MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (NEOGURI) WARNING NR 008// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM (TS) 21W (NEOGURI), LOCATED APPROXIMATELY 401 NM SOUTH-SOUTHEAST OF KADENA AB, HAS TRACKED NORTHWESTWARD AT 03 KNOTS (KTS) OVER THE PAST SIX HOURS. THERE IS FAIR CONFIDENCE IN THE INITIAL POSITION BASED ON ANIMATED ENHANCED INFRARED SATELLITE IMAGERY, REVEALING A SMALL SYSTEM WITH DEEP CONVECTION PRIMARILY NEAR THE CENTER AND IN THE NORTHEAST QUADRANT. THE LOW LEVEL CIRCULATION CENTER WAS DETERMINED BY FOLLOWING SHALLOW BANDING TO THE WEST AND SOUTH UNDER THE CIRRUS SHIELD. THE INITIAL INTENSITY OF 45 KTS IS CONSISTENT WITH THE PGTW DVORAK CURRENT INTENSITY ESTIMATE OF T3.0 (45 KTS) AND A 180050Z METOP-B ASCAT PASS SHOWING A VERY SMALL WIND FIELD AND AN AREA OF 45 KT WINDS TO THE NORTH. WITH LOW (10-15 KT) VERTICAL WIND SHEAR, GOOD POLEWARD OUTFLOW, FAIR EQUATORWARD OUTFLOW, AND WARM (27-28 CELSIUS) SEA SURFACE TEMPERATURE, THE ENVIRONMENT APPEARS FAVORABLE. HOWEVER, UPPER LEVEL CONVERGENCE TO THE WEST IS CAUSING SUBSIDENCE AND DRY AIR ENTRAINMENT THAT IS LIKELY PARTIALLY OFFSETTING THE OTHERWISE FAVORABLE ENVIRONMENT TO LIMIT INTENSIFICATION. TS 21W IS TRACKING SLOWLY ALONG THE SOUTHWESTERN PERIPHERY OF A SUBTROPICAL RIDGE (STR). 3. FORECAST REASONING. A. THERE IS NO SIGNIFICANT CHANGE TO THE FORECAST PHILOSOPHY FROM THE PREVIOUS PROGNOSTIC REASONING MESSAGE. B. THROUGH TAU 36, TS 21W WILL CONTINUE TO TRACK ALONG THE SOUTHWESTERN PERIPHERY OF THE STR. AFTER TAU 36, THE SYSTEM WILL BEGIN GRADUALLY TURNING POLEWARD AS IT ROUNDS THE STR AXIS. THE MARGINALLY FAVORABLE ENVIRONMENT SHOULD ALLOW SOME INTENSIFICATION THROUGH TAU 36. AFTER TAU 36, HIGH VWS SHOULD PREVENT FURTHER INTENSIFICATION DESPITE A BRIEF PERIOD WITH ENHANCED POLEWARD OUTFLOW AS THE SYSTEM BEGINS ROUNDING THE STR AXIS. THERE IS LOW CONFIDENCE IN THE NEAR TERM INTENSITY FORECAST DUE TO THE COMPETING INFLUENCES OF VWS AND OUTFLOW NEAR TAU 48. DESPITE THE BULK OF THE GUIDANCE NOW DEPICTING A RECURVE SCENARIO, THE TRACK FORECAST IS HIGHLY UNCERTAIN AS THE ECMWF AND ECMWF ENSEMBLE STILL SUGGEST A WESTWARD TRACK OR EVEN A TURN SOUTHWESTWARD IS POSSIBLE. THROUGH TAU 72, THE JTWC TRACK FORECAST CLOSELY FOLLOWS THE CONSENSUS WITH LOW CONFIDENCE DUE TO HIGH MODEL UNCERTAINTY. C. BEYOND TAU 72, TS 21W IS EXPECTED TO TRACK NORTHEASTWARD AND, BY TAU 120, BEGIN EXTRATROPICAL TRANSITION AS IT ENCOUNTERS THE MID- LATITUDE BAROCLINIC ZONE. THE INTENSITY FORECAST HAS VERY LARGE SPREAD WITH MEMBERS TRACKING ANYWHERE FROM A RECURVE SITUATION (UKMET ENSEMBLE MEAN, JGSM, GALWEM, UKMET) TO A WESTWARD TRACK (ECMWF, ECMWF ENSEMBLE MEAN). ADDITIONALLY, THE MEMBERS DEPICTING A RECURVE HAVE SIGNIFICANTLY DIFFERENT ALONG TRACK SPEEDS. THE ACTUAL TRACK WILL DICTATE THE INTENSITY BUT WITH THE CURRENT FORECAST TRACK, HIGH VWS WILL WEAKEN THE SYSTEM CONSIDERABLY. BY TAU 120, SPREAD EXCEEDS 1400 NM BETWEEN THE MOST EXTREME MEMBERS AND CONFIDENCE IN THE TRACK FORECAST IS VERY LOW.// NNNN NNNN