456 
 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
  
  
 $$