WDPN32 PGTW 221500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 18W (NARRA) WARNING
NR 008//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 21.0N 108.0E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 118 NM EAST OF HANOI, VIETNAM
   MOVEMENT PAST 6 HOURS: SOUTHWESTWARD AT 01 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
DEPRESSION 18W WITH DEEP FLARING CONVECTION DISPLACED TO THE WEST
OF THE PARTIALLY EXPOSED LOW-LEVEL CIRCULATION CENTER (LLCC) OVER
NORTHEASTERN VIETNAM. CLOUD TOPS ARE MEASURED TO BE NEARLY -90 C IN
SOME AREAS. SURFACE OBSERVATIONS FROM BACH LONG VI REPORTED 29 KTS
FROM THE WEST-NORTHWEST AT 221200Z, SUPPORTING THE INITIAL
INTENSITY OF 30 KTS WITH MEDIUM CONFIDENCE. CIMSS INTENSITY
ESTIMATES ARE ANALYZED TO BE OVERESTIMATING THE CURRENT INTENSITY,
LIKELY DUE TO THE VIGOROUS BURSTS OF CONVECTION ASSOCIATED WITH THE
CIRCULATION. ENVIRONMENTAL ANALYSIS INDICATES THAT 18W IS IN A
MARGINAL ENVIRONMENT CHARACTERIZED BY WEAK EQUATORWARD OUTFLOW
ALOFT, LOW TO MODERATE (15-20 KTS) NORTHEASTERLY SHEAR, AND VERY
WARM (30-31 C) SEA SURFACE TEMPERATURES.

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

CURRENT STEERING MECHANISM: COMPETING STEERING ENVIRONMENT WITH A
NEAR EQUATORIAL RIDGE (NER) TO THE SOUTH AND A SUBTROPICAL RIDGE
(STR) TO THE NORTHWEST

AGENCY DVORAK AND AUTOMATED FIXES:
   PGTW: NO CURRENT ESTIMATE
   CIMSS ADT: 39 KTS AT 221210Z
   CIMSS AIDT: 38 KTS AT 221210Z
   CIMSS D-MINT: 41 KTS AT 221016Z
   CIMSS D-PRINT: 41 KTS AT 221210Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 15-20 KTS
   SST: 30-31 CELSIUS
   OUTFLOW: MODERATE EQUATORWARD
   OTHER FACTORS: DRY AIR ENTRAINMENT FROM THE SOUTH

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: TD 18W IS FORECAST TO MAKE A SLOW
COUNTERCLOCKWISE LOOP OVER THE NEXT 72 HOURS AS THE SYSTEM REMAINS
PLACED WITHIN A COMPETING STEERING PATTERN WITH RIDGING TO BOTH THE
NORTH AND SOUTH. IN REALITY THE ACTUAL TRACK OF 18W IS LIKELY TO BE
HIGHLY ERRATIC DURING THIS TIME. AFTER TAU 72, 18W IS FORECAST TO
BE DRIVEN NORTHWARD INTO SOUTHERN CHINA UNDER THE INFLUENCE OF THE
LOW-LEVEL FLOW. THE ENVIRONMENT IS EXPECTED TO REMAIN MARGINAL WITH
MODERATE NORTHEASTERLY SHEAR CONTINUING TO IMPACT THE VORTEX WITH
DRY AIR ENTRAINMENT FROM THE SOUTH. HOWEVER, THESE FACTORS ARE
OFFSET BY THE VERY WARM WATERS WITHIN THE GULF OF TONKIN, WHICH IS
FOREACAST TO ALLOW FOR SLIGHT DEVELOPMENT OVER THE NEXT 36 HOURS.
SOME MODELS, SUCH AS ECMWF, SHOW THE NORTHEASTERLY SHEAR DECREASING
AND THE SYSTEM REMAINING QUASISTATIONARY OVER THE GULF OF TONKIN.
IF THE SHEAR LESSENS, THEN FURTHER FURTHER INTENSIFICATION IS
POSSIBLE. AS IT STANDS, A PEAK INTENSITY OF 40 KTS IS FORECAST
BEFORE 18W IS DRIVEN NORTH-NORTHEASTWARD INTO SOUTHERN CHINA.
DISSIPATION OVER LAND IS EXPECTED OCCUR AROUND TAU 96, QUICKLY
AFTER LANDFALL.

MODEL DISCUSSION: MODEL TRACK GUIDANCE CONTINUES TO BE IN VERY POOR
AGREEMENT, BUT IT GENERALLY SHOWS A SOUTHWARD TURN THROUGH TAU 48
FOLLOWED BY A TURN BACK TO THE NORTH. THE JTWC TRACK FORECAST IS
PLACED NEAR THE MULTI-MODEL CONSENSUS WITH VERY LOW CONFIDENCE DUE
TO THE LARGE SPREAD IN GUIDANCE. GFS SHOWS A TRACK THAT SKIRTS THE
COAST OF VIETNAM, AND THEREFORE WEAKENS THE VORTEX MUCH MORE
QUICKLY THAN ECMWF, WHICH KEEPS THE SYSTEM CENTERED OVER WATER.
INTENSITY GUIDANCE IS ALSO IN POOR AGREEMENT DUE TO DIFFERING
AMOUNTS OF LAND INTERACTION AND AMOUNT OF TIME OVER WATER BETWEEN
MODELS. THE JTWC INTENSITY FORECAST IS ALSO PLACED NEAR THE
CONSENSUS THROUGHOUT THE FORECAST PERIOD WITH LOW CONFIDENCE.

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