000
WTPZ44 KNHC 110231
TCDEP4
HURRICANE HERNAN DISCUSSION NUMBER  18
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   EP092008
800 PM PDT SUN AUG 10 2008
 
SATELLITE IMAGES INDICATE THAT THE EYE OF HERNAN HAS SHRUNK IN
DIAMETER OVER THE PAST FEW HOURS...THOUGH THE EYE HAS ALSO BECOME A
LESS WELL-DEFINED. SINCE DVORAK T-NUMBERS HAVE NOT CHANGED...THE
INITIAL INTENSITY WILL REMAIN 75 KT. A STEADY DEMISE OVER COLD
WATERS FOR HERNAN SEEMS LIKELY FOR THE FORECAST PERIOD. IN ABOUT 24
HOURS...UPPER-LEVEL WINDS ARE FORECAST TO SHIFT FROM LIGHT EASTERLY
TO LIGHT WESTERLY...WHICH MAY HASTEN THE WEAKENING OF THE TROPICAL
CYCLONE. THE INTENSITY FORECAST IS CLOSE TO A CONSENSUS OF THE
LGEM/GFDL MODELS AND THE PREVIOUS FORECAST. ALTHOUGH MOST GUIDANCE
SUGGESTS THE SYSTEM WILL REMAIN A TROPICAL CYCLONE FOR THE NEXT 5
DAYS...IT SEEMS UNLIKELY THAT PERSISTENT CONVECTION WILL CONTINUE
NEAR HERNAN DUE TO A COMBINATION OF COLD WATERS AND INCREASING
SHEAR AT THE END OF THE PERIOD. THUS...THE FORECAST WILL CALL FOR
THE SYSTEM TO DISSIPATE AROUND THE TIME IT REACHES THE CENTRAL
PACIFIC.
 
THE INITIAL MOTION ESTIMATE IS 285/9...AND THE CYCLONE APPEARS TO BE
GRADUALLY TURNING TO THE LEFT DUE TO SUBTROPICAL RIDGING BUILDING
TO ITS NORTHWEST. GLOBAL MODELS ARE IN SURPRISINGLY GOOD AGREEMENT
ON AN ATYPICAL WEST-SOUTHWESTWARD MOTION BEGINNING IN DAY OR TWO.
THE NEW FORECAST TRACK IS JUST A HAIR SOUTH OF THE PREVIOUS
FORECAST AND IS PRETTY CLOSE TO THE 12Z ECMWF MODEL NEAR THE MIDDLE
OF THE TRACK GUIDANCE ENVELOPE.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      11/0300Z 18.4N 128.4W    75 KT
 12HR VT     11/1200Z 18.6N 129.7W    65 KT
 24HR VT     12/0000Z 18.5N 131.1W    55 KT
 36HR VT     12/1200Z 18.2N 132.5W    45 KT
 48HR VT     13/0000Z 17.7N 133.8W    40 KT
 72HR VT     14/0000Z 16.5N 136.5W    30 KT
 96HR VT     15/0000Z 16.0N 139.5W    25 KT...DISSIPATING
120HR VT     16/0000Z 15.5N 143.5W    20 KT...REMNANT LOW
 
$$
FORECASTER BLAKE