MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TYPHOON 17W (SANBA) WARNING NR 25// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TYPHOON 17W (SANBA), LOCATED APPROXIMATELY 205 NM SOUTH-SOUTHWEST OF PUSAN, SOUTH KOREA, HAS TRACKED NORTHWARD AT 17 KNOTS OVER THE PAST SIX HOURS. THE CURRENT POSITION IS BASED ON RECENT SATELLITE FIXES FROM PGTW AND RJTD AND A 19800Z RADAR FIX FROM JAPAN. THE CURRENT INTENSITY OF 85 KNOTS IS CONSISTENT WITH DVORAK ESTIMATES FROM KNES, PGTW, AND RJTD. RECENT ANIMATED INFRARED SATELLITE IMAGERY SHOWS THE CONVECTIVE STRUCTURE OF 17W BEGINNING TO BREAK DOWN AS THE SYSTEM ENTERS AN AREA OF INCREASING SOUTHWESTERLY VERTICAL WIND SHEAR AHEAD OF A MIDLATITUDE TROUGH TO THE NORTHWEST. DESPITE INCREASING SHEAR, THE CYCLONE HAS MANAGED TO MAINTAIN A ROUGHLY 8 NM RAGGED EYE FEATURE FOR THE PAST SIX HOURS. TY 17W CONTINUES TO TRACK POLEWARD AHEAD OF THE AFOREMENTIONED MIDLATITUDE TROUGH UNDER THE STEERING INFLUENCE OF A DEEP-LAYERED SUBTROPICAL RIDGE POSITIONED TO THE EAST OF JAPAN. 3. FORECAST REASONING. A. NO SIGNIFICANT CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING MESSAGE. B. TY 17W WILL ACCELERATE NORTH-NORTHEASTWARD ACROSS THE KOREAN PENINSULA OVER THE NEXT 24 HOURS IN THE GRADIENT BETWEEN THE CURRENT STEERING RIDGE AND THE APPROACHING MIDLATITUDE TROUGH THE NORTHWEST. THE SYSTEM WILL BEGIN EXTRATROPICAL TRANSITION AS IT ENTERS A BAROCLINIC ZONE WITHIN THE NEXT 12 HOURS AND WILL RAPIDLY TRANSITION INTO A FULLY EXTRATROPICAL LOW BY TAU 24. ALTHOUGH INTERACTION WITH THE TAEBAEK MOUNTAIN RANGE AND INCREASING VERTICAL WIND SHEAR ARE EXPECTED TO INDUCE STEADY WEAKENING, THE SYSTEM SHOULD MAINTAIN GALE- FORCE INTENSITY UNDER THE INFLUENCE STRONG POLEWARD DIVERGENCE ALOFT DURING EXTRATROPICAL TRANSITION. THE AVAILABLE NUMERICAL GUIDANCE REMAINS IN VERY TIGHT AGREEMENT. THE CURRENT FORECAST LIES SLIGHTLY TO THE RIGHT AND FASTER THAN THE MODEL CONSENSUS IN ANTICIPATION OF A SLIGHTLY STRONGER NORTH-NORTHEASTWARD STEERING FLOW THAN DEPICTED IN MANY OF THE CONSENSUS MEMBER MODEL FIELDS. THERE IS HIGH CONFIDENCE IN THE TRACK FORECAST.// NNNN NNNN