WDPN31 PGTW 301500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 17W (SANVU) WARNING NR 
09//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 17W (SANVU), LOCATED APPROXIMATELY 48 NM 
SOUTHWEST OF CHICHI JIMA, HAS TRACKED WESTWARD AT 12 KNOTS OVER THE 
PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY DEPICTS 
IMPROVED DEEP CONVECTIVE BANDING WRAPPING INTO THE SOUTH QUADRANT OF 
THE LOW-LEVEL CIRCULATION CENTER. A 301124Z METOP-A 89GHZ COLOR 
COMPOSITE IMAGE INDICATES TIGHTLY-CURVED SHALLOW BANDING WRAPPING 
INTO A WELL-DEFINED CENTER WITH DEEP CONVECTION OVER THE SOUTHWEST 
SEMI-CIRCLE. UPPER-LEVEL ANALYSIS NOW INDICATES AN IMPROVED 
ENVIRONMENT WITH GOOD OUTFLOW OVER ALL QUADRANTS EXCEPT THE SOUTH 
QUADRANT DUE TO CONVERGENT FLOW ASSOCIATED WITH A WEAKENING UPPER-
LOW TO THE SOUTH. THE INITIAL POSITION IS BASED ON THE METOP-A IMAGE 
AND A 301219Z ASCAT AMBIGUITY ANALYSIS WITH EXCELLENT CONFIDENCE. 
ADDITIONALLY, SURFACE OBSERVATIONS FROM CHICHI-JIMA NOW SHOW 
SOUTHEASTERLY SUSTAINED 10-MINUTE AVERAGE WINDS OF 36 KNOTS 
(MINIMUM SLP THUS FAR OF 973.9MB). THE INITIAL INTENSITY REMAINS 
ASSESSED AT 55 KNOTS BASED ON DVORAK CURRENT INTENSITY ESTIMATES OF 
3.5 (55 KNOTS) FROM ALL AGENCIES AND A 30/0858Z SATCON ESTIMATE OF 
56 KNOTS. TS 17W IS TRACKING UNDER THE STEERING INFLUENCE OF AN 
EXTENSION OF THE SUBTROPICAL RIDGE (STR) TO THE NORTH. 
3. FORECAST REASONING.
   A. NO SIGNIFICANT CHANGE TO THE FORECAST PHILOSOPHY FROM THE 
PREVIOUS PROGNOSTIC REASONING MESSAGE. THE 34-KNOT WIND RADII HAVE 
BEEN ADJUSTED BASED ON A DETAILED ANALYSIS OF THE AFOREMENTIONED 
ASCAT IMAGE. 
   B. THE STR IS FORECAST TO WEAKEN AS A MIDLATITUDE SHORTWAVE 
TROUGH DEEPENS OVER CENTRAL JAPAN BY TAU 24. THEREFORE, AFTER A 
BRIEF PERIOD OF QUASI-STATIONARY MOTION, TS 17W WILL TURN SHARPLY 
POLEWARD AND WILL ACCELERATE NORTH-NORTHEASTWARD. THE SYSTEM IS 
FORECAST TO BEGIN EXTRA-TROPICAL TRANSITION BY TAU 72 AS IT BEGINS 
TO INTERACT WITH THE BAROCLINIC ZONE AND SHORTWAVE TROUGH. DYNAMIC 
MODEL GUIDANCE IS IN GOOD AGREEMENT THROUGH TAU 72 WITH A SPREAD IN 
SOLUTIONS AT TAU 72 OF 170NM.  
   C. AFTER TAU 72, TS SANVU WILL SIGNIFICANTLY ACCELERATE
NORTH-NORTHEASTWARD AS IT BECOMES EMBEDDED WITHIN THE LEADING EDGE 
OF THE MAJOR SHORTWAVE TROUGH AND RAPIDLY GAINS FRONTAL 
CHARACTERISTICS. THE SYSTEM IS FORECAST TO COMPLETE EXTRA-TROPICAL 
TRANSITION BY TAU 96 BUT IS EXPECTED TO REMAIN A STRONG COLD-CORE 
LOW WITH EXPANSIVE GALE-FORCE WINDS. DYNAMIC MODEL GUIDANCE REMAINS 
IN GOOD AGREEMENT, THEREFORE, THERE IS HIGH CONFIDENCE IN THE JTWC 
FORECAST TRACK.//
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