WDPN31 PGTW 171500
MSGID/GENADMIN/NAVMARFCSTCEN PEARL HARBOR HI/JTWC//
SUBJ/PROGNOSTIC REASONING FOR SUPER TYPHOON 15W (CHOI-WAN)/
WARNING NR 23//
RMKS/
1. FOR METEOROLOGISTS.
2. 12 HOUR SUMMARY AND ANALYSIS.
    A. SUPER TYPHOON (STY) 15W (CHOI-WAN) HAS WEAKENED SLIGHTLY BUT
REMAINS A FULLY DEVELOPED CYCLONE WITH A VERY DISTINCT SYMMETRICAL
EYE. ANIMATED WATER VAPOR SATELLITE IMAGERY SHOWS SOME EROSION OF DEEP
CONVECTION TO THE NORTHWEST QUADRANT OF THE SYSTEM. HOWEVER,
ENVIRONMENTAL ANALYSIS INDICATES THE SYSTEM REMAINS IN A CORRIDOR OF
LOW VERTICAL WIND SHEAR AND WARM SEA SURFACE TEMPERATURES. THE CURRENT
INTENSITY AND POSITION ARE BASED ON DVORAK FIXES FROM PGTW, RJTD AND
KNES WITH A HIGH DEGREE OF CONFIDENCE. STY 15W IS TRACKING NORTHWEST-
WARD ALONG THE WESTERN PERIPHERY OF THE SUBTROPICAL STEERING RIDGE
(STR) TO THE EAST.
3. FORECAST REASONING.
    A. THE FORECAST PHILOSOPHY HAS NOT SIGNIFICANTLY CHANGED SINCE THE
PREVIOUS PROGNOSTIC REASONING BULLETIN. HOWEVER, THE EXTENDED TRACK
FORECAST AT AND PAST TAU 72 IS RIGHT OF THE PREVIOUS FORECAST. 
    B. STY 15W WILL CONTINUE TO TRACK ALONG THE WESTERN EDGE OF THE
STR TO THE EAST, CRESTING THE AXIS AND INCREASING IN FORWARD MOTION
BETWEEN TAU 12 AND 24. AFTER TAU 12, THE SYSTEM WILL GRADUALLY WEAKEN
AS THE ENVIRONMENTAL CONDITION DETERIORATES WITH INCREASING VERTICAL
WIND SHEAR. BY TAU 48, THE SYSTEM WILL BEGIN EXTRATROPICAL TRANSITION
AS IT GETS CAUGHT IN THE STRONG UPPER LEVEL WESTERLIES.
    C. AFTER TAU 72, STY CHOI-WAN WILL TRACK ON A MORE EASTWARD
TRAJECTORY AS IT GETS EMBEDDED IN THE BAROCLINIC ZONE AND BECOME FULLY
EXTRATROPICAL BY TAU 96. THE NUMERICAL AIDS ARE OVERALL IN VERY GOOD
AGREEMENT UP TO TAU 72. AT TAU 72, THE MODEL AIDS DIVERGE WITH GFDN
AND NGPS ON THE FAR LEFT AND THE EUROPEAN MODELS - EGRR AND ECMF, ON
THE FAR RIGHT OF THE ENVELOPE. THIS TRACK FORECAST IS IN THE MIDDLE OF
THE PACKING UP TO TAU 48 THEN DIVERGES TO THE RIGHT OF CONSENSUS BY
TAU 72.//
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