Hurricane HERNAN

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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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