WDPN31 PGTW 151500 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TYPHOON 17W (SANBA) WARNING NR 20// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TYPHOON (TY) 17W (SANBA), LOCATED APPROXIMATELY 115 NM SOUTH-SOUTHEAST OF KADENA AIR BASE, OKINAWA, JAPAN, HAS TRACKED NORTH-NORTHWESTWARD AT 12 KNOTS OVER THE PAST SIX HOURS. ANIMATED INFRARED (IR) SATELLITE IMAGERY SHOWS THE SYSTEM HAS MAINTAINED VERY DEEP CONVECTIVE BANDS TIGHTLY WRAPPED INTO A SYMMETRICAL 30-NM EYE. A 151146Z 91 GHZ SSMI-S MICROWAVE IMAGE SHOWS THE DEEPEST CONVECTION IS ALONG THE NORTHEASTERN SEMICIRCLE AND SHOWING SIGNS OF ELONGATION. THE 37 GHZ CHANNEL OF THE SAME SSMI-S PASS, WHICH IS INTERROGATING CLOSER TO THE SURFACE, PAINTS THE LOW LEVEL CIRCULATION SLIGHTLY TO THE SOUTHWEST OF THE 91 GHZ CHANNEL. THIS INDICATES THE SYSTEM IS TILTED TO THE NORTHEAST, LIKELY DUE TO SOUTHWESTERLY VERTICAL WIND SHEAR (VWS). THE INITIAL POSITION IS BASED ON THE 37 GHZ SSMI-S MICROWAVE EYE WITH HIGH CONFIDENCE. THE INITIAL INTENSITY IS BASED ON DVORAK ESTIMATES FROM PGTW, RJTD AND KNES. UPPER LEVEL ANALYSIS SHOWS TY 17W HAS CONTINUED TO MAINTAIN GOOD RADIAL OUTFLOW AND A STRONG POLEWARD OUTFLOW, ENHANCED BY ITS INITIAL EXPOSURE TO THE SUBTROPICAL JET STREAM. THE DEEP-LAYERED SUBTROPICAL RIDGE (STR) TO THE EAST OF JAPAN REMAINS THE PREDOMINANT STEERING MECHANISM. 3. FORECAST REASONING. A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING MESSAGE. B. TY 17W IS FORECAST TO CONTINUE TRACKING NORTH-NORTHWESTWARD ALONG THE WESTERN PERIPHERY OF THE STR. A MID-LATITUDE TROUGH APPROACHING FROM THE NORTHWEST WILL SLIGHTLY WEAKEN THE RIDGE AND CAUSE THE SYSTEM TO TRACK ON A MORE NORTH-NORTHEASTWARD TRAJECTORY AFTER TAU 24. AS TY 17W TRACKS DEEPER INTO THE SUBTROPICAL JET STREAM, THE VWS WILL SIGNIFICANTLY INCREASE AND WILL BE THE PREDOMINANT FACTOR IN THE GRADUAL WEAKENING OF THE SYSTEM. TYPHOON SANBA WILL MAKE LANDFALL WEST OF PUSAN, SOUTH KOREA, JUST BEFORE TAU 36 THEN COMMENCE EXTRA-TROPICAL (ET) TRANSITION BY TAU 48. THE CYCLONE WILL BECOME A FULL ET LOW BY TAU 72 AFTER IT BRIEFLY CROSSES THE SEA OF JAPAN AND MAKES A SECOND AND FINAL LANDFALL NEAR THE NORTH KOREA/RUSSIA BORDER. THE AVAILABLE NUMERICAL GUIDANCE IS IN TIGHT AGREEMENT WITH WBAR THE SOLE LEFT OUTLIER. THERE IS HIGH CONFIDENCE IN THE TRACK FORECAST, WHICH IS ADJUSTED SLIGHTLY RIGHT AND AHEAD OF CONSENSUS TO OFFSET WBAR. // NNNN