WDPN31 PGTW 202100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 30W (FENGSHEN) WARNING NR 
012//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.9N 112.9E
   INITIAL INTENSITY: 45 KTS
   GEOGRAPHIC REFERENCE: 288 NM EAST-NORTHEAST OF DA NANG, VIETNAM
   MOVEMENT PAST 6 HOURS: WESTWARD AT 10 KTS
   SIGNIFICANT WAVE HEIGHT: 22 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS CYCLING
DEEP CONVECTION CONCENTRATED TO THE NORTH AND EAST OF A LOW-LEVEL
CIRCULATION CENTER EVIDENT IN A 201650Z OCEANSAT-3 PASS AND 201830Z
CIRA PROXY VISIBLE IMAGERY. THE INITIAL POSITION IS PLACED WITH
MEDIUM CONFIDENCE BASED ON THE OCEANSAT-3 AND CIRA PROXY VISIBLE
DATA. THE INITIAL INTENSITY OF 45 KTS IS ASSESSED WITH HIGH
CONFIDENCE BASED ON A CONSENSUS OF THE SUBJECTIVE DVORAK AND
AUTOMATED INTENSITY ESTIMATES LISTED BELOW. TS 30W HAS TURNED TO AN
ANTICIPATED SOUTH OF WESTWARD TRAJECTORY OVER THE PAST 12 HOURS AND
IS INTERACTING DIRECTLY WITH THE SOUTHERN EXTENSION OF A
NORTHEASTERLY COLD SURGE ANALYZED OVER THE NORTHWESTERN SOUTH CHINA
SEA.

INITIAL WIND RADII BASIS: 201650Z OCEANSAT-3 SCATTEROMETER DATA AND
OBJECTIVE BEST TRACK 

CURRENT STEERING MECHANISM: SUBTROPICAL RIDGING TO THE NORTHWEST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS
   RJTD: T3.0 - 45 KTS
   RCTP: T3.0 - 45 KTS
   KNES: T3.5 - 55 KTS
   DEMS: T3.0 - 45 KTS
   CIMSS SATCON: 46 KTS AT 201828Z
   CIMSS ADT: 47 KTS AT 201620Z
   CIMSS AIDT: 38 KTS AT 201620Z
   CIMSS D-MINT: 35 KTS AT 201828Z
   CIMSS D-PRINT: 35 KTS AT 201900Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 15-20 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: INTERACTION WITH COLD SURGE FLOW OVER THE
NORTHWESTERN SOUTH CHINA SEA

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THE ASSESSED PREVIOUS POSITION HAS
SHIFTED ABOUT 25 NM TO THE SOUTH, RESULTING IN A SMALL SOUTHWARD
SHIFT IN THE NEAR-TERM FORECAST TRACK. OTHERWISE, THERE ARE NO
SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: TS 30W WILL CONTINUE TO FOLLOW THE FLOW
ASSOCIATED WITH THE DEEP LAYER SUBTROPICAL STEERING RIDGE CURRENTLY
SITUATED TO THE NORTHWEST OF THE SYSTEM THROUGHOUT THE FORECAST
PERIOD. SLIGHT NEAR-TERM INTENSIFICATION IS EXPECTED AS FAVORABLE
POLEWARD OUTFLOW AS PASSAGE OVER WARM WATER OFFSET PERSISTENT,
MODERATE SOUTHERLY VERTICAL WIND SHEAR. ADDITIONALLY, AN
ACCELERATION OF LOW-LEVEL WIND SPEEDS IS ANTICIPATED WITHIN THE 
WESTERN PORTION OF THE CIRCULATION DUE TO GRADIENT TIGHTENING AS THE 
SYSTEM CONTINUES TO INTERACT WITH THE AMBIENT COLD SURGE. HOWEVER, BY 
TAU 36, RAPID WEAKENING WILL OCCUR, DRIVEN BY AN INFLUX OF DRY MID-
LEVEL AIR. THE SYSTEM IS EXPECTED TO MAKE LANDFALL IN CENTRAL VIETNAM 
AS A MINIMAL TROPICAL STORM OR STRONG TROPICAL DEPRESSION AROUND TAU 
48 AND DISSIPATE FULLY BY TAU 72 AS IT PROGRESSES INLAND.

MODEL DISCUSSION: CONSENSUS GUIDANCE IS IN GOOD AGREEMENT REGARDING
THE DIRECTION OF MOTION, WITH SIGNIFICANTLY LARGER SPREAD IN
FORWARD TRACK SPEED. THE SOLUTIONS ARE BRACKETED BY THE GEFS
ENSEMBLE MEAN, WHICH DEPICTS LANDFALL JUST AFTER TAU 36, AND THE
UKMET MODEL SOLUTION, WHICH DEPICTS LANDFALL JUST PRIOR TO TAU 60.
TRACK SPEEDS ARE A BIT FASTER IN THE CURRENT FORECAST THAT THE
PREVIOUS, CONSERVATIVELY NUDGED TOWARD THE CURRENT CONSENSUS
FORECAST SOLUTION WHICH IS ALSO FASTER THAN THE PREVIOUS RUN.
DYNAMIC INTENSITY FORECAST GUIDANCE REMAINS IN VERY GOOD AGREEMENT,
SUPPORTING A NEAR TERM BUMP IN INTENSITY FOLLOWED BY RAPID
DISSIPATION AFTER TAU 24. THE CURRENT INTENSITY FORECAST IS
CONSISTENT WITH THE DYNAMIC MODEL GROUPING AND PREVIOUS JTWC
FORECAST.

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