WDPN33 PGTW 231500
MSGID/GENADMIN/NAVPACMETOCCEN PEARL HARBOR HI/JTWC//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 20W
/WARNING NR 17//
RMKS/
1. FOR METEOROLOGISTS.
2. PROGNOSTIC REASONING FOR 231200Z8 TO 261200Z1 AUG 2004.
   A. TYPHOON (TY) 20W (AERE), LOCATED APPROXIMATELY 220 NM
SOUTHWEST OF NAHA, OKINAWA, HAS TRACKED NORTHWESTWARD AT 05
KNOTS OVER THE PAST 06 HOURS. THE WARNING POSITION IS BASED
ON 231130Z0 INFRARED SATELLITE IMAGERY. THE WARNING INTENSITY
IS BASED ON SATELLITE CURRENT INTENSITY ESTIMATES OF 55, 65
AND 75 KNOTS. 
   B. TY 20W IS FORECAST TO CONTINUE TO TRACK NORTHWESTWARD
ALONG THE SOUTHWEST PERIPHERY OF THE MID LEVEL STEERING
RIDGE LOCATED TO THE NORTHEAST. HIGH PRESSURE IS BUILDING IN
FROM CHINA TRANSITIONING TY 20W INTO A WEAK STEERING ENVIRONMENT
CAUSING IT TO SLOW DOWN. A MAJOR SHORTWAVE CURRENTLY PASSING
NORTH OF THE SYSTEM WILL PROPAGATE EASTWARD ALLOWING HIGH
PRESSURE TO BUILD BACK IN FROM THE EAST TOWARDS THE YELLOW SEA.
THIS CHANGE WILL SEND TY 20W INTO A MORE WESTWARD TRACK INTO
NORTHERN TAIWAN. THE AVAILABLE DYNAMICS AIDS CONSISTING OF
NOGAPS, UKMET EGRR, JGSM, JTYM, WBAR, GFDN, TCLAPS, NCEP GFS, AND
COAMPS ARE IN GOOD AGREEMENT WITH THIS SCENARIO; ONLY THE UKMET
KEEPS TY 20W NORTH OF THE TAIWANESE COAST. THIS FORECAST IS BASED
ON A CONSENSUS OF ALL AVAILABLE DYNAMIC AIDS.
   C. TY 20W IS FORECAST TO BRIEFLY INTENSIFY FOR THE NEXT 12 TO
18 HOURS AS THE MAJOR SHORTWAVE ENHANCES POLEWARD OUTFLOW. TY 20W
WILL SIGNIFICANTLY WEAKEN WHEN IT TRANSITS THE HIGHER TERRAIN
OF TAIWAN AND MAY NOT SURVIVE THE PASSAGE. 
   D. CURRENT WIND AND FORECASTED WIND RADII ARE BASED ON A 
CLIMATOLOGY FOR AN AVERAGE TO LARGE SIZED SYSTEM.
   E. AFTER TAU 72, THE REMNANTS OF TY 20W IS FORECAST TO MAKE
LANDFALL INTO EASTERN CHINA AND BEGIN TO RAPIDLY DISSIPATE.
3. FORECAST TEAM: GRUBER/KLINZMAN/HEILER//