MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (TWENTYONE) WARNING NR 006// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM (TS) 21W (NEOGURI), LOCATED APPROXIMATELY 408 NM SOUTH-SOUTHEAST OF KADENA AB, HAS TRACKED NORTH-NORTHWESTWARD AT 06 KNOTS (KTS) OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY REVEALS TWO AREAS OF DEEP CONVECTION THAT ARE OBSCURING THE LOW LEVEL CIRCULATION CENTER. THE POSITION IS PLACED WITH LOW CONFIDENCE BETWEEN THESE TWO LOBES. THE INITIAL INTENSITY OF 40 KTS IS SET ABOVE THE MULTI-AGENCY DVORAK ESTIMATES OF T2.0-T2.5 (30-35 KTS), BUT BELOW THE 172010Z ADVANCED DVORAK TECHNIQUE ESTIMATE 53 KTS. THE ENVIRONMENT IS FAVORABLE WITH MODERATE (15-20 KTS) VERTICAL WIND SHEAR (VWS), GOOD POLEWARD OUTFLOW, AND WARM (28-29 CELSIUS) SEA SURFACE TEMPERATURE (SST). TS 21W IS TRACKING SLOWLY NORTH-NORTHWESTWARD TOWARDS A BREAK IN A SUBTROPICAL RIDGE (STR) WITH LIMITED STEERING. 3. FORECAST REASONING. A. THE FORECAST TRACK HAS BEEN SHIFTED TO THE RECURVE SCENARIO. B. IN THE NEAR TERM, TS 21W IS EXPECTED TO GRADUALLY SHIFT TO A NORTHWESTERLY TRACK AS RIDGING SLOWLY BUILDS TO THE NORTHEAST AND TAKES OVER STEERING. THERE IS SIGNIFICANT UNCERTAINTY IN THE TIMING AND STRENGTH OF THE STR. DUE TO THIS UNCERTAINTY, THERE IS POOR CONFIDENCE IN THE TRACK GUIDANCE MEMBERS WITH A SPREAD OF APPROXIMATELY 400NM BY TAU 72. THE UPPER LEVEL ENVIRONMENT HAS LIMITED THE INTENSIFICATION OF TS 21W TO THIS POINT. HOWEVER, WITH THE APPROACHING TROUGH TO THE NORTHWEST OF TS 21W, THIS MAY STRENGTHEN THE POLEWARD OUTFLOW AND CAUSE SIGNFICANT INTENSITY CHANGES. TS 21W IS EXPECTED TO CROSS THE STR AXIS BY TAU 72 AND BEGIN RECURVING TO THE NORTHEAST. ECMWF SHOWS A WESTWARD TRACK THAT HAS YET TO MATERIALIZE. THE REMAINING CONSENSUS MEMBERS HAVE SHIFTED TO A RECURVE SCENARIO. THE JTWC TRACK FORECAST IS PLACED TO THE NORTHEAST OF THESE REMAINING MEMBERS AS TS 21W MAY CONTINUE TO TRACK TOWARDS THIS BREAK IN THE STR BEFORE GAINING STEERING GUIDANCE. DUE TO THE AFOREMENTIONED FACTORS, THERE IS OVERALL LOW TRACK AND INTENSITY CONFIDENCE IN THIS PORTION OF THE FORECAST. C. BEYOND TAU 72, TS 21W IS EXPECTED TO RECURVE AS THE APPROACHING TROUGH WEAKENS THE STR AND TS 21W ENCOUNTERS SOUTHWESTERLY FLOW. TS 21W IS CURRENTLY EXPECTED TO ENCOUNTER INCREASED SHEAR WHICH WILL LIKELY WEAKEN TS 21W ALONG ITS POLEWARD TRACK. WITH SPREAD STILL EXCEEDING 500 NM BY TAU 120, THERE IS LOW CONFIDENCE IN THE EXTENDED PORTION OF THE JTWC TRACK FORECAST.// NNNN NNNN