MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TYPHOON 20W (KROVANH) WARNING NR 18// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TYPHOON (TY) 20W (KROVANH), LOCATED APPROXIMATELY 60 NM NORTHEAST OF CHICHI JIMA, HAS TRACKED NORTH-NORTHEASTWARD AT 11 KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY SHOWS CURVED CONVECTIVE BANDING WRAPPING TIGHTLY INTO A PARTIALLY-EXPOSED LOW- LEVEL CIRCULATION CENTER (LLCC). A 182215Z SSMIS 91GHZ IMAGE DEPICTS A MICROWAVE EYE FEATURE, WHICH SUPPORTS THE CURRENT POSITION AND TRACK MOTION WITH GOOD CONFIDENCE. UPPER-LEVEL ANALYSIS SHOWS A MARGINALLY-FAVORABLE ENVIRONMENT WITH STRONG SOUTHWESTERLY VERTICAL WIND SHEAR OFFSET BY VIGOROUS POLEWARD OUTFLOW. THE INITIAL INTENSITY IS ASSESSED AT 75 KNOTS BASED ON DVORAK CURRENT INTENSITY ESTIMATES OF 77 KNOTS FROM PGTW AND RJTD. TY 20W IS TRACKING ALONG THE NORTHWESTERN PERIPHERY OF A DEEP-LAYERED STR. 3. FORECAST REASONING. A. THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING MESSAGE. B. TY 20W WILL RE-CURVE NORTHEASTWARD THROUGH THE FORECAST PERIOD AS IT TRACKS POLEWARD OF THE STR AND BEGINS TO ENCOUNTER MIDLATITUDE WESTERLIES. TY 20W IS FORECAST TO BEGIN EXTRA-TROPICAL TRANSITION (ETT) BY TAU 48 AS IT BECOMES EMBEDDED WITHIN THE MIDLATITUDE WESTERLIES. THE SYSTEM SHOULD COMPLETE ETT BY TAU 72 AS IT GAINS FRONTAL CHARACTERISTICS. DYNAMIC GUIDANCE IS IN GOOD AGREEMENT THROUGH TAU 48 AND SUPPORTS THE JTWC FORECAST TRACK WITH HIGH CONFIDENCE. TY 20W IS FORECAST TO CONTINUE TO WEAKEN DUE TO THE STRONG VWS AND COOLER SST AFTER TAU 48. AFTER TAU 48, DYNAMIC GUIDANCE DIVERGES DUE TO AN ATYPICAL SYNOPTIC PATTERN, WHICH WILL SERVE TO WEAKEN THE MIDLATITUDE WESTERLIES, PRODUCING A SLOWER RE- CURVE SCENARIO. BASICALLY, A MIDLATITUDE HIGH TO THE NORTH WILL RESULT IN A WEAKER WESTERLY STEERING ENVIRONMENT BUT SHOULD STILL STEER THE SYSTEM EASTWARD. OVERALL, THERE IS HIGH CONFIDENCE IN THE JTWC FORECAST TRACK.// NNNN NNNN