THE FORECAST PERIOD.

FORECASTER BEVEN

FORECAST POSITIONS AND MAX WINDS

INITIAL      09/0300Z 13.6N  69.1W   125 KT
 12HR VT     09/1200Z 14.5N  71.0W   135 KT
 24HR VT     10/0000Z 15.9N  73.4W   130 KT
 36HR VT     10/1200Z 17.4N  75.6W   125 KT
 48HR VT     11/0000Z 18.6N  77.5W   120 KT...NEAR JAMAICA
 72HR VT     12/0000Z 21.0N  80.0W   125 KT
 96HR VT     13/0000Z 23.5N  82.0W   120 KT
120HR VT     14/0000Z 27.0N  82.5W   115 KT


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