Tropical Storm NANGKA Advisory gio, 09.07.

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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