WDPN31 PGTW 210300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 30W (FENGSHEN) WARNING NR 
013//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.4N 112.4E
   INITIAL INTENSITY: 45 KTS
   GEOGRAPHIC REFERENCE: 250 NM EAST-NORTHEAST OF DA NANG, VIETNAM
   MOVEMENT PAST 6 HOURS: SOUTHWESTWARD AT 07 KTS
   SIGNIFICANT WAVE HEIGHT: 22 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A PARTIALLY
EXPOSED LOW-LEVEL CIRCULATION WITH ASSOCIATED DEEP CONVECTION
LIMITED TO THE NORTHEASTERN PERIPHERY. THE INITIAL POSITION IS
PLACED WITH MEDIUM CONFIDENCE BASED ON THE ANIMATED MSI, 202236Z
WSF-M 37 GHZ MICROWAVE SATELLITE IMAGERY, AND A 202330Z PGTW
SATELLITE FIX. MSI INDICATE THE PRESENCE OF MULTIPLE VORTICES
WITHIN THE EXPOSED CIRCULATION, COMPLICATING PRECISE POSITIONING OF
THE CENTER. THE INITIAL INTENSITY OF 45 KTS IS ASSESSED WITH MEDIUM
CONFIDENCE BASED ON A CONSENSUS OF SUBJECTIVE DVORAK AND AUTOMATED
INTENSITY ESTIMATES LISTED BELOW. TS 30W CONTINUES TO INTERACT WITH
THE SOUTHERN EXTENSION OF A NORTHEASTERLY COLD SURGE ANALYZED OVER
THE NORTHWESTERN SOUTH CHINA SEA, WITH ASSOCIATED STRATOCUMULUS
CLOUD LINES CLEARLY EVIDENT IN THE MSI.

INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK AND PERSISTENCE 

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
   CIMSS ADT: 47 KTS AT 202030Z
   CIMSS AIDT: 40 KTS AT 202030Z
   CIMSS D-MINT: 35 KTS AT 202133Z
   CIMSS D-PRINT: 38 KTS AT 202300Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 10-15 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: 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: SUBTROPICAL RIDGING CURRENTLY ANALYZED TO THE
NORTHWEST OF TS 30W WILL STEER THE SYSTEM ON A WEST-SOUTHWESTWARD
TRACK THROUGHOUT THE FORECAST PERIOD. SLIGHT INTENSIFICATION IS
POSSIBLE IN THE NEAR-TERM AS POLEWARD OUTFLOW AIDS CONVECTIVE
DEVELOPMENT AND A TIGHTENING GRADIENT WITH THE AMBIENT COLD SURGE
SUPPORTS POTENTIAL ACCELERATION OF WIND SPEEDS WITHIN THE WESTERN
PORTION OF THE CIRCULATION. HOWEVER, PERSISTENT SOUTHERLY VERTICAL 
WIND SHEAR AND INFUSION OF MORE STABLE AIR FROM THE WEST COULD VERY 
WELL STIFLE INTENSIFICATION. IN ANY CASE, THE BALANCE OF OFFSETTING 
FACTORS IS EXPECTED TO LEAD TO A NEARLY STEADY STATE INTENSITY OVER
THE NEXT 24 HOURS. THEREAFTER, INCREASING INGESTION OF MORE STABLE
AIR AND PERSISTENT SHEAR WILL OVERCOME SUPPORTIVE FACTORS, LEADING
TO STEADY WEAKENING. THE SYSTEM IS EXPECTED TO MAKE LANDFALL IN
CENTRAL VIETNAM AS A MINIMAL TROPICAL STORM OR STRONG TROPICAL
DEPRESSION BETWEEN TAU 36 AND 48 AND DISSIPATE FULLY BY TAU 72 AS
IT PROGRESSES INLAND.

MODEL DISCUSSION: CONSENSUS TRACK FORECAST GUIDANCE IS IN GOOD
AGREEMENT REGARDING THE OVERALL SCENARIO AND GENERAL DIRECTION OF
MOTION PRIOR TO LANDFALL, ALTHOUGH SIGNIFICANT SPREAD RELATED TO 
FORWARD TRACK SPEED PERSISTS. GIVEN ANTICIPATED RAPID CHANGES IN STORM
STRUCTURE AND UNCERTAINTIES RELATED TO INTERACTIONS WITH THE
AMBIENT COLD SURGE AND RESULTANT CHANGES IN STEERING LEVEL OVER
TIME, NO INDIVIDUAL MODEL SOLUTION OR GROUPING STANDS OUT AS MOST
VIABLE IN THE CURRENT RUN. THE TRACK IS PLACED JUST TO THE SOUTH OF
THE PREVIOUS FORECAST, ACCOUNTING FOR RECENT STORM MOTION, AND IS A
BIT SLOWER IN THE FINAL OVER LAND PHASE CONSISTENT WITH CHANGES IN
THE MULTI-MODEL CONSENSUS DURING THAT PERIOD. INTENSITY FORECAST
GUIDANCE IS IN GOOD AGREEMENT ON THIS RUN, AND THE CURRENT
INTENSITY FORECAST IS CONSISTENT WITH BOTH THE INTENSITY CONSENSUS
AND PREVIOUS JTWC INTENSITY FORECAST.

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