WDPN33 PGTW 090300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 11W (NANGKA) WARNING NR 23//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TYPHOON 11W (NANGKA), LOCATED APPROXIMATELY 125 NM NORTH-
NORTHEAST OF SAIPAN, HAS TRACKED WEST-NORTHWESTWARD AT 10 KNOTS
OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY 
(MSI) DEPICTS TIGHT SPIRAL BANDING WRAPPING INTO A 19 NM DIAMETER 
EYE. THE 082159Z SSMIS MICROWAVE IMAGE REVEALS THAT THE INNER 
EYEWALL HAS SLIGHTLY WEAKENED OVER THE PAST SIX HOURS. THE OUTER 
EYEWALL, WHICH HAS BEEN IN THE PROCESS OF FORMING, APPEARS TO HAVE 
WEAKENED SLIGHTLY AS WELL. THE INITIAL POSITION IS BASED ON THE MSI 
ANIMATION AND PGTW FIX WITH GOOD CONFIDENCE. THE INITIAL INTENSITY 
IS ASSESSED AT 125 KNOTS BASED ON THE STRONG STRUCTURE OF THE STORM 
AS WELL AS CONSISTENT DVORAK CURRENT INTENSITY ESTIMATES OF 6.0 PGTW 
AND RJTD AND 6.5 FROM KNES. ANIMATED WATER VAPOR IMAGERY INDICATES 
THAT THE DEEP CONVECTION REMAINS FAIRLY SYMMETRIC, AND OUTFLOW 
CONTINUES TO BE ENHANCED BY A TUTT CELL AHEAD OF THE SYSTEM. 
VERTICAL WIND SHEAR (VWS) IS LOW TO MODERATE (10-20 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 SLIGHTLY 
INTENSIFY TO A PEAK NEAR 130 KNOTS. IF AN OUTER EYEWALL CONTINUES TO 
FORM, AS INDICATED BY A SERIES OF RECENT MICROWAVE IMAGES, THE 
SYSTEM MAY UNDERGO AN EYEWALL REPLACEMENT CYCLE WHICH MAY PAUSE 
INTENSIFICATION OR EVEN LEAD TO THE ONSET OF AN EARLIER WEAKENING 
PHASE. TY NANGKA WILL BEGIN TO ENCOUNTER INCREASING VWS AND POORER 
OUTFLOW CONDITIONS AFTER TAU 36, AS RIDGING BUILDS IN THE WAKE OF TY 
09W. AVAILABLE DETERMINISTIC AND ENSEMBLE MODELS HAVE BEEN GRADUALLY 
FLATTENING AND SLOWING THE TRACK OVER THE PAST SEVERAL CYCLES AS THE 
STR STRENGTHENS, BUT REMAIN IN GOOD AGREEMENT.
   C. IN THE EXTENDED TAUS, TY 11W WILL 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 SIGNIFICANTLY INCREASE THE VWS BY THE 
END OF THE FORECAST PERIOD. THESE EFFECTS WILL SERVE TO HASTEN A 
WEAKENING TREND. MODELS ARE INCREASINGLY SPREAD AFTER TAU 72, 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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