Advisories

WDPN32 PGTW 220900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 18W (NARRA) WARNING 
NR 007//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 21.1N 108.1E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 123 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 MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
DEPRESSION 18W (NARRA) WITH AN ELONGATED PARTIALLY-EXPOSED
LOW-LEVEL CIRCULATION CENTER (LLCC) JUST OFF THE COAST OF NORTHERN 
VIETNAM. BURSTS OF CONVECTION HAVE BEEN FORMING ALONG THE COAST OVER 
THE PAST 4 HOURS, WEST OF THE ASSESSED CENTER. A 220257Z METOP-C ASCAT 
SCATTEROMETER PASS REVEALED THE POORLY-DEFINED CIRCULATION WITH A 
LARGE SWATH OF 25-30 KTS ALONG THE SOUTHERN SEMICIRCLE. THE INITIAL 
POSITION IS PLACED WITH MEDIUM CONFIDENCE BASED ON THE RAGGED LLCC IN 
ANIMATED RADAR IMAGERY AND MSI. THE INITIAL INTENSITY OF 30 KTS IS 
ASSESSED WITH MEDIUM CONFIDENCE BASED PRIMARILY ON THE AFOREMENTIONED 
ASCAT PASS. 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: T2.5 - 35 KTS
   RJTD: T2.0 - 30 KTS
   RCTP: T1.5 - 25 KTS
   KNES: T3.0 - 45 KTS
   CIMSS ADT: 47 KTS AT 220540Z
   CIMSS AIDT: 41 KTS AT 220540Z
   CIMSS D-MINT: 31 KTS AT 220618Z
   CIMSS D-PRINT: 33 KTS AT 220540Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 15-20 KTS
   SST: 30-31 CELSIUS
   OUTFLOW: WEAK 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: 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. 
HOWEVER, 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. THESE FACTORS ARE OFFSET BY THE VERY WARM WATERS WITHIN THE 
GULF OF TONKIN. SOME MODELS, SUCH AS ECMWF, DO SHOW THE NORTHEASTERLY 
SHEAR DECREASING AND THE SYSTEM REMAINING QUASISTATIONARY OVER THE 
GULF OF TONKIN. IF THIS SCENARIO OCCURS, THEN FURTHER INTENSIFICATION 
COULD BE POSSIBLE. AS IT STANDS, THE JTWC FORECAST CALLS FOR A STEADY 
INTENSITY THROUGH TAU 60 BEFORE WEAKENING AS THE VORTEX TRACKS INTO 
SOUTHERN CHINA WITH DISSIPATION AT TAU 96. 

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. GFS SHOWS A TRACK THAT SKIRTS
THE COAST OF VIETNAM, AND THEREFORE WASHES OUT THE CIRCULATION AT
AROUND TAU 48. THE JTWC TRACK FORECAST IS PLACED NEAR THE MULTI-MODEL 
CONSENSUS WITH VERY LOW CONFIDENCE. INTENSITY GUIDANCE IS ALSO IN POOR 
AGREEMENT DUE TO DIFFERING AMOUNTS OF LAND INTERACTION. THE JTWC 
INTENSITY FORECAST IS PLACED NEAR THE HAFS-A 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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