WDPN31 PGTW 191500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 09W (WIPHA) WARNING NR 
005//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 21.0N 117.7E
   INITIAL INTENSITY: 55 KTS
   GEOGRAPHIC REFERENCE: 210 NM EAST-SOUTHEAST OF HONG KONG
   MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 12 KTS
   SIGNIFICANT WAVE HEIGHT: 24 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY REVEALS AN
OBSCURED LOW-LEVEL CIRCULATION CENTER (LLCC) EMBEDDED WITHIN CURVED
BANDS OF DEEP CONVECTIVE ACTIVITY ASSOCIATED WITH TROPICAL STORM
(TS) 09W (WIPHA). THE INITIAL POSITION IS ASSIGNED WITH HIGH
CONFIDENCE SUPPORTED BY A 191235Z METOP-B ASCAT PASS. THE INITIAL
INTENSITY ESTIMATE OF 55 KTS IS ALSO LAID WITH HIGH CONFIDENCE,
BASED ON A SYNTHESIS OF SUBJECTIVE DVORAK INTENSITY ESTIMATES FROM
MULTIPLE AGENCIES AND OBJECTIVE SATELLITE-DERIVED WIND PRODUCTS,
INCLUDING THE ABOVE-MENTIONED ASCAT DATA. ENVIRONMENTAL ANALYSIS
INDICATES A FAVORABLE SYNOPTIC SETUP CHARACTERIZED BY HIGH (29-30 C) 
SEA SURFACE TEMPERATURES (SST), ABUNDANT TROPOSPHERIC MOISTURE, AS 
WELL AS AN ENHANCED EQUATORWARD OUTFLOW EVIDENT IN WATER VAPOR 
IMAGERY, INDICATIVE OF ROBUST UPPER-TROPOSPHERIC DIVERGENCE. THESE
POSITIVE PARAMETERS ARE PARTIALLY OFFSET BY MODERATE TO STRONG
(20-25 KTS) VERTICAL WIND SHEAR (VWS).

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM:  DEEP-LAYER SUBTROPICAL RIDGE (STR) TO
THE NORTHEAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.5 - 55 KTS
   RJTD: T3.0 - 45 KTS
   RCTP: T3.5 - 55 KTS
   KNES: T3.5 - 55 KTS
   DEMS: T3.5 - 55 KTS
   CIMSS ADT: 53 KTS AT 191200Z
   CIMSS AIDT: 48 KTS AT 191200Z
   CIMSS D-MINT: 56 KTS AT 190956Z
   CIMSS D-PRINT: 49 KTS AT 191300Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 20-25 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG EQUATORWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: HIGH
   INITIAL WIND RADII: LOW

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: TS 09W IS ANTICIPATED TO TRACK GENERALLY
WEST-NORTHWESTWARD UNDER THE STEERING INFLUENCE OF A SUBTROPICAL
RIDGE (STR) POSITIONED TO THE NORTHEAST. BETWEEN TAUS 12 AND 24,
THE STR IS EXPECTED TO AMPLIFY AND EXTEND WESTWARD, PROMOTING A
SLIGHT WESTWARD SHIFT IN THE STORM MOTION. PRIOR TO THAT, THE
CYCLONE WILL REMAIN EMBEDDED WITHIN A MARGINALLY CONDUCIVE
ENVIRONMENT, SUPPORTIVE OF MAINTAINED OR SLIGHTLY INCREASED PEAK
SUSTAINED WINDS. SUBSEQUENTLY, AS THE RIDGE BUILDS, THE TRACK IS 
FORECAST TO VEER WEST-SOUTHWESTWARD, BRINGING THE SYSTEM ALONG THE 
SOUTHERN COASTLINE OF CHINA TOWARD A LANDFALL IN NORTHEASTERN VIETNAM. 
AFTER TAU 24, A GRADUAL WEAKENING TREND IS EXPECTED DUE TO TERRAIN
INTERACTION AND INCREASING VWS. HOWEVER, UPON ENTERING THE GULF OF
TONKIN, TS 09W WILL TRAVERSE VERY WARM SEA SURFACE TEMPERATURES OF 30-
31 C, WHICH MAY TEMPORARILY SUSTAIN OR SLIGHTLY INCREASE MAXIMUM WIND 
SPEEDS. BEYOND THAT POINT, RAPID WEAKENING AND EVENTUAL DISSIPATION 
ARE ANTICIPATED SHORTLY AFTER INLAND INGRESS EAST OF HANOI, VIETNAM, 
WITH FULL DISSIPATION EXPECTED AROUND TAU 96.

MODEL DISCUSSION: NUMERICAL MODEL TRACK GUIDANCE SHOWS STRONG
AGREEMENT IN THE NEAR TERM, DEMONSTRATED BY A CROSS-TRACK SPREAD
OF APPROXIMATELY 80 NM AT TAU 72. AROUND TAU 48 ALONG-TRACK
UNCERTAINTY INCREASES, AS INDICATED BY A SPREAD OF 230 NM BETWEEN
THE FASTEST GFS AND SLOWEST NAVGEM. AS A RESULT, LONG TERM TRACK IS
ASSESSED WITH MEDIUM CONFIDENCE. INTENSITY GUIDANCE IS IN MODERATE
AGREEMENT, WITH MOST MODELS PROJECTING SUSTAINED WINDS AROUND 50
KTS, FOLLOWED BY A WEAKENING TREND AND EVENTUAL DISSIPATION. THE
JTWC FORECAST IS LAID ON THE HIGHER SIDE OF THE MULTI-MODEL
CONSENSUS THROUGH THE INITIAL STAGES OF THE FORECAST DUE TO HIGH
CONFIDENCE IN CURRENT ASSESSMENT, WHICH PLACES THE WIND MAXIMA 5-10
KTS ABOVE THE AVAILABLE MODEL GUIDANCE. ADDITIONALLY, LONG TERM
INTENSITY FORECAST CONFIDENCE IS LOW DUE TO THE UNCERTAINTY IN THE
TRACK SPEED AND POSITIONING NEAR AND AFTER LANDFALL.

FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: HIGH
   TRACK 72-120 HR: MEDIUM
   INTENSITY 0 - 72 HR: MEDIUM
   INTENSITY 72-120 HR: LOW
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