WDPN31 PGTW 182100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 30W (FENGSHEN) WARNING NR 
004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 13.9N 122.6E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 102 NM EAST-SOUTHEAST OF MANILA,
   PHILIPPINES
   MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 12 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A FULLY
OBSCURED LOW-LEVEL CIRCULATION CENTER (LLCC) AND A POORLY ORGANIZED
UPPER-LEVEL WITH FRAGMENTED BANDS OF DEEP CONVECTION ALONG THE
PERIPHERY OF THE SYSTEM. ENVIRONMENTAL ANALYSIS INDICATES A
MARGINALLY UNFAVORABLE ENVIRONMENT DUE TO LAND INTERACTION WITH
LUZON, OFFSET BY THE STRONG OUTFLOW, LOW (0-5 KTS) VERTICAL WIND
SHEAR, AND WARM (29-30 C) SEA SURFACE TEMPERATURES. DAET, PHILIPPINES 
(98440) IS REPORTING SURFACE WIND OBSERVATIONS OF 8KTS FROM THE 
SOUTHEAST AT 181900Z, WHILE ALANBAT, PHILIPPINES (98435) IS REPORTING 
4KTS FROM THE NORTHEAST AT 181900Z, INDICATING THE LLCC MUST BE SOUTH 
OF BOTH OBSERVATIONS. THE INITIAL POSITION WAS PLACED BY USING THE 
SURFACE OBSERVATIONS TO TRIANGULATE THE LOCATION WITH OVERALL LOW 
CONFIDENCE. THE INITIAL INTENSITY OF 35 KTS IS ASSESSED WITH MEDIUM 
CONFIDENCE BASED ON THE AGENCY DVORAK FIXES AND OBJECTIVE
INTENSITY ESTIMATES LISTED BELOW.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: SOUTHWESTWARD EXTENSION OF THE
SUBTROPICAL RIDGE TO THE NORTHEAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS
   RJTD: T2.0 - 30 KTS
   RCTP: T2.5 - 35 KTS
   KNES: T3.0 - 45 KTS
   CIMSS ADT: 43 KTS AT 181730Z
   CIMSS AIDT: 37 KTS AT 181730Z
   CIMSS D-MINT: 32 KTS AT 181725Z
   CIMSS D-PRINT: 38 KTS AT 181800Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY
UNFAVORABLE
   VWS: 0-5 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG RADIAL
   OTHER FACTORS: TERRAIN INTERACTION WITH SOUTHERN LUZON

ANALYSIS CONFIDENCE:
   INITIAL POSITION: LOW
   INITIAL INTENSITY: MEDIUM
   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 TRACKING NORTHWESTWARD THROUGH LUZON FOR THE NEXT 12 HOURS
BEFORE REEMERGING OVER WATER IN THE WESTERN PHILIPPINE SEA. AFTER
REEMERGING JUST AFTER TAU 12, 30W WILL REMAIN IN A NORTHWESTWARD
TRAJECTORY WHILE UNDER THE INFLUENCE OF THE STR TO ITS NORTHEAST.
BEGINNING AROUND TAU 48, 30W WILL ENCOUNTER A WEAKNESS IN THE STEERING 
RIDGE, CAUSING IT TO SLOW AND POSSIBLY BECOME QUASI-STATIONARY. 
BETWEEN TAU 48-72, FENGSHEN WILL TRANSITION TO THE STEERING INFLUENCE 
OF THE STR THAT WILL BUILD IN OVER SOUTHEAST ASIA, CAUSING IT TO CURVE 
SOUTHWESTWARD. SIMULTANEOUSLY, A COLD SURGE FUNNELING SOUTHWESTWARD 
THROUGH THE TAIWAN STRAIT WILL PUMMEL THE SYSTEM WITH COLD, DRY AIR. 
30W IS FORECAST TO STEADILY INTENSIFY WHILE TRACKING NORTHWESTWARD FOR 
THE NEXT 48 HOURS AND WILL BRIEFLY REACH TYPHOON STRENGTH AS A RESULT 
OF THE ENHANCED WINDS FROM THE COLD SURGE WITH A PEAK AT 70 KTS 
BETWEEN TAU 48-72. AFTER TRANSITIONING STEERING RIDGES AND BEGINNING 
ITS SOUTHWESTWARD TRACK, 30W IS FORECAST TO RAPIDLY WEAKEN FOR THE 
REMAINDER OF THE FORECAST PERIOD DUE TO THE SUDDEN SURGE OF STABLE AIR 
ENTRAINING INTO THE SYSTEM. 

MODEL DISCUSSION: DETERMINISTIC MODEL TRACK GUIDANCE IS IN GOOD
AGREEMENT IN A NORTHWESTWARD TRACK, FOLLOWED BY A BRIEF STALL AND A
SOUTHWESTWARD TRACK FOR THE REMAINDER OF THE FORECAST PERIOD. THERE
IS SOME DISAGREEMENT IN THE TIMING OF THE STALL AND SOUTHWESTWARD
CURVE, INJECTING UNCERTAINTY INTO THE JTWC TRACK FORECAST. THE JTWC
TRACK FORECAST IS PLACED WITH HIGH CONFIDENCE FROM TAU 0-72, AND
MEDIUM CONFIDENCE FROM TAU 72-120. RELIABLE INTENSITY GUIDANCE IS
IN GOOD AGREEMENT THAT 30W WILL STEADILY INTENSIFY AFTER REEMERGING
OVER WATER AND REACHING A PEAK BETWEEN TAU 60-72. THE JTWC
INTENSITY FORECAST LIES SLIGHTLY HIGHER THAN THE MULTI-MODEL
CONSENSUS WITH OVERALL MEDIUM CONFIDENCE.

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