MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 31W (YUTU) WARNING NR 42// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM (TS) 31W (YUTU), LOCATED APPROXIMATELY 258 NM SOUTHEAST OF HONG KONG, HAS TRACKED NORTH-NORTHWESTWARD AT 04 KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY SHOWS SLIGHT RE-CONSOLIDATION WITH FLARING CONVECTION REVIVING ALONG THE SOUTHERN HALF OF A BROAD CIRCULATION THAT REMAINS PARTIALLY EXPOSED. THE INITIAL POSITION IS EXTRAPOLATED FROM A LOW BRIGHTNESS TEMPERATURE CIRCULATION FEATURE IN THE 311516Z 36GHZ GPM WITH FAIR CONFIDENCE. THE INITIAL INTENSITY OF 50 KNOTS IS AVERAGED FROM DVORAK ESTIMATES OF T3.0/45KTS FROM RJTD AND T3.5/55KTS FROM PGTW AND REFLECTS THE SLIGHT INTENSIFICATION. THE SYSTEM IS CURRENTLY IN MODERATE (15-20 KNOT) VERTICAL WIND SHEAR (VWS), THAT IS BEING OFFSET BY A STRONG POLEWARD OUTFLOW. SST AND OHC VALUES ARE CONDUCIVE. THE CYCLONE IS TRACKING ALONG THE WESTERN PERIPHERY OF A DEEP-LAYERED SUBTROPICAL RIDGE (STR) TO THE EAST. 3. FORECAST REASONING. A. THE FORECAST TRACK IS CUT TO 48 HOURS AND TURNS MORE NORTHWESTWARD AT THE TERMINUS TO REFLECT A MORE RAPID DISSIPATION, OTHERWISE, THERE IS NO SIGNIFICANT CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING MESSAGE. B. TS YUTU IS FORECAST TO CONTINUE ON A NORTH-NORTHWESTWARD TRACK UP TO TAU 36 THEN NORTHWESTWARD AS IT COLLAPSES DOWN TO A LOWER LAYER OF THE STEERING STR. INCREASING VWS THEN DECREASING SST AND OHC WILL GRADUALLY THEN RAPIDLY WEAKEN THE SYSTEM TO DISSIPATION BY TAU 48, POSSIBLY SOONER. MODEL GUIDANCE SIGNIFICANTLY SPREADS TO OVER 200 NM BY TAU 48, LENDING LOW CONFIDENCE IN THE JTWC TRACK FORECAST THAT FAVORS THE WEAKER LEFT SIDE OF THE MODEL ENVELOPE.// NNNN NNNN