WDPN31 PGTW 191500
MSGID/GENADMIN/NAVMARFCSTCEN PEARL HARBOR HI/JTWC//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 15W (CHOI-WAN)/
WARNING NR 31//
RMKS/
1. FOR METEOROLOGISTS.
2. 12 HOUR SUMMARY AND ANALYSIS.
    A. OVER THE PAST 12 HOURS TY 15W HAS STARTED TO RAPIDLY ACCELER-
ATE TO THE NORTHEAST UNDER THE COMBINED STEERING INFLUENCE OF THE
NORTHWESTERN PERIPHERY OF THE SUBTROPICAL RIDGE (STR) AND AN APPRO-
ACHING MID-LATITUDE TROUGH. ANIMATED INFRARED HAS SHOWN A CONTINUAL
DECREASE IN DEEP CONVECTION NEAR THE CENTER OF THE SYSTEM WITH A 
OVERALL DECREASE IN AREA EXPANSE AS WELL. DVORAK INTENSITY ESTIMATES
CONTINUE TO SUPPORT THIS OBSERVED WEAKENING TREND OVER THE PAST 24
HOURS. AN INCREASINGLY UNFAVORABLE ENVIRONMENT WITH INCREASING
VERTICAL WIND SHEAR (VWS), DECREASING SEA SURFACE TEMPERATURES (SST)
HAVE BEEN THE MAJOR FACTORS IN THE RECENT DECLINE IN SYSTEM STRUCTURE
AND INTENSITY. A POLEWARD OUTFLOW CHANNEL HAS BEEN THE KEY FACTOR IN
KEEPING TY 15W FROM WEAKENING FASTER THAN CURRENTLY OBSERVED. THE
CURRENT INTENSITY OF 65 KNOTS IS BASED ON A 4.0/5.0 DVORAK INTENSITY
ESTIMATE FROM PGTW AND THE OBSERVED INCREASE IN CLOUD TOP TEMPERA-
TURES OVER THE PAST 12 HOURS.
    A. THE FORECAST PHILOSOPHY FOR INTENSITY HAS CHANGED SINCE THE
PREVIOUS PROGNOSTIC REASONING BULLETIN, WITH A MORE RAPID DECREASE
IN INTENSITY THAN PREVIOUSLY FORECAST.
    B.  TYPHOON CHOI-WAN WILL TRACK ALONG THE NORTHWESTERN SIDE OF
THE STR AND MAINTAIN A WEAKENING TREND THROUGH THE FORECAST PERIOD.
TY 15W WILL QUICKLY BEGIN TO UNDERGO EXTRATROPICAL TRANSITION (ET),
WITH SOME INDICATIONS ET HAS ALREADY BEGUN. CURRENT MODEL GUIDANCE
INDICATES THAT ET WILL BE COMPLETE BY TAU 36, AND AT THE CURRENT RATE
OF WEAKENING THIS APPEARS TO BE A FAIR ASSESSMENT OF THE TROPICAL
POTENTIAL OF TY 15W. INCREASED INTERACTION WITH THE MID-LATITUDE
TROUGH WILL CONTINUE TO INCREASE THE TRACK SPEED THROUGH THE FORECAST
PERIOD, AND ALONG WITH INCREASING VWS AND DECREASING SST WILL HELP IN
THE RAPID TRANSITION TO AN ET SYSTEM. THE OBJECTIVE AIDS ARE IN GOOD
AGREEMENT, AND THIS FORECAST TRACK FAVORS CONSENSUS, BUT ALONG TRACK
SPEED IS FASTER THAN CONSENSUS TO CORRECT FOR KNOWN BIASES.
FORECAST TEAM: BRAVO.//
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