WDPN31 PGTW 180300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 31W (KIROGI) WARNING 
NR 03//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
TROPICAL STORM 31W (KIROGI), LOCATED APPROXIMATELY 391 NM WEST-
SOUTHWEST OF MANILA, PHILIPPINES, HAS TRACKED WEST-NORTHWESTWARD AT 
17 KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE 
IMAGERY DEPICTS A PARTIALLY EXPOSED LOW LEVEL CIRCULATION CENTER 
WITH DEEP FLARING CENTRAL CONVECTION. THERE IS FAIR CONFIDENCE IN 
THE INITIAL POSITION WHICH IS BASED ON SATELLITE FIXES AND A 172001Z 
89GHZ AMSU-B IMAGE. THE INITIAL INTENSITY IS ASSESSED AT 40 KNOTS 
WHICH IS BASED ON MULTI- AGENCY DVORAK ESTIMATES RANGING FROM T2.0 
TO T3.0 (30-45 KNOTS) AND A 172002Z SATCON ESTIMATE OF 38 KNOTS. 
UPPER-LEVEL ANALYSIS SHOWS TS 31W SLIGHTLY EQUATORWARD OF A STRONG 
RIDGE AXIS WITH MODERATE DIFFLUENCE ENHANCING THE CONVECTIVE 
ACTIVITY. EASTERLY FLOW ALOFT IS INDUCING LOW TO MODERATE WIND SHEAR 
AS THE SYSTEM TRACKS IN PHASE WITH THE SHEAR VECTOR. SSTS IN THE 
REGION ARE ALSO CONDUCIVE FOR DEVELOPMENT AROUND 29 CELSIUS. 
CURRENTLY TS 31W IS TRACKING WEST-NORTHWESTWARD ALONG THE SOUTHERN 
BOUNDARY OF A DEEP-LAYERED EAST-WEST ORIENTED SUBTROPICAL RIDGE TO 
THE NORTH.   
3. FORECAST REASONING.
   A. THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE 
PREVIOUS PROGNOSTIC REASONING MESSAGE.
   B. TS 31W WILL CONTINUE TO TRACK WEST-NORTHWEST ALONG THE 
SOUTHERN BOUNDARY OF THE CURRENT STEERING RIDGE THROUGH THE FORECAST 
PERIOD. ENVIRONMENTAL CONDITIONS ARE FORECAST TO REMAIN MARGINAL TO 
FAVORABLE AS THE SYSTEM TRACKS OVER THE WARM WATERS OF THE SOUTH 
CHINA SEA WITH MODERATE WIND SHEAR FROM THE UPPER-LEVEL RIDGE TO THE 
NORTHEAST. THE PEAK INTENSITY OF TS 31W IS FORECAST TO REACH 50 
KNOTS JUST BEFORE MAKING LANDFALL OVER VIETNAM SHORTLY AFTER TAU 24. 
THE SYSTEM WILL CONTINUE TO WEAKEN AS IT TRACKS INLAND OVER THE 
MOUNTAINOUS TERRAIN. DYNAMIC MODEL GUIDANCE SHOWS VERY GOOD 
AGREEMENT FOR THE FIRST 24 HOURS WHILE THE SYSTEM IS OVER WATER DUE 
TO THE WELL-ESTABLISHED STEERING RIDGE TO THE NORTH. OVERALL THERE 
IS HIGH CONFIDENCE IN THE FORECAST TRACK.//
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