WDPN31 PGTW 180900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 16W (SANBA) WARNING NR 
004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.9N 108.7E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 116 NM NORTH OF DA NANG, VIETNAM
   MOVEMENT PAST 6 HOURS: NORTHWESTWARD AT 12 KTS
   SIGNIFICANT WAVE HEIGHT: 13 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) SHOWS A
MEDIUM-SIZED, DEVELOPING SYSTEM IN THE SOUTH CHINA SEA SOUTH IF 
HAINAN WITH FLARING AND DEEPENED CONVECTION NEAR THE CENTRAL DENSE 
OVERCAST (CDO) AND FORMATIVE BANDS WRAPPING TIGHTER FROM THE 
NORTHEASTERN PERIPHERY INTO THE CDO THAT IS OBSCURING THE LOW LEVEL 
CIRCULATION (LLC). THE INITIAL POSITION IS PLACED WITH HIGH 
CONFIDENCE BASED ON EXTRAPOLATION OF A WELL-DEFINED LLC IN THE 
180255Z ASCAT BULLSEYE PASS. THE INITIAL INTENSITY OF 35KTS IS ALSO 
PLACED WITH HIGH CONFIDENCE BASED ON THE HIGH END OF AGENCY AND 
AUTOMATED DVORAK ESTIMATES, SUPPORTED BY THE ASCAT PASS, AND REFLECTS 
THE 6-HR IMPROVEMENT. ANALYSIS INDICATES A MARGINALLY FAVORABLE 
ENVIRONMENT WITH LOW VWS, WARM SST, AND MODERATE POLEWARD OUTFLOW, 
TEMPERED BY PROXIMITY TO LAND.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA.

CURRENT STEERING MECHANISM: SUBTROPICAL RIDGE (STR) TO THE
EAST-NORTHEAST.


AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   RJTD: T2.0 - 30 KTS
   KNES: T2.5 - 35 KTS
   DEMS: T2.0 - 30 KTS
   CIMSS ADT: 37 KTS AT 180220Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 0-5 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE POLEWARD

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: UNDER THE STEERING INFLUENCE OF THE STR TO THE
ENE, TS SANBA WILL CONTINUE TO TRACK NORTHWESTWARD INTO THE GULF OF
TONKIN. AFTER TAU 36, A SECONDARY STR APPROACHING FROM THE WEST
WILL COMPETE FOR STEERING AND FORCE THE CYCLONE IN A
QUASI-STATIONARY (QS) MODE UP TO TAU 60 BEFORE SLOWLY DRIVING IT
SOUTHWARD. THE MARGINALLY FAVORABLE ENVIRONMENT WILL FUEL A MODEST
INTENSIFICATION TO A PEAK 0F 45KTS AT TAUS 36-48. AFTERWARD, LAND
INTERFERENCE FROM NORTHERN VIETNAM TO THE WEST AND HAINAN ISLAND TO
THE EAST, SST COOLING DUE TO CYCLONIC UPWELLING DUE TO THE QS STORM
MOTION, INTRUSION OF COOL DRY AIR FROM CHINA, THEN EVENTUAL
LANDFALL INTO CENTRAL VIETNAM GRADUALLY WEAKEN THE SYSTEM TO
DISSIPATION TAU 96.  

MODEL DISCUSSION: NUMERICAL MODELS ARE IN FAIRLY GOOD AGREEMENT UP
TO TAU 36 THEN DIVERGE SIGNIFICANTLY AFTERWARD AS THE MODELS
DIVERGE ALONG AND ACROSS TRACK DURING THE QS PHASE AND SLOW
SOUTHWARD DRIFT, LENDING OVERALL MEDIUM CONFIDENCE TO THE JTWC
TRACK AND INTENSITY FORECASTS. 

FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: MEDIUM
   TRACK 72-120 HR: MEDIUM
   INTENSITY 0 - 72 HR: MEDIUM
   INTENSITY 72-120 HR: MEDIUM//
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