WDPN31 PGTW 200300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 30W (FENGSHEN) WARNING NR 
009//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 18.0N 116.7E
   INITIAL INTENSITY: 45 KTS
   GEOGRAPHIC REFERENCE: 497 NM EAST-NORTHEAST OF DA NANG, VIETNAM
   MOVEMENT PAST 6 HOURS: NORTHWESTWARD AT 10 KTS
   SIGNIFICANT WAVE HEIGHT: 16 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A
CONSOLIDATING SYSTEM WITH BUILDING DEEP CONVECTION ALONG THE
EASTERN SEMICIRCLE BEGINNING TO WRAP AROUND THE NORTHERN QUADRANT.
BUILDING DEEP CONVECTION IN THE SOUTHERN QUADRANT HAS ALSO FORMED
WITHIN THE PAST 6 HOURS. STRONG OUTFLOW HAS DEVELOPED, WITH UPPER-
LEVEL STRIATIONS EVIDENT IN MSI. A 192203Z RCM-3 SAR WINDSPEED
PARTIAL PASS REVEALS A BAND OF 40-45 KTS ALONG THE NORTHEASTERN
QUADRANT, SUPPORTING THE INITIAL INTENSITY. ENVIRONMENTAL ANALYSIS
INDICATES A FAVORABLE ENVIRONMENT, CHARACTERIZED BY LOW (10-15 KTS)
VERTICAL WIND SHEAR, VERY WARM (28-29 C) SEA SURFACE TEMPERATURES,
AND GOOD DIVERGENCE ALOFT. THE INITIAL POSITION IS PLACED WITH
MEDIUM CONFIDENCE BASED ON THE AFOREMENTIONED PARTIAL SAR DATA. THE
INITIAL INTENSITY OF 45 KTS IS ASSESSED WITH HIGH CONFIDENCE BASED
ON THE AGENCY DVORAK FIXES AND THE CIMSS AUTOMATED OBJECTIVE
INTENSITY ESTIMATES LISTED BELOW, CORROBORATED BY THE PARTIAL SAR
PASS.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: SOUTHWESTERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) TO THE NORTHEAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS
   RJTD: T3.0 - 45 KTS
   CIMSS SATCON: 38 KTS AT 192300Z
   CIMSS ADT: 37 KTS AT 200030Z
   CIMSS AIDT: 36 KTS AT 200030Z
   CIMSS D-MINT: 34 KTS AT 192256Z
   CIMSS D-PRINT: 30 KTS AT 200030Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 10-15 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: STRONG RADIAL

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TROPICAL STORM (TS) 30W (FENGSHEN) WILL
CONTINUE WEST-NORTHWESTWARD FOR THE NEXT 24 HOURS WHILE IT REMAINS
UNDER THE STEERING INFLUENCE OF THE STR TO ITS NORTHEAST. AROUND
TAU 12, 30W WILL ENTER INTO A COMPETING STEERING ENVIRONMENT AS IT
ENCOUNTERS A RIDGE BUILDING OVER SOUTHEAST ASIA. AT THAT POINT,
FENGSHEN WILL TRANSITION STEERING RIDGES, CURVE SOUTHWESTWARD, THEN
SUBSEQUENTLY TRACK SOUTHWESTWARD FOR THE REMAINDER OF THE FORECAST
PERIOD. LEADING UP TO TAU 24-36, THE SOUTH CHINA SEA IS AN OVERALL
FAVORABLE ENVIRONMENT FOR DEVELOPMENT, ALLOWING 30W TO STEADILY
INTENSIFY TO A PEAK OF 65KTS AT TAU 36. BETWEEN TAU 12-36, FENGSHEN
WILL EXPERIENCE IMPACTS ASSOCIATED WITH A COLD SURGE FUNNELING
SOUTHWESTWARD THROUGH THE TAIWAN STRAIT. THE COLD SURGE WILL
INTRODUCE COPIOUS AMOUNTS OF DRY AIR TO THE ENVIRONMENT, ALTHOUGH IT
WILL ALSO TEMPORARILY EXPAND THE GALE FORCE WINDS ALONG THE
NORTHEASTERN PERIPHERY. AROUND TAU 36, SIGNIFICANT DRY AIR
ENTRAINMENT IN THE MID- TO UPPER-LEVELS WILL SUPPRESS DEVELOPMENT
AND BEGIN TO WEAKEN 30W AS IT ENTRAINS INTO THE CORE. 30W WILL
CONTINUE TO STEADILY WEAKEN WHILE TRACKING SOUTHWESTWARD, MAKING
LANDFALL AROUND DA NANG, VIETNAM AS A WEAK TROPICAL STORM AFTER TAU
72. DISSIPATION OVER LAND IS FORECAST NO LATER THAN TAU 96.

MODEL DISCUSSION: DETERMINISTIC MODEL TRACK GUIDANCE IS IN GOOD
AGREEMENT THROUGH TAU 24, BUT DIVERGES FAIRLY QUICKLY IN BOTH
ALONG-TRACK AND CROSS-TRACK SPREADS DUE TO DISAGREEMENT IN THE
TIMING AND SPEED OF THE STEERING TRANSITION. THE MAJOR OUTLIER IS
GFS, WHICH TRACKS 30W SIGNIFICANTLY FARTHER INLAND THAN OTHER
MODELS, PREDICTING 30W WILL MAKE IT ALL THE WAY TO THE BAY OF
BANGKOK IN 96 HOURS. GFS IS THE MAIN CONTRIBUTOR TO THE OVER 500NM
ALONG-TRACK SPREAD AT TAU 96. CROSS-TRACK SPREAD AT TAU 96 HAS
REMAINED CONSISTENT FOR THE PAST 6 HOURS, HOVERING AROUND 200NM. AS
A RESULT OF THE SIGNIFICANT ALONG-TRACK UNCERTAINTY IN THE LATER
TAUS, THE JTWC TRACK FORECAST IS PLACED WITH MEDIUM CONFIDENCE FOR
TAU 0-72 AND LOW CONFIDENCE FOR TAU 72-120. RELIABLE INTENSITY
GUIDANCE IS IN GOOD AGREEMENT THAT 30W WILL STEADILY INTENSIFY
THROUGH TAU 36 AND PEAK AROUND TAU 36-48. A LIMITED NUMBER OF RAPID 
INTENSIFICATION AIDS HAVE TRIGGERED, INDICATING A SLIGHT POSSIBILITY 
OF 30W UNDERGOING RAPID INTENSIFICATION OR REACHING A PEAK GREATER 
THAN 65 KTS. THE JTWC INTENSITY FORECAST LIES SLIGHTLY HIGHER THAN THE 
BULK OF MULTI-MODEL CONSENSUS MEMBERS, WITH OVERALL MEDIUM CONFIDENCE.

FORECAST CONFIDENCE:
   TRACK 00-72 HR: MEDIUM
   TRACK 72-120 HR: LOW
   INTENSITY 00-72 HR: MEDIUM
   INTENSITY 72-120 HR: MEDIUM//
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