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 COMPETEING 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 DOES OCCUR, 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// NNNN