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 917 
 WTPZ44 KNHC 102036
 TCDEP4
 HURRICANE HERNAN DISCUSSION NUMBER  17
 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   EP092008
 200 PM PDT SUN AUG 10 2008
  
 A SERIES OF MICROWAVE IMAGES DURING THE PAST 24 HOURS INDICATES THAT
 HERNAN HAS UNDERGONE AN EYEWALL REPLACEMENT...AS THE INNER EYEWALL
 THAT BEGAN TO ERODE AT ABOUT 15Z YESTERDAY IS ESSENTIALLY GONE. 
 HERNAN EXHIBITS A WELL-DEFINED EYE...WITH DIAMETER OF ABOUT 20 N
 MI...SURROUNDED BY CLOUD TOPS THAT ARE STILL QUITE COLD BUT HAVE
 BEEN GRADUALLY WARMING TODAY.  SUBJECTIVE DVORAK INTENSITY
 ESTIMATES AT 18Z WERE A CONSENSUS 77 KT...AND THE ADVISORY
 INTENSITY IS ADJUSTED SLIGHTLY DOWNWARD TO 75 KT.  THE INTENSITY
 FORECAST HINGES PRIMARILY ON WATER TEMPERATURE CHANGES ALONG THE
 FORECAST TRACK...SINCE DYNAMICAL MODELS SHOW WEAK VERTICAL WIND
 SHEAR PERSISTING THROUGH ALMOST THE ENTIRE FIVE-DAY FORECAST
 PERIOD.  HERNAN IS CURRENTLY OVER SEA-SURFACE TEMPERATURES OF ABOUT
 24.5 CELSIUS...AND WHILE THE UNDERLYING WATERS WILL GET COOLER
 DURING THE NEXT 24 HOURS...THEY WILL THEN GET SLIGHTLY WARMER AGAIN
 AND APPROACH 25 CELSIUS AT DAYS 3 THROUGH 5.  AS A RESULT...HERNAN
 IS LIKELY TO CONTINUE TO STEADILY WEAKEN FOR THE NEXT COUPLE OF
 DAYS...AND THEN WEAKEN MORE GRADUALLY.  THE OFFICIAL FORECAST
 CLOSELY FOLLOWS THE SHIPS AND LGEM GUIDANCE IN FORECASTING HERNAN
 TO WEAKEN TO A DEPRESSION AFTER ABOUT 72 HOURS...EVEN THOUGH THE
 GFDL AND HWRF SUGGEST IT COULD STILL BE A MARGINAL TROPICAL STORM
 AT FIVE DAYS.
 
 THE INITIAL MOTION ESTIMATE IS 295/9...BUT THE CYCLONE IS GRADUALLY
 TURNING TO THE LEFT AS SUBTROPICAL RIDGING BUILDS TO ITS NORTHWEST. 
 THE TRACK FORECAST IS FAIRLY STRAIGHTFORWARD...WITH THE MODELS
 AGREEING WELL ON A GENERAL WEST-SOUTHWESTWARD MOTION BEGINNING IN
 ABOUT 48 HOURS.  NEAR THE END OF THE FORECAST PERIOD...HERNAN
 SHOULD BE A MUCH WEAKER SYSTEM MOVING ALONG IN THE LOW- TO
 MID-LEVEL TRADE WIND FLOW.  THE NEW TRACK IS JUST ABOUT IDENTICAL
 TO THE PREVIOUS ONE AND IS CLOSE TO THE MODEL CONSENSUS.
  
  
 FORECAST POSITIONS AND MAX WINDS
  
 INITIAL      10/2100Z 18.2N 127.5W    75 KT
  12HR VT     11/0600Z 18.5N 128.7W    65 KT
  24HR VT     11/1800Z 18.5N 130.2W    55 KT
  36HR VT     12/0600Z 18.3N 131.6W    50 KT
  48HR VT     12/1800Z 17.9N 133.0W    45 KT
  72HR VT     13/1800Z 17.0N 135.5W    35 KT
  96HR VT     14/1800Z 16.5N 138.5W    30 KT
 120HR VT     15/1800Z 16.0N 142.0W    30 KT
  
 $$
 FORECASTER KNABB
  
 
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