MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (PHANFONE) WARNING NR 05// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM 18W (PHANFONE), LOCATED APPROXIMATELY 225 NM EAST- SOUTHEAST OF SAIPAN, HAS TRACKED WEST-NORTHWESTWARD AT 09 KNOTS OVER THE PAST SIX HOURS. THE CURRENT POSITION ESTIMATE IS BASED ON A RECENT SATELLITE FIX FROM PGTW AND A 291633Z AMSU IMAGE. THE INTENSITY ESTIMATE OF 45 KNOTS IS CONSISTENT WITH DVORAK T-NUMBERS FROM PGTW, KNES, AND RJTD AND WITH A CIMSS 291633Z AUTOMATED SATELLITE INTENSITY CONSENSUS ESTIMATE OF 43 KNOTS. RECENT MICROWAVE SATELLITE DATA, INCLUDING THE 211633Z AMSU PASS, INDICATE IMPROVING LOW-LEVEL STRUCTURE DURING THE PAST SIX HOURS. DEEP CONVECTION HAS PERSISTED OVER THE CENTER OF THE CYCLONE AND INCREASED WITHIN CONVECTIVE BANDING ALONG THE NORTHERN AND WESTERN PERIPHERY OF THE CIRCULATION. UPPER-LEVEL ANALYSIS SHOWS AN ANTICYCLONE DEVELOPING OVER THE LOW-LEVEL CIRCULATION CENTER AND A TROPICAL UPPER TROPOSPHERIC TROUGH (TUTT) CELL TO THE WEST, WHICH IS ENHANCING OUTFLOW ALOFT. THE SYSTEM CONTINUES TO TRACK ALONG THE SOUTHERN PERIPHERY OF AN EXTENSIVE SUBTROPICAL STEERING RIDGE. 3. FORECAST REASONING. A. THE EXTENDED TRACK FORECAST HAS SHIFTED POLEWARD TO INDICATE A SLIGHTLY TIGHTER RECURVATURE THAN PREVIOUSLY ANTICIPATED. OTHERWISE, THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING MESSAGE. B. TS 18W IS FORECAST TO TRACK WEST-NORTHWESTWARD ALONG THE SOUTHERN AND SOUTHWESTERN PERIPHERY OF THE SUBTROPICAL RIDGE THROUGH TAU 72. THE DYNAMIC MODEL GUIDANCE REMAINS IN GOOD AGREEMENT DURING THIS PERIOD. THUS, THERE IS HIGH CONFIDENCE IN THE JTWC FORECAST TRACK THROUGH TAU 72. TS 18W IS EXPECTED TO STEADILY INTENSIFY THROUGH TAU 72 DUE TO PASSAGE OVER VERY WARM WATER AND FAVORABLE UPPER-LEVEL CONDITIONS, INCLUDING LOW VERTICAL WIND SHEAR AND STRONG OUTFLOW ALOFT. GIVEN THESE FAVORABLE CONDITIONS, A PERIOD OF RAPID INTENSIFICATION, GREATER THAN CURRENTLY FORECAST, IS POSSIBLE. C. TS 18W IS EXPECTED TO TURN POLEWARD AS A DEVELOPING MID- LATITUDE TROUGH INDUCES A WEAKNESS IN THE SUBTROPICAL STEERING RIDGE DURING THE EXTENDED FORECAST PERIOD. CONSENSUS MODEL DEPICTIONS OF THIS DEVELOPING MID-LATITUDE TROUGH VARY, RESULTING IN INCREASED TRACK FORECAST UNCERTAINTY FOR TS 18W IN THE EXTENDED PERIOD. THE GFS, GFS ENSEMBLE, AND HWRF INDICATE AN EARLIER BREAK AND FASTER RECURVATURE, WHILE THE MAJORITY OF THE GUIDANCE SHOW THE BREAK DEVELOPING LATER AND A SLOWER CONSEQUENT RECURVATURE. THE CURRENT FORECAST FAVORS THE MAJORITY OF THE GUIDANCE DEPICTING A SLOWER RECURVATURE. GIVEN THE CURRENT MODEL FORECAST UNCERTAINTY, CONFIDENCE IN THE EXTENDED FORECAST TRACK IS LOW. TS 18W IS EXPECTED TO REACH PEAK INTENSITY NEAR THE RIDGE AXIS AROUND TAU 96 AS POLEWARD OUTFLOW IMPROVES AHEAD OF THE DEVELOPING MID-LATITUDE TROUGH. THEREAFTER, SLOW WEAKENING SHOULD BEGIN AS WESTERLY VERTICAL WIND SHEAR BEGINS TO IMPINGE ON THE SYSTEM.// NNNN NNNN