Tropical Storm NANGKA Advisory Wed Jul 08

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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