WDPN31 PGTW 240900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 28W (PABUK) WARNING 
NR 008//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 11.8N 111.9E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 312 NM EAST-NORTHEAST OF HO CHI MINH CITY,
VIETNAM
   MOVEMENT PAST 6 HOURS: NORTHWESTWARD AT 05 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A PARTIALLY
EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC), WHICH HAS MOVED UNDER
THE EASTERN EDGE OF CIRRUS SHIELD ASSOCIATED WITH DEEP, FLARING
CONVECTION, OVER THE PAST FEW HOURS. THE LLCC HAS EXHIBITED ERRATIC
MOTION OVER THE PAST 12 HOURS, AS IT HAS RUN UP AGAINST THE STRONG
BELT OF COASTALLY TRAPPED SURGE WINDS LYING ALONG THE EASTERN COAST
OF VIETNAM. THESE COASTALLY TRAPPED WINDS ARE ACTING LIKE A LANE
BUMPER AT THE BOWLING ALLEY, AND THE LLCC HAS RUN INTO IT, THEN
LOOPED BACK AROUND TO THE SOUTH AND GIVEN IT ANOTHER GO. THE
INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE
PARTIALLY EXPOSED LLCC IN THE MSI. THE INITIAL INTENSITY IS
ASSESSED WITH HIGH CONFIDENCE BASED ON THE GENERAL AGREEMENT OF THE
SUBJECTIVE AND OBJECTIVE INTENSITY ESTIMATES NOTED ABOVE.
ADDITIONALLY, A SHIP OBSERVATION LOCATED APPROXIMATELY 80NM
SOUTHWEST OF THE ASSESSED CENTER IS REPORTING NORTH-NORTHWESTERLY
WINDS OF 32 KNOTS, WHICH PROVIDES ADDED CONFIDENCE TO THE INTENSITY
ESTIMATE. THE ENVIRONMENT IS ASSESSED AS UNFAVORABLE, WITH LOW TO
MODERATE VWS, MARGINAL (27-28C) SSTS AND WEAK OUTFLOW ALOFT.

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

CURRENT STEERING MECHANISM: WESTERN SIDE OF A LOW- TO MID-LEVEL
SUBTROPICAL RIDGE (STR) CENTERED FAR TO THE EAST.  

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T1.5 - 25 KTS
   RJTD: T2.0 - 30 KTS
   KNES: T2.0 - 30 KTS
   CIMSS SATCON: 37 KTS AT 240430Z
   CIMSS ADT: 33 KTS AT 240600Z
   CIMSS AIDT: 31 KTS AT 240600Z
   CIMSS D-PRINT: 30 KTS AT 240600Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 15-20 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: WEAK POLEWARD
   OTHER FACTORS: SURGE FLOW TO THE WEST, TRAPPED ALONG THE COAST
OF VIETNAM. 

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: HIGH
   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: WHILE THE SYSTEM HAS RECENTLY EXPERIENCED A
BIT OF AN ERRATIC TRACK MOTION DUE TO THE SURGE FLOW TO THE EAST,
THE STRENGTH OF THE SURGE FLOW IS FORECAST TO WEAKEN OVER THE NEXT
24 HOURS. THIS WILL ALLOW TD 28W TO TURN ONTO A MORE SOUTHWESTWARD
TRACK, MOVING ALONG THE EASTERN EDGE OF THE WEAKENED SURGE FLOW.
UPPER-LEVEL WINDS REMAIN SOUTHEASTERLY AT THE MOMENT, INDUCING
ABOUT 20 KNOTS OF SHEAR OVER TOP OF TD 28W, KEEPING THE NEWLY
FORMED CONVECTION, WHICH IS FORMING ALONG THE CONVERGENCE LINES
BETWEEN THE LLCC AND THE SURGE FLOW, DISPLACED TO THE NORTHWEST.
OVER THE NEXT 24 HOURS, ENVIRONMENTAL CONDITIONS WILL DETERIORATE,
WITH THE SOUTHEASTERLY FLOW ALOFT EXPECTED TO INCREASE, RESULTING
IN EVEN HIGHER SHEAR, AS WELL AS USHERING IN A WEDGE OF DRY
MID-LEVEL AIR. THE COMBINED EFFECTS OF INCREASED SHEAR AND DRY AIR
ENVELOPMENT WILL RESULT IN A WEAKENING OF TD 28W AFTER TAU 12. THE
SYSTEM WILL STEADILY WEAKEN THROUGH THE FORECAST PERIOD, AND TD 28W
IS FORECAST TO DISSIPATE NO LATER THAN TAU 48 OVER THE MEKONG DELTA
REGION. IT IS POSSIBLE THAT THE SYSTEM COULD WEAKEN BELOW WARNING
STRENGTH AS EARLY AS TAU 36, WHILE IT REMAINS OFFSHORE TO THE EAST
OF HO CHI MINH CITY.  

MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE IS IN GOOD AGREEMENT
THAT TD 28W WILL TURN TOWARDS THE SOUTHWEST WITHIN THE NEXT 12
HOURS, THEN TRACK ALONG THE COAST OF SOUTHEAST VIETNAM THROUGH THE
REST OF THE FORECAST PERIOD. THE ECMWF, GEFS ENSEMBLE MEAN AND
NAVGEM, KEEP THE VORTEX OFFSHORE OF VIETNAM THROUGH THE FORECAST
PERIOD, WHILE THE REMAINDER OF THE CONSENSUS MEMBERS TURN IT
INLAND, CROSSING THE COAST IN AN ENVELOPE THAT IS ROUGHLY CENTERED
ON HO CHI MINH CITY. THE JTWC FORECAST LIES CLOSE TO THE CONSENSUS
MEAN THROUGH THE FORECAST PERIOD, WITH LOW CONFIDENCE, ESPECIALLY
AFTER TAU 24 AS THE MODELS START TO LOSE THE VORTEX AND CONFIDENCE
IN THE VORTEX TRACKER DROPS SHARPLY. INTENSITY GUIDANCE IS IN GOOD
AGREEMENT THAT THE SYSTEM WILL STEADILY WEAKEN AFTER TAU 12, WITH
DISSIPATION EXPECTED BETWEEN TAU 36 AND TAU 48. 

FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: LOW
   INTENSITY 0 - 72 HR: HIGH//
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