WDIO31 PGTW 252100 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 04B (FOUR) WARNING NR 002// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 4.7N 98.4E INITIAL INTENSITY: 45 KTS GEOGRAPHIC REFERENCE: 614 NM SOUTHWEST OF HO CHI MINH CITY, VIETNA MOVEMENT PAST 6 HOURS: WESTWARD AT 07 KTS SIGNIFICANT WAVE HEIGHT: 16 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A RAGGED AND ELONGATED CENTRAL DENSE OVERCAST (CDO) FEATURE, WITH A SHARP EASTERN EDGE INDICATIVE OF THE EASTERLY SHEAR VECTOR. ANIMATED RADAR DATA FROM INDONESIA SHOWS A VERY WEAK LOW LEVEL CIRCULATION CENTER (LLCC) MOVING SLOWLY WESTWARD TOWARDS THE COAST, WITH DEEP CONVECTION PRIMARILY LOCATED ALONG THE EASTERN FLANK. A 251858Z NOAA-21 ATMS 183GHZ MICROWAVE IMAGE REVEALED THAT UNDER THE HOOD, THE CONVECTIVE STRUCTURE IS HIGHLY ELONGATED ALONG A GENERAL NORTH-SOUTH AXIS TO THE EAST OF THE LLCC. ANIMATED WATER VAPOR IMAGERY AND CIMSS ATMOSPHERIC MOTION VECTOR (AMV) ANALYSIS INDICATE A GENERAL EASTERLY FLOW PATTERN UPSTREAM (EAST) OF TC 04B, AND SPLIT FLOW OVER THE STRAITS OF MALACCA AND IN THE VICINITY OF TC 04B, WHICH HAS ALLOWED FOR THE PERSISTENCE OF THE DEEP CONVECTION AND INTENSIFICATION OF THE SYSTEM OVER THE PAST DAY. THE INITIAL POSITION IS ASSESSED WITH MEDIUM CONFIDENCE BASED PRIMARILY ON THE RADAR DATA ANALYSIS. THE INITIAL INTENSITY IS ASSESSED WITH MEDIUM CONFIDENCE, HEDGED ABOVE THE AGENCY DVORAK CURRENT INTENSITY ESTIMATES AND CONSISTENT WITH THE OBJECTIVE ESTIMATES NOTED BELOW. ENVIRONMENTAL CONDITIONS ARE MARGINALLY FAVORABLE, WITH LOW TO MODERATE EASTERLY SHEAR, WARM SSTS AND MODERATE WESTWARD AND EQUATORWARD OUTFLOW. HOWEVER, THE PROXIMITY TO LAND IS INHIBITING THE SYSTEM SOMEWHAT. INITIAL WIND RADII BASIS: PERSISTENCE CURRENT STEERING MECHANISM: SOUTHERN PERIPHERY OF A DEEP-LAYER SUBTROPICAL RIDGE (STR) OVER THAILAND. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T2.0 - 30 KTS KNES: T2.0 - 30 KTS DEMS: T2.0 - 30 KTS CIMSS SATCON: 46 KTS AT 251858Z CIMSS ADT: 43 KTS AT 251730Z CIMSS AIDT: 33 KTS AT 251730Z CIMSS D-MINT: 51 KTS AT 251858Z CIMSS D-PRINT: 51 KTS AT 251900Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE VWS: 15-20 KTS SST: 29-30 CELSIUS OUTFLOW: MODERATE EQUATORWARD OTHER FACTORS: PROXIMITY TO LAND ANALYSIS CONFIDENCE: INITIAL POSITION: MEDIUM INITIAL INTENSITY: MEDIUM INITIAL WIND RADII: LOW 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: THE LLCC ASSOCIATED WITH TC 04B HAS TRACKED WESTWARD OVER THE PAST 12 HOURS AND IS EXPECTED TO CONTINUE DOING SO OVER THE NEXT 12. LANDFALL TO THE NORTH OF LANGSA, INDONESIA IS EXPECTED WITHIN THE NEXT FEW HOURS. THE TRACK AFTER TAU 12 BECOMES HIGHLY UNCERTAIN DUE TO UNCERTAINTIES IN HOW THE LLCC WILL INTERACT WITH THE RUGGED AND HIGH TERRAIN FEATURES OF NORTHERN SUMATRA AND THE FACT THAT THIS REGION HAS NEVER HAD A DIRECT TC IMPACT BEFORE, SO THERE IS NO DATA TO SUPPORT HOW THE VORTEX WILL MOVE AS IT CROSSES THE TERRAIN FEATURES. THE GFS JUMPS THE MID-LEVEL VORTEX TO THE INDIAN OCEAN SIDE OF SUMATRA AND THEN REDEVELOPS IT, WHILE THE ECMWF KEEPS THE LLCC ON THE EAST SIDE OF SUMATRA THROUGH THE ENTIRE FORECAST PERIOD. THE CONSENSUS IS THAT THE SYSTEM WILL TRACK WESTWARD TO THE MOUNTAIN RIDGELINE, THEN TURN SHARPLY TO THE SOUTHEAST ALONG THE RIDGELINE, WHILE STEADILY WEAKENING DUE PRIMARILY TO FRICTIONAL EFFECTS AND TERRAIN INDUCED VORTEX DECOUPLING. DISSIPATION OVER THE MOUNTAINS IS EXPECTED WITHIN THE NEXT 36 HOURS. REDEVELOPMENT OF THE SYSTEM IN THE INDIAN OCEAN BY TAU 36 IS POSSIBLE BUT AT THE MOMENT, THIS IS CONSIDERED AN UNLIKELY SCENARIO. MODEL DISCUSSION: DETERMINISTIC MODEL GUIDANCE IS WIDELY DISPERSED, WITH LITTLE TO NO AGREEMENT BETWEEN THE VARIOUS MODELS OF THE CONSENSUS. AS MENTIONED ABOVE, THE ECMWF, ECEPS AND EC-AIFS TRACK THE SYSTEM SOUTHEAST FROM THE START OF THE FORECAST, KEEPING THE SYSTEM OVER THE STRAITS OF MALACCA FOR THE NEXT FEW DAYS. THE GFS, NAVGEM, GEFS MEANWHILE JUMP THE VORTEX TO THE INDIAN OCEAN, REDEVELOP IT, THEN CROSS BACK TO THE EAST, ULTIMATELY CROSSING SUMATRA AGAIN AND MAKING LANDFALL IN WESTERN MALAYSIA AROUND DAY 5. THE MULTI-MODEL ENSEMBLE GUIDANCE SHOWS A VERY SIMILAR TRACK TO THE JTWC FORECAST AND PROVIDES AT LEAST SOME LEVEL OF CONFIDENCE IN THE FORECAST TRACK, WHICH IS PLACED NEAR THE CONSENSUS MEAN WITH LOW CONFIDENCE. INTENSITY GUIDANCE INDICATES A FLAT OR SLIGHTLY WEAKENING TREND THROUGH TAU 12, FOLLOWED BY MORE SIGNIFICANT WEAKENING THEREAFTER, THOUGH THE MODELS THAT TRACK THE SYSTEM INTO THE INDIAN OCEAN INDICATE REINTENSIFICATION. OVERALL, THE JTWC FORECAST IS MOST CONSISTENT WITH THE HWRF SOLUTION, THOUGH CONFIDENCE IS LOW. FORECAST CONFIDENCE: TRACK 00-72 HR: LOW INTENSITY 00-72 HR: LOW// NNNN