WDPN32 PGTW 180300
MSGID/GENADMIN/NAVMARFCSTCEN PEARL HARBOR HI/JTWC//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 13W WARNING NR 05//
RMKS/
1. FOR METEOROLOGISTS.
2. 12 HOUR SUMMARY AND ANALYSIS.
A. TROPICAL STORM (TS) 13W HAS CONTINUED TO SLOWLY DEVELOP DISPLAYING 
MARKEDLY IMPROVED ORGANIZATION OF THE LOW-LEVEL CIRCULATION CENTER
(LLCC) AND UPPER-LEVEL EQUATORWARD OUTFLOW OVER THE PAST 12 HOURS.
ANIMATED METSAT IMAGERY REVEALED IMPROVED CONVECTIVE BANDING OVER THE
WESTERN AND SOUTHERN PORTIONS OF THE SYSTEM WHICH CONTINUES TO TRACK
WESTWARD AT 15 KNOTS UNDER THE STEERING INFLUENCE OF THE LOW- TO MID-
LEVEL SUBTROPICAL RIDGE NORTH OF THE STORM. THE AVAILABLE DYNAMIC 
MODEL AIDS ARE COMING INTO BETTER AGREEMENT REGARDING THE TRACK FORE-
CAST, HOWEVER A SIGNIFICANT DIFFERENCE STILL EXISTS IN THE MODEL 
INTENSITY FORECAST WITH NOGAPS REMAINING THE MOST AGGRESSIVE OF ALL
DYNAMIC AIDS.
B. TS 13W IS TRACKING QUICKLY WESTWARD UNDER THE STEERING INFLUENCE
OF THE LOW- TO MID-LEVEL SUBTROPICAL RIDGE (STR) POSITIONED NORTH
OF THE SYSTEM. 500 MB ANALYSIS CONTINUES TO DEPICT A BROAD STR EX-
TENDING WEST INTO SOUTHEASTERN CHINA AND A DEEP MIDLATITUDE SHORTWAVE
TROUGH CURRENTLY NEAR 115E MOVING EASTWARD AT APPROXIMATELY 15 KNOTS.
ANIMATED WATER VAPOR IMAGERY INDICATES VERY GOOD EQUATORWARD OUTFLOW
ASSOCIATED WITH A STRONG DIVERGENT REGION OF EASTERLY FLOW STEMMING 
FROM AN ANTICYCLONE TO THE NORTH. THIS DIVERGENCE APPEARS TO BE EN-
HANCED BY A WEAK TROUGH LOCATED TO THE SOUTHEAST OF THE SYSTEM. POLE-
WARD OUTFLOW HOWEVER, APPEARS LIMITED DUE TO THE LACK OF A SUFFICIENT
EXHAUST MECHANISM NORTH OF THE SYSTEM. THERE IS GOOD CONFIDENCE IN 
THE CURRENT POSITION AND MOTION BASED ON THE IMPROVED LLCC EVIDENT 
IN SATELLITE IMAGERY. THE CURRENT INTENSITY IS BASED ON DVORAK EST-
IMATES OF 55 KNOTS FROM PGTW, A 172208Z AERIAL RECONNAISSANCE DROP-
SONDE NEAR THE CIRCULATION REPORTING 50 KNOT SOUTHERLY WINDS AT THE
SURFACE AND A 172048Z QSCAT IMAGE SHOWING 35 KNOT UNFLAGGED WINDS
ALONG PERIPHERY OF THE SYSTEM.
3. FORECAST REASONING
A. THERE HAS BEEN LITTLE SIGNIFICANT CHANGE IN THE FORECAST PHILOSO-
PHY SINCE THE PREVIOUS ISSUANCE OF THIS PROGNOSTIC REASONING WITH THE 
EXCEPTION OF A SLIGHT SOUTHWARD SHIFT IN TRACK AND A MORE SHARPLY 
DEPICTED RECURVATURE NEAR TAU 72 DUE TO CHANGES IN THE EXPECTED IN-
FLUENCE OF THE APPROACHING MID-LEVEL SHORTWAVE TROUGH IN THE EXTENDED
TAUS.  
B. IN THE NEAR TERM, THE STR IS FORECAST TO REMAIN STRONG ALLOWING 
THE SYSTEM TO CONTINUE TRACKING QUICKLY WESTWARD OVER THE NEXT 24 TO 
48 HOURS. DURING THIS TIMEFRAME, GOOD EQUATORWARD OUTFLOW ASSOCIATED
WITH STRONG EASTERLY FLOW AND GOOD DIVERGENCE ALOFT WILL CONTINUE,
HOWEVER LACK OF A POLEWARD OUTFLOW MECHANISM WILL SEVERELY LIMIT RAPID
DEVELOPMENT ALLOWING ONLY MODERATE INTENSIFICATION TO TYPHOON STRENGTH
BY TAU 48. AFTER TAU 48, THE TROUGH CURRENTLY OVER EASTERN CHINA IS 
FORECAST TO ERODE THE WESTERN PERIPHERY OF THE STR ALLOWING THE SYSTEM 
TO BEGIN RE-CURVING EAST OF TAIWAN. THE SYSTEM WILL SLOW DURING RECURV-
ATURE, AND GENERALLY FAVORABLE CONDITIONS WHICH INCLUDE THE INTRODUC-
TION OF ENHANCED POLEWARD OUTFLOW WILL LEAD TO RAPID INTENSIFICATION 
WITH 20 KNOT INCREASES IN INTENSITY PER 24 HOURS EXPECTED TO BEGIN NEAR
TAU 72. THE AVAILABLE MODEL GUIDANCE IS IN GENERAL AGREEMENT WITH THIS
FORECAST THROUGH TAU 72.
C. IN THE EXTENDED PERIOD, THE SYSTEM WILL CONTINUE TO RE-CURVE EAST
OF TAIWAN AS IT RAPIDLY INTENSIFIES DUE TO IMPROVED POLEWARD OUTFLOW
AHEAD OF THE SHORTWAVE TROUGH. IN GENERAL, THE NOGAPS AND GFS MODEL 
TRACKERS REMAIN UNRELIABLE AFTER TAU 72 THEREFORE THERE IS INCREASED
UNCERTAINTY IN THE FORECAST IN THE EXTENDED TAUS. THE GFS MODEL LOSES 
THE CIRCULATION AFTER TAU 72 AND NOGAPS IS AGGRESSIVELY DEPICTING A
SHARP NORTHWARD TURN BUT QUESTIONS REMAIN REGARDING ITS DEEPER THAN
EXPECTED DEPICTION OF THE APPROACHING MIDLATITUDE TROUGH.
FORECAST TEAM: CHARLIE//
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