WDPN33 PGTW 081500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 11W (NANGKA) WARNING NR 21//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TYPHOON 11W (NANGKA), LOCATED APPROXIMATELY 195 NM EAST-NORTHEAST 
OF SAIPAN, HAS TRACKED NORTHWESTWARD AT 09 KNOTS OVER THE PAST SIX 
HOURS. ANIMATED ENHANCED INFRARED IMAGERY (EIR) DEPICTS TIGHT SPIRAL 
BANDING WRAPPING INTO A 17 NM DIAMETER CLOUD FILLED EYE. THE INITIAL 
POSITION IS BASED ON THE PGTW FIX AND ERI, WITH GOOD CONFIDENCE. A 
081057Z ASCAT PASS ALSO SUPPORTS THE INITIAL POSITION AND PROVIDES 
GOOD ESTIMATES OF THE WIND STRUCTURE. THE INITIAL INTENSITY IS 
ASSESSED AT 115 KNOTS BASED ON DVORAK CURRENT INTENSITY ESTIMATES 
FROM PGTW AND RJTD. ANIMATED WATER VAPOR IMAGERY INDICATES DEEP 
CONVECTION IS BECOMING MORE SYMMETRIC, AND OUTFLOW IS NOW BEING 
ENHANCED  BY A TUTT CELL AHEAD OF THE SYSTEM, INDICATING THE SYSTEM 
IS LIKELY INTENSIFYING. VERTICAL WIND SHEAR (VWS) IS CURRENTLY LIGHT 
(5-10 KNOTS), AND SSTS WILL REMAIN FAVORABLE THROUGHOUT THE FORECAST 
PERIOD. TY NANGKA IS TRACKING ALONG THE SOUTHERN PERIPHERY OF AN 
EXTENSION OF THE DEEP LAYERED SUB-TROPICAL RIDGE (STR) TO THE 
NORTHEAST.
3. FORECAST REASONING.
   A. THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE 
PREVIOUS PROGNOSTIC REASONING MESSAGE.
   B. TY 11W WILL CONTINUE TO TRACK GENERALLY NORTHWESTWARD THROUGH 
TAU 72 UNDER THE DOMINANT STEERING STR. ROBUST RADIAL OUTFLOW IS 
EXPECTED TO CONTINUE THROUGH TAU 24, ALLOWING TY 11W TO INTENSIFY  
TO A PEAK NEAR 125 KNOTS. TY NANGKA WILL BEGIN TO ENCOUNTER 
INCREASING VWS  AND POORER OUTFLOW CONDITIONS BEGINNING AROUND TAU 
36, AS RIDGING BUILDS IN THE WAKE OF TY 09W. AVAILABLE DETERMINISTIC 
AND ENSEMBLE MODELS HAVE BEEN GRADUALLY FLATTENING THE TRACK OVER 
THE PAST SEVERAL CYCLES AS THE STR STRENGTHENS, BUT REMAIN IN GOOD 
AGREEMENT.
   C. IN THE EXTENDED TAUS, TY 11W WILL BEGIN TO SLOW AND TURN 
POLEWARD IN RESPONSE TO AN APPROACHING MID-LATITUDE TROUGH WHICH 
WILL WEAKEN THE STEERING STR. ADDITIONALLY, RIDGING BUILDING AHEAD 
OF THE SYSTEM IS EXPECTED TO HAMPER OUTFLOW AND SIGNFICANTLY 
INCREASE THE VWS TO NEAR 50 KNOTS BY THE END OF THE FORECAST PERIOD. 
THESE EFFECTS WILL SERVE TO HASTEN A WEAKENING TREND. 
MODELS ARE INCREASINGLY SPREAD AFTER TAU 72, WITH HWRF, EGRR AND 
ACES MAINTAINING A NORTHWESTWARD MOTION THROUGHOUT THE PERIOD, WHILE 
THE REMAINING GUIDANCE HAS VARYING DEGREES OF RECURVATURE. DUE TO 
THE SPREAD IN THE NUMERICAL GUIDANCE SOLUTIONS AND THEIR DEPICTION 
OF THE APPROACHING TROUGH, THERE IS LOW CONFIDENCE IN THIS PORTION 
OF THE JTWC FORECAST.//
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