MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (PHANFONE) WARNING NR 06// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM 18W (PHANFONE), LOCATED APPROXIMATELY 145 NM EAST OF SAIPAN, HAS TRACKED NORTHWESTWARD AT 13 KNOTS OVER THE PAST SIX HOURS. THE CURRENT POSITION ESTIMATE IS BASED ON A RECENT SATELLITE FIX FROM PGTW AND A 292310Z AMSU PASS. THE INTENSITY ESTIMATE OF 45 KNOTS REPRESENTS AN AVERAGE OF DVORAK T-NUMBERS FROM PGTW, KNES, AND RJTD AND A CIMSS 292023Z AUTOMATED SATELLITE INTENSITY CONSENSUS ESTIMATE OF 50 KNOTS. DEEP CONVECTION HAS WANED A BIT OVER THE PAST SIX HOURS, BUT THE LOW-LEVEL CIRCULATION CENTER (LLCC) REMAINS WELL-DEFINED IN MICROWAVE AND MULTISPECTRAL SATELLITE DATA. UPPER- LEVEL ANALYSIS INDICATES FAVORABLE CONDITIONS FOR FURTHER DEVELOPMENT, INCLUDING RIDGING ALOFT, LOW VERTICAL WIND SHEAR, AND A TROPICAL UPPER TROPOSPHERIC TROUGH (TUTT) TO THE WEST ENHANCING OUTFLOW. THE SYSTEM CONTINUES TO TRACK ALONG THE SOUTHERN PERIPHERY OF AN EXTENSIVE SUBTROPICAL STEERING RIDGE. 3. FORECAST REASONING. A. TS 17W HAS TRACKED A BIT MORE POLEWARD AND FASTER THAN PREVIOUSLY EXPECTED OVER THE PAST SIX HOURS. HOWEVER, STORM MOTION IS EXPECTED TO SETTLE BACK TOWARD PREVIOUSLY-FORECASTED MOTION. THEREFORE, THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING MESSAGE. B. TS 18W IS EXPECTED 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, LENDING HIGH CONFIDENCE TO THE JTWC FORECAST TRACK THROUGH TAU 72. TS 18W IS EXPECTED TO STEADILY INTENSIFY THROUGH TAU 72 AS IT PASSES OVER VERY WARM WATER AND UPPER-LEVEL CONDITIONS REMAIN FAVORABLE. 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 HAVE COME INTO BETTER AGREEMENT DURING THE LATEST FORECAST CYCLE, WITH ONLY THE GFDN MODEL DEPICTING AN OUTLIER SCENARIO TO THE WEST OF THE CONSENSUS GROUPING. THE CURRENT TRACK FORECAST IS CONSISTENT WITH THE MULTI-MODEL CONSENSUS. GIVEN INCREASING AGREEMENT REGARDING TIMING AND LOCATION OF THE POLEWARD TURN, CONFIDENCE IN THE EXTENDED TRACK FORECAST IS HIGH. 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