Advisories

WDPN32 PGTW 210300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 18W (EIGHTEEN) 
WARNING NR 002//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 20.2N 108.6E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 159 NM EAST-SOUTHEAST OF HANOI, VIETNAM
   MOVEMENT PAST 6 HOURS: EAST-NORTHEASTWARD AT 02 KTS
   SIGNIFICANT WAVE HEIGHT: 10 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS PERSISTENT
DEEP CONVECTION DISPLACED TO THE SOUTHWEST OF THE ASSESSED CENTER
LOCATION DUE TO APPARENT PRESSURE FROM NORTHEASTERLY SHEAR. SOME
FORMATIVE CONVECTIVE BANDING CAN ALSO BE SEEN IN THE NORTHEAST
QUADRANT IN THE ANIMATED MSI. ANIMATED RADAR IMAGERY DEPICTS A
LOOSELY DEFINED CYCLONIC ROTATION IN THE GULF OF TONKIN WITH THE
HEAVIEST PRECIPITATION DISPLACED TO THE SOUTHWEST UNDER THE
CONVECTION, AND IN THE FORMATIVE CONVECTIVE BANDING. THE INITIAL
POSITION IS PLACED WITH LOW CONFIDENCE BASED ON THE ROUGH CENTER OF
ROTATION IN ANIMATED RADAR IMAGERY. THE INITIAL INTENSITY OF 30 KTS
IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE AGENCY DVORAK FIXES
AND CIMSS INTENSITY ESTIMATES, ALTHOUGH THERE HAS BEEN A LACK OF
RECENT WIND SPEED DATA TO CONFIRM. THE ENVIRONMENT IS CHARACTERIZED
AS MARGINALLY FAVORABLE, WITH WARM SEA SURFACE TEMPERATURES AND
MODERATE EQUATORWARD OUTFLOW BEING MITIGATED BY MODERATE-HIGH
VERTICAL WIND SHEAR (VWS).

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

CURRENT STEERING MECHANISM: WEAK STEERING ENVIRONMENT

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   RJTD: T1.5 - 25 KTS
   KNES: T2.0 - 30 KTS
   CIMSS ADT: 27 KTS AT 210010Z
   CIMSS AIDT: 26 KTS AT 210010Z
   CIMSS D-MINT: 27 KTS AT 202318Z
   CIMSS D-PRINT: 35 KTS AT 210110Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 20-25 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE EQUATORWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: LOW
   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: TROPICAL DEPRESSION (TD) 18W HAS MOVED LITTLE
OVER THE PAST 6 HOURS, TAKING UP A QUASI-STATIONARY POSITION IN THE
GULF OF TONKIN. THE SYSTEM IS EXPECTED TO MIGRATE VERY SLOWLY
THROUGH THE GULF OF TONKIN WITH A PRIMARILY NORTHEASTWARD COMPONENT
AS THERE ARE NO SYNOPTIC FEATURES PRESENT TO DICTATE A STEERING
REGIME. THE SYSTEM IS NOT EXPECTED TO INTENSIFY BEYOND 30 KTS AS
MODERATE-HIGH SHEAR IS EXPECTED TO PERSIST OVER THE GULF OF TONKIN
 THROUGHOUT THE FORECAST PERIOD. THAT SAID, THE ASSOCIATED CONVECTION 
IS 
QUITE POTENT AND SHOULD BE ABLE TO SUSTAIN TROPICAL DEPRESSION 
INTENSITY. 
THE MOST LIKELY SCENARIO AT THIS TIME IS THAT THE SYSTEM WILL 
EVENTUALLY BE
OVERCOME BY THE SHEAR AND DISSIPATE IN THE GULF OF TONKIN BY TAU
120.

MODEL DISCUSSION: MODEL TRACK GUIDANCE SOLUTIONS ALL DEPICT SOME
VERSION OF QUASI-STATIONARY MOTION WITHIN THE GULF OF TONKIN,
HOWEVER, THERE IS NO CONSENSUS ON A SPECIFIED TRACK. THE GENERAL
TREND OF GUIDANCE AND GLOBAL MODEL FIELDS SUGGESTS A SLOW NORTHWARD
MIGRATION WITH A SMALL EASTWARD COMPONENT. THE JTWC FORECAST IS
REPRESENTATIVE OF THIS TREND BUT IS PLACED WITH VERY LOW
CONFIDENCE. MODEL INTENSITY GUIDANCE ALL REMAINS BELOW 35 KTS FOR
THE ENTIRE FORECAST PERIOD, WITH THE EXCEPTION OF GFS-BASED
COAMPS-TC, WHICH DEPICTS AN UNREALISTIC PEAK NEAR 50 KTS.
CONSIDERING THE VWS ENVIRONMENT, THE JTWC INTENSITY FORECAST IS
PLACED ON THE UPPER END OF THE REMAINING GUIDANCE. 

FORECAST CONFIDENCE:
   TRACK 00-72 HR: LOW
   TRACK 72-120 HR: LOW
   INTENSITY 00-72 HR: MEDIUM
   INTENSITY 72-120 HR: MEDIUM//
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