MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 20W (TALIM) WARNING NR 35// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM 20W (TALIM), LOCATED APPROXIMATELY 114 NM SOUTH OF IWAKUNI, JAPAN, HAS TRACKED NORTHEASTWARD AT 21 KNOTS OVER THE PAST SIX HOURS. THE CURRENT POSITION IS BASED ON RECENT AGENCY SATELLITE POSITION FIXES AND OBSERVATIONAL AND RADAR DATA FROM THE JAPAN METEOROLOGICAL AGENCY. THE INTENSITY OF 55 KNOTS IS CONSISTENT WITH SUBJECTIVE DVORAK INTENSITY ESTIMATES FROM MULTIPLE REPORTING AGENCIES, A VARIETY OF AUTOMATED SATELLITE INTENSITY ESTIMATES AND SCATTEROMETER DATA FROM A 170130Z ASCAT PASS. TS 20W IS ACCELERATING NORTHEASTWARD ALONG THE NORTHWESTERN PERIPHERY OF A SUBTROPICAL STEERING RIDGE, AND IS MERGING INTO THE FLOW ASSOCIATED WITH AN APPROACHING MID-LATITUDE TROUGH. STRONG UPPER-LEVEL DIVERGENCE AHEAD OF THE TROUGH HAS FUELED A FLAIR OF DEEP CONVECTION OVER THE NORTHERN PORTION OF THE CIRCULATION OVER THE PAST SEVERAL HOURS. 3. FORECAST REASONING. A. THE FORECAST TRACK HAS SHIFTED SLIGHTLY TO THE RIGHT BASED ON RECENT STORM MOTION RELATIVE TO THE PREVIOUS FORECAST. HOWEVER, THERE IS NO SIGNIFICANT CHANGE TO THE FORECAST PHILOSOPHY FROM THE PREVIOUS PROGNOSTIC REASONING MESSAGE. B. TS 20W WILL CONTINUE TO ACCELERATE NORTHEASTWARD AS IT MERGES WITH THE AFOREMENTIONED MID-LATITUDE TROUGH OVER THE NEXT DAY AND A HALF. EXTRATROPICAL TRANSITION WILL BEGIN SHORTLY, COMPLETING BY TAU 24. FAVORABLE BAROCLINIC INTERACTION BETWEEN TS 20W AND THE TROUGH DURING EXTRATROPICAL TRANSITION WILL ALLOW THE SYSTEM TO MAINTAIN A STRONG AND EXPANSIVE LOW-LEVEL WIND FIELD. IN FACT, THE OVER-WATER AREA INFLUENCED BY GALE-FORCE WINDS IS EXPECTED TO GROW THROUGHOUT THE FORECAST PERIOD. GIVEN THE SIZE OF THIS WIND FIELD, POTENTIALLY SIGNIFICANT WEATHER IMPACTS SHOULD BE ANTICIPATED THROUGHOUT MAINLAND JAPAN - PARTICULARLY IN COASTAL AREAS - FOR THE NEXT 36 HOURS. GIVEN CONTINUED STRONG AGREEMENT AMONG FORECAST MODELS, THERE IS HIGH CONFIDENCE IN THE CURRENT JTWC OFFICIAL TRACK FORECAST.// NNNN NNNN