WDPN31 PGTW 182100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 16W (SANBA) WARNING NR 
006//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 18.9N 108.2E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 181 NM SOUTHEAST OF HANOI, VIETNAM
   MOVEMENT PAST 6 HOURS: NORTH-NORTHWESTWARD AT 05 KTS
   SIGNIFICANT WAVE HEIGHT: 13 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A
PARTIALLY EXPOSED LLC WITH DEEP FLARING CONVECTION BEING SHEARED TO
THE NORTH. PRECISE LOCALIZATION OF 16W IS PROVING DIFFICULT AS
RECENT POLAR ORBITING SATELLITES HAVE ENTIRELY MISSED THE SYSTEM.
UTILIZING SURFACE OBSERVATIONS FROM THE REGION, WE CAN OBSERVE
NORTHWESTERLY WINDS TRANSITIONING TO SOUTHWESTERLY WINDS AS YOU
TRAVEL DOWN THE COAST OF VIETNAM AS WELL AS EASTERLY FLOW ACROSS
HAINAN ISLAND. BASED ON THIS INFORMATION, 16W CAN BE TRIANGULATED
TO A NARROW REGION OFF THE WESTERN COAST OF HAINAN ISLAND. THE
INITIAL POSITION IS PLACED WITH LOW CONFIDENCE HOWEVER, BASED ON A
LACK OF POLAR ORBITING DATA. THE INITIAL INTENSITY OF 35 KTS IS
ASSESSED WITH MEDIUM CONFIDENCE BASED ON A GENERAL AGREEMENT
AMONGST AGENCY DVORAK INTENSITY ESTIMATES OF 35KTS, WHILE CIMSS
AUTOMATED INTENSITY ESTIMATES INDICATE SLIGHTLY HIGHER.

INITIAL WIND RADII BASIS: PERSISTENCE 

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
   CIMSS SATCON: 49 KTS AT 181402Z
   CIMSS ADT: 53 KTS AT 181730Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 5-10 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: MODERATE POLEWARD

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TS 16W (SANBA) IS FORECAST TO CONTINUE ITS
GENERALLY POLEWARD MARCH INTO THE GULF OF TONKIN UNDER THE STEERING
INFLUENCE OF THE STR TO THE EAST-NORTHEAST. NEAR TAU 24, AN
ADDITIONAL STR FORMS WEST OF THE SYSTEM, GENERALLY NEGATING FURTHER
POLEWARD MOVEMENT AND BRINGING ABOUT A QS PERIOD. GLOBAL MODELS
GENERALLY AGREE THAT THE RIDGING TO THE WEST AND EAST OF 16W JOIN
FORCES AND FORCE THE SYSTEM EQUATORWARD THROUGH THE FORECAST
PERIOD. IN TERMS OF INTENSITY, AFTER CLEARING HAINAN ISLAND NEAR
TAU 12 AND FUELED BY HIGH OHC VALUES, LOW VERTICAL WIND SHEAR (VWS)
AND A MOIST AIR COLUMN, 16W IS FORECAST TO GRADUALLY INTENSIFY TO
45KTS BY TAU 36. AFTER REACHING ITS APOGEE, INCREASING VWS AND DRY
AIR ENTRAINMENT WILL STEADILY WEAKENING THE SYSTEM AS IT APPROACHES
THE COAST OF TAIWAN, EVENTUALLY DISSIPATING NEAR TAU 72.

MODEL DISCUSSION: NUMERICAL MODELS ARE IN FAIR AGREEMENT WITH ALL
MEMBERS INDICATING A SLOW TREK INTO THE GULF OF TONKIN THROUGH TAU
36, AFTER WHICH A SHARP EQUATORWARD TURN THROUGH DISSIPATION. DUE
TO MILD ALONG AND CROSS TRACK SPREADING, THE JTWC FORECAST TRACK IS
PLACED WITH MEDIUM CONFIDENCE. RELIABLE MODEL INTENSITY GUIDANCE IS
IN GOOD AGREEMENT WITH ALL MEMBERS INDICATING A GRADUAL
INTENSIFICATION TO 40-45KTS BY TAU 36 AND EVENTUAL DISSIPATION BY
TAU 72. FOR THIS REASON THE JTWC INTENSITY FORECAST IS PLACED WITH
HIGH CONFIDENCE. 

FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: MEDIUM
   INTENSITY 0 - 72 HR: HIGH//
NNNN