MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 27W (SAOLA) WARNING NR 21// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM 27W (SAOLA), LOCATED APPROXIMATELY 256 NM NORTH OF YAP, HAS TRACKED WESTWARD AT 24 KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY DEPICTS THE MAIN CONVECTIVE AREA SHEARED SOUTHWESTWARD OF THE WEAK AND POORLY ORGANIZED LOW LEVEL CIRCULATION CENTER (LLCC). OVER THE PAST 6 HOURS, THERE HAS BEEN A FLARE UP OF WEAK CONVECTION NEAR THE LLCC, BUT THE MAJORITY OF THE DEEPEST ACTIVITY REMAINS ISOLATED TO THE SOUTHWEST QUADRANT. A 241036Z OSCAT SCATTEROMETER PASS INDICATED THAT THE LLCC WAS FURTHER SOUTH THAN THE ORIGINAL 241200Z INITIAL POSITION, LEADING TO A RELOCATION OF THE 1200Z BEST TRACK POSITION SIGNIFICANTLY SOUTHWARD. THE CURRENT (241800Z) INITIAL POSITION CONTINUES THE TREND OF WESTWARD MOVEMENT SEEN OVER THE PREVIOUS 12 HOURS, THOUGH THE POSITION IS PLACED WITH RELATIVELY LOW CONFIDENCE BASED ON THE AVAILABLE INFRARED IMAGERY. A 241536Z NPP ATMS MICROWAVE IMAGE SHOWED A BROAD, ILL-DEFINED LLCC, WHICH ASSISTED IN THE PLACEMENT OF THE INITIAL POSITION. THE INITIAL INTENSITY OF 35 KNOTS IS BASED ON A COMBINATION OF DVORAK CURRENT INTENSITIES OF 2.5 (35 TO 45 KTS) FROM PGTW AND RJTD, AND AUTOMATED DVORAK INTENSITIES OF 2.3 (33 KTS). THE INTENSITY ESTIMATE IS ALSO SUPPORTED BY THE OSCAT PASS SHOWING MOSTLY 30 KNOT WIND BARBS WITH ONLY A COUPLE 35 KNOT BARBS. ENVIRONMENTAL CONDITIONS REMAIN MARGINAL, WITH MODERATE TO UNFAVORABLE (15-25 KNOTS) EASTERLY VERTICAL WIND SHEAR (VWS) AND HIGH SEA SURFACE TEMPERATURES (31 DEG CELSIUS). THE MAIN LIMITING FACTOR IS THE RAPID MOVEMENT AND THE LIMITED OUTFLOW, WHICH IS BEING HINDERED BY AN UPPER-LEVEL INVERTED TROUGH JUST EAST OF THE SYSTEM, BRINGING CONVERGENT FLOW TO THE NORTH SIDE OF THE SYSTEM. CURRENTLY THERE ARE INDICATIONS OF A WEAK POLEWARD CHANNEL DEVELOPING TO THE NORTHWEST OF THE SYSTEM. TS SAOLA CONTINUES TO TRACK RAPIDLY WESTWARD UNDER THE INFLUENCE OF A DEEP SUBTROPICAL RIDGE (STR) LOCATED TO THE NORTHEAST. THE TRACK HAS FLATTENED SIGNIFICANTLY OVER THE PAST TWELVE HOURS AS THE STR HAS BUILT WESTWARD, FORCING 27W TO A WESTWARD TRACK. 3. FORECAST REASONING. A. THE SHORT-TERM TRACK HAS SHIFTED SIGNIFICANTLY SOUTHWARD BASED ON THE UPDATED INITIAL POSITION. B. TS 27W WILL CONTINUE TO TRACK TO THE WEST-NORTHWEST FOR THE NEXT 12 HOURS ALONG THE SOUTHWESTERN PERIPHERY OF THE STR. BY TAU 24, THE STR BEGINS TO REORIENT TO A MORE NORTH-SOUTH ORIENTATION, WHILE SIMULTANEOUSLY A BREAK IN THE RIDGE PATTERN DEVELOPS NEAR 13OE LONGITUDE, ALLOWING SAOLA TO TURN NORTHWARD AFTER TAU 36. TRACK SPEEDS WILL SLOW FROM TAU 12 THROUGH 72, AS THE SYSTEM ENTERS A WEAKER STEERING ENVIRONMENT AS IT APPROACHES THE WEAKNESS IN THE RIDGE. THE GENERALLY NORTHWARD TRACK WILL PERSIST UNTIL THE SYSTEM ROUNDS THE RIDGE AXIS NEAR TAU 60, SOUTHEAST OF OKINAWA, THEREAFTER ACCELERATING AND TURNING NORTHEASTWARD. THE SYSTEM IS FORECAST TO MAINTAIN INTENSITY FOR 12 HOURS DUE TO THE PERSISTENCE OF THE CONVERGENT FLOW ON THE NORTH SIDE AND WEAK OUTFLOW CONDITIONS. SLOW INTENSIFICATION RESUMES THROUGH TAU 72, AS THE SYSTEM MOVES INTO A BETTER OUTFLOW ENVIRONMENT, ESTABLISHES A POLEWARD CHANNEL AND VWS DECREASES, PEAKING AT 70 KNOTS BY TAU 72 AS IT ROUNDS THE RIDGE AXIS. DYNAMIC MODEL GUIDANCE IS IN GOOD AGREEMENT THROUGH TAU 24, THOUGH THE JTWC FORECAST IS TO SOUTH OF ALL MODEL GUIDANCE THROUGH TAU 24, IN RESPONSE TO THE CURRENT MOTION TO THE WEST. BEYOND THIS POINT THE JTWC TRACK BECOMES MORE IN LINE WITH THE MULTI-MODEL CONSENSUS, THOUGH THE OVERALL SPREAD OF THE MODELS INCREASES AFTER TAU 48. ECMWF AND GFS ARE THE WESTERN OUTLIERS, SHOWING A LANDFALL OVER OKINAWA AROUND TAU 84. NAVGEM AND HWRF ARE THE EASTERN OUTLIERS, SOME 400NM EAST OF OKINAWA. WITH THE HIGH DEGREE OF UNCERTAINTY BEYOND TAU 48, AND THE RECENT TRACK CHANGES, THERE IS LOW CONFIDENCE IN THIS PORTION OF THE JTWC TRACK FORECAST. C. AFTER TAU 72, TS 27W WILL ROUND THE STR AXIS, ACCELERATING RAPIDLY TO THE NORTHEAST THROUGH THE END OF THE FORECAST PERIOD. AS THE SYSTEM BEGINS TO INTERACT WITH AN APPROACHING MID-LATITUDE TROUGH, IT COULD POTENTIALLY REACH A SLIGHTLY HIGHER INTENSITY THAN SHOWN ON THE FORECAST, AFTER TAU 72, BEFORE THE BEGINNING OF EXTRATROPICAL TRANSITION (ET). AS THE SYSTEM PASSES 30 DEG NORTH BY TAU 96, IT WILL BEGIN TO UNDERGO ET BECOMING FULLY ET BY TAU 120. DYNAMIC MODEL GUIDANCE HAS SIGNIFICANT SPREAD AND UNCERTAINTY IN THE LATER PERIOD OF THE FORECAST, BOTH CROSS AND ALONG TRACK. THE JTWC TRACK IS NEAR THE CONSENSUS, BUT SLIGHTLY FASTER. THE PRINCIPLE OUTLIERS ARE GFS AND HWRF, BOTH INDICATING A SHARPER TURN AND LYING TO THE SOUTH OF THE JTWC TRACK, WHILE UKMET INDICATES A SHALLOWER TURN AND TAKES THE SYSTEM OVER THE KANTO PLAN. ADDITIONALLY, THE MODELS BEGIN TO DIVERGE ON SYSTEM SPEED AFTER TAU 72, CREATING A HIGH DEGREE OF UNCERTAINTY BY THE END OF THE FORECAST PERIOD. DUE TO THE VARIABILITY IN BOTH TRACK AND SPEED, THERE IS LOW CONFIDENCE IN THIS PORTION OF THE JTWC OFFICIAL FORECAST.// NNNN NNNN