[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 122100
TCDAT4
HURRICANE IVAN DISCUSSION NUMBER  42
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
5 PM EDT SUN SEP 12 2004

THE INITIAL MOTION ESTIMATE IS 295/09 OVER THE PAST 12 HOURS.  IVAN
HAS WOBBLED A LITTLE LEFT OF TRACK...SO THE FORECAST TRACK WAS
SHIFTED WESTWARD...ESPECIALLY IN THE FIRST 24 HOURS. THE NHC MODEL
CONSENSUS HAS ALSO SHIFTED SLIGHTLY WESTWARD...ALTHOUGH SOME OF THE
INDIVIDUAL MODELS HAVE SWAPPED POSITIONS...WITH THE UKMET SHIFTING
EASTWARD WHILE NOGAPS SHIFTED WESTWARD. THE LATEST 18Z UPPER-AIR
DATA INDICATE A SHARP SHORTWAVE TROUGH OVER THE SOUTH CENTRAL U.S.
HAS CONTINUED TO PUSH SLOWLY SOUTHEASTWARD INTO THE CENTRAL GULF OF
MEXICO. THIS HAS ERODED THE MID-LEVEL RIDGE ALONG 90W LONGITUDE...
AND THE SLOW EASTWARD EROSION SHOULD CONTINUE FOR THE NEXT 24-48
HOURS. THIS SHOULD ALLOW IVAN TO SLOWLY MOVE NORTHWESTWARD TO NEAR
WESTERN CUBA AND THEN TURN MORE NORTHWARD BY 36 HOURS AFTER THE
HURRICANE EMERGES OVER THE SOUTHEASTERN GULF OF MEXICO. THE
FORECAST TRACK IS SIMILAR TO THE GFS AND UKMET SOLUTIONS...AND
REMAINS TO THE RIGHT OF THE NHC MODEL CONSENSUS.

THE CURRENT INTENSITY WAS DECREASED TO 130 KT BASED ON RECENT AIR
FORCE AND NOAA RECON REPORTS. HAVING SAID THAT...A NEW RECON REPORT
INDICATES THE PRESSURE HAS DECREASED 4 MB IN THE PAST 2 HOURS DOWN
TO 916 MB. THE OUTER CONCENTRIC EYEWALL HAS ALSO DECREASED FROM 60
NMI DOWN TO 30 NMI DURING THE PAST 6 HOURS...SO THE RECENT PRESSURE
FALL IS ABOUT ON TARGET FOR THE OUTER EYEWALL TO BECOME STABLE
AROUND 20 NMI. AT THAT POINT...RE-INTENSIFICATION TO CATEGORY 5
STRENGTH MAY BEGIN LIKE IT DID ABOUT THIS TIME YESTERDAY. THERE
WILL CONTINUE TO BE PERIODIC FLUCTUATIONS IN THE INTENSITY...MAINLY
DUE TO ADDITIONAL EYEWALL REPLACEMENT CYCLES. BUT THE LOW SHEAR
CONDITIONS AND THE VERY WARM SSTS AHEAD OF IVAN SHOULD KEEP THE
HURRICANE AT LEAST AT CATEGORY 4 STRENGTH UNTIL THE CENTRAL GULF OF
MEXICO IS REACHED IN 36-48 HOURS. SOME COOL UPWELLING IS EXPECTED
FROM THAT POINT UNTIL U.S. LANDFALL OCCURS...WHICH SHOULD BRING
ABOUT GRADUAL WEAKENING DESPITE THE VERY FAVORABLE UPPER-LEVEL
OUTFLOW PATTERN THAT ALL OF THE GLOBAL MODELS ARE FORECASTING.

BECAUSE STEERING ARE CURRENTLY WEAK AND MAY WEAKEN EVEN MORE...THERE
REMAINS CONSIDERABLE UNCERTAINTY AS TO JUST WHEN AND WHERE...AND AT
WHAT INTENSITY...IVAN WILL MAKE LANDFALL IN THE UNITED STATES IN
THE LONGER TIME PERIODS.

FORECASTER STEWART

FORECAST POSITIONS AND MAX WINDS

INITIAL      12/2100Z 19.3N  82.5W   130 KT
 12HR VT     13/0600Z 19.9N  83.5W   140 KT
 24HR VT     13/1800Z 21.3N  84.7W   145 KT
 36HR VT     14/0600Z 23.1N  85.5W   140 KT
 48HR VT     14/1800Z 25.4N  86.0W   125 KT
 72HR VT     15/1800Z 28.0N  86.0W   110 KT
 96HR VT     16/1800Z 32.5N  85.0W    55 KT...INLAND
120HR VT     17/1800Z 37.5N  81.0W    25 KT...INLAND


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