Advisories

WDPN32 PGTW 202100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 18W (EIGHTEEN) 
WARNING NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 20.1N 108.4E
   INITIAL INTENSITY: 25 KTS
   GEOGRAPHIC REFERENCE: 150 NM EAST-SOUTHEAST OF HANOI, VIETNAM
   MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 08 KTS
   SIGNIFICANT WAVE HEIGHT: 9 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS POORLY
ORGANIZED DEEP CONVECTION DISPLACED TO THE SOUTHWEST OF THE
ASSESSED CENTER LOCATION DUE TO APPARENT PRESSURE FROM
NORTHEASTERLY SHEAR. 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. THE
INITIAL POSITION IS PLACED WITH LOW CONFIDENCE BASED ON THE
ROTATION IN ANIMATED RADAR. THE INITIAL INTENSITY OF 25 KTS IS
ASSESSED WITH LOW CONFIDENCE BASED ON EARLIER SCATTEROMETRY DATA
AND T1.0-1.5 AGENCY DVORAK FIXES. 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: T1.5 - 25 KTS
   RJTD: T1.0 - 25 KTS

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: LOW
   INITIAL WIND RADII: NOT APPLICABLE

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING
MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY.

FORECAST DISCUSSION: TROPICAL DEPRESSION (TD) 18W HAS TRAVERSED
WESTWARD OFF THE COAST OF HAINAN OVER THE PAST 6 HOURS AND APPEARS
TO BE 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 NORTHWARD COMPONENT AS THERE ARE NO
SYNOPTIC FEATURES PRESENT TO DICTATE A STEERING REGIME. THE SYSTEM
IS NOT EXPECTED TO INTENSIFY BEYOND 30 KTS BASED ON THE INITIAL
PROGNOSIS OF THE ENVIRONMENT, WITH MODERATE-HIGH SHEAR 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.

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//
NNNN