WDPN31 PGTW 151500 MSGID/GENADMIN/NAVMARFCSTCEN PEARL HARBOR HI/JTWC// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (ROKE) WARNING NR 17// RMKS/ 1. FOR METEOROLOGISTS. 2. 12 HOUR SUMMARY AND ANALYSIS. A. TROPICAL STORM (TS) 18W (ROKE), LOCATED APPROXIMATELY 175 NM EAST OF KADENA AB, JAPAN, HAS TRACKED WESTWARD AT 06 KTS OVER THE PAST SIX HOURS. ANIMATED INFRARED SATELLITE IMAGERY SHOWS LITTLE CHANGE OVER THE PAST SIX HOURS. CONVECTION NEAR THE CORE CONTINUES AND SOME DEEP CONVECTION IS SCATTERED OVER THE NORTHERN SEMICIRCLE. DRY AIR IS WRAPPING INTO THE LOW LEVEL CIRCULATION CENTER (LLCC) ON THE SOUTHWEST SIDE. ANIMATED WATER VAPOR IMAGERY AND THE 150000Z PGTW UPPER LEVEL STREAMLINE ANALYSIS INDICATE THAT A TUTT CELL IS DIRECTLY OVER THE LOW LEVEL CIRCULATION CENTER (LLCC). TS 18W APPEARS TO BE ACQUIRING SUBTROPICAL CHARACTERISTICS, YET A 150800Z AMSU CROSS SECTION CONFIRMS THE PERSISTENCE OF A WEAK MID-LEVEL WARM ANOMALY. A 151127Z SSMIS IMAGE SHOWS CURVED CONVECTIVE BANDING ON THE POLEWARD SIDE OF THE LLCC. A COMPARISON BETWEEN THE SSMIS IMAGE AND AN EARLIER AMSRE IMAGE INDICATES THE LLCC MAY BE BECOMING MORE ELONGATED. 27 DEGREE SEA SURFACE TEMPERATURES IN THE AREA ARE FAVORABLE FOR SLOW DEVELOPMENT AND THERE IS ALMOST NO VERTICAL WIND SHEAR OVER THE SYSTEM. THE LACK OF CONVECTION IS CAUSING AGENCY DVORAK ESTIMATES TO LAG ACTUAL INTENSITY. THE CURRENT INTENSITY ANALYSIS IS BASED ON A 151146Z PARTIAL ASCAT IMAGE, WHICH SHOWS 25 TO 30 KNOT WINDS EXTENDING FROM THE SYSTEM CENTER AS FAR NORTH AS KYUSHU. TS 18W IS BEING STEERING BY TWO ANTICYCLONES ALONG THE 140TH MERIDIAN. THE FIRST, CENTERED NEAR 15N 140E, IS ASSOCIATED WITH THE NEAR EQUATORIAL RIDGE AND IS GIVING TS 18W A NUDGE TO THE NORTHWEST. THE SECOND ANTICYCLONE IS ASSOCIATED WITH THE SUBTROPICAL RIDGE, JUST OFF-SHORE OF TOKYO AND MOVING SEAWARD. 3. FORECAST REASONING. A. THE FORECAST REASONING HAS NOT CHANGED SINCE THE PREVIOUS PROGNOSTIC REASONING BULLETIN. B. DURING THE NEXT 24 HOURS, THE NEAR EQUATORIAL RIDGE WILL RELAX AND RETROGRADE TOWARDS THE PHILIPPINE SEA WHILE THE SUBTROPICAL RIDGE WILL SHIFT EASTWARD. THIS WILL ALLOW TS 18W TO SUSTAIN NORTHWESTWARD MOVEMENT. AFTER TAU 24, THE SUBTROPICAL RIDGE WILL SURGE BACK OVER THE NORTHERN RYUKYUS AND DRIVE TS 18W ON A SOUTHWESTWARD COURSE ALONG THE EAST COAST OF OKINAWA. ALTHOUGH TS 18W WILL REMAIN IN A LOW SHEAR ENVIRONMENT THROUGH TAU 48, SUBSIDENCE ASSOCIATED WITH THE TUTT CELL WILL BALANCE A SLIGHT RISE IN SEA SURFACE TEMPERATURE, MAKING FOR A FLAT INTENSITY TREND THROUGH TAU 72. C. THE EXTENDED RANGE FORECAST FOR TS 18W IS ESPECIALLY CHALLENGING. THE WEAK STEERING ENVIRONMENT RAISES THE RISK OF THE SYSTEM STALLING, MEANDERING ALONG TRACK, OR EVEN REVERSING COURSE. LONG RANGE PROGS INDICATE THAT NEAR TAU 72, TS 18W WILL EXIST IN A COL BETWEEN SPLIT CELLS OF THE SUBTROPICAL RIDGE THE OCEANIC CELL CENTERED TO THE DISTANT NORTHEAST, AND A DEVELOPING ANTICYCLONE OVER EASTERN CHINA. A DEEP, SLOW-MOVING TROUGH WILL FILL THE WEAKNESS BETWEEN THE TWO RIDGES. AT THIS JUNCTURE THE DEGREE OF INTERACTION BETWEEN TS 18W AND THE TROUGH IS UNCERTAIN, BUT THERE WILL DOUBTEDLY BE A BOOST TO POLEWARD OUTFLOW. THE LARGE SIZE, COMPLEX UPPER LEVEL ENVIRONMENT, AND RELATIVELY LOW INTENSITY OF TS 18W HAVE RESULTED IN UNUSUALLY POOR MODEL GUIDANCE THUS FAR. THE LONG RANGE FORECAST STAYS CLOSER TO ECMWF, GFDN AND EGRR THAN CONSENSUS, AS CONSENSUS IS BEING SKEWED BY GFS, WBAR, AND NOGAPS, WHICH MAKE SHARP POLEWARD TURNS AWAY FROM THE MAIN BODY OF GUIDANCE AFTER TAU 48.// NNNN