WDPN31 PGTW 200900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 16W (SANBA)
WARNING NR 012//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 20.8N 109.8E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 219 NM EAST OF HANOI, VIETNAM
   MOVEMENT PAST 6 HOURS: SOUTH-SOUTHEASTWARD AT 04 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
DEPRESSION (TD) 16W (SANBA) EXHIBITING A MOSTLY EXPOSED LOW LEVEL
CIRCULATION CENTER (LLCC) TRACKING SOUTHWARD AT 4 KNOTS OVER THE
PAST 6 HOURS.  NEAR 200430Z, THE LLCC EMERGED FROM THE DEEP
CONVECTIVE PLUME ALOFT, WHICH IS EXPERIENCING INTENSE SOUTHWESTERLY
SHEAR IN THE UPPER LEVELS.  SSTS REMAIN MARGINALLY FAVORABLE, WHICH 
COULD CONTRIBUTE TO A BRIEF SUSTAINMENT PERIOD.  HOWEVER, VERTICAL 
WIND SHEAR (VWS) CONTINUES TO INCREASE AND IS LIKELY A PRIMARY FACTOR 
IN THE AFOREMENTIONED DECOUPLING. THE INITIAL POSITION IS PLACED WITH
MEDIUM CONFIDENCE BASED ON A 200600Z HIMAWARI-9 VISIBLE 1KM
SATELLITE IMAGE. THE INITIAL INTENSITY OF 30 KTS IS ASSESSED WITH
MEDIUM CONFIDENCE BASED ON SUBJECTIVE DVORAK INTENSITY ESTIMATES
LISTED BELOW AND SURFACE OBSERVATIONS IN VICINITY OF TD 16W.
SURFACE OBSERVATIONS VALID AT 200600Z USED FOR ANALYSIS WERE BEIHAI
(N 30KTS), ZHANJIANG (E 19KTS), AND HAIKOU (SW 11KTS).

INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND
RADII).

CURRENT STEERING MECHANISM: LOW TO MID-LEVEL NORTHEASTERLY FLOW

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS
   RJTD: T1.5 - 25 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 20-25 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: MODERATE POLEWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: NOT APPLICABLE

3. FORECAST REASONING.

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

FORECAST DISCUSSION: NOW THAT TD 16W HAS RETURNED TO NEAR
RECIPROCAL TRACK, AS HAS BEEN FORECAST FOR SEVERAL CONSECUTIVE
WARNINGS, THE WEAKENING VORTEX IS ANTICIPATED TO FULLY DISSIPATE BY
TAU 24.  MODERATE TO HIGH VERTICAL WIND SHEAR (20-25 KTS), MARGINAL
SST (27C TO 28C), LOW RELATIVE HUMIDITY (LESS THAN 50 PERCENT), AND
DECOUPLING OF MID AND UPPER LEVEL SUPPORT LEAVES LITTLE ROOM FOR THE
STRUGGLING SYSTEM TO GROW. AFTER A POSSIBLE SHORT-LIVED SUSTAINMENT 
THROUGH TAU 6, TD 16W WILL GRADUALLY WEAKEN TO DISSIPATION BY TAU 24.

MODEL DISCUSSION: GLOBAL NUMERICAL MODEL GUIDANCE IS IN GOOD
AGREEMENT THAT TD 16W WILL TAKE A SOUTHWESTWARD TRACK THROUGH THE
FORECAST PERIOD (TAU 24) TO DISSIPATION.  THERE IS SUBSTANTIAL 
DISAGREEMENT IN TRACK SPEED, HOWEVER, WITH AN ALONG-TRACK SPREAD AT 
TAU 24 OF 130NM BETWEEN THE GFS AND ECMWF SOLUTIONS. COINCIDENTALLY, 
THE SAME TWO SOLUTIONS OFFER THE LARGEST CROSS-TRACK SPREAD OF 63NM 
AT TAU 24. THE UNCERTAINTY PRESENTED CONTRIBUTES TO AN OVERALL 
FORECAST CONFIDENCE OF MEDIUM.

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