MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 20W (NURI) WARNING NR 06// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM (TS) 20W (NURI), LOCATED APPROXIMATELY 358 NM NORTHWEST OF YAP, HAS TRACKED WEST-NORTHWESTWARD AT 06 KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) CONTINUES TO SHOW CONSOLIDATING FEEDER BANDS SPIRALING TIGHTER INTO THE LOW LEVEL CIRCULATION CENTER THAT IS OBSCURED BY A COMPACT CENTRAL DENSE OVERCAST FEATURE. THE INITIAL POSITION IS BASED ON THE MSI LOOP AND A 010443Z AMSU-B MICROWAVE IMAGE. THE INITIAL INTENSITY OF 55 KNOTS IS BASED ON AN OVERALL ASSESSMENT OF DVORAK ESTIMATES FROM PGTW, RJTD, AND KNES. UPPER LEVEL ANALYSIS INDICATES THE SYSTEM IS AN AREA WITH MODERATE (10-20 KNOT) VERTICAL WIND SHEAR AND EXCELLENT DUAL OUTFLOW CHANNELS. TS 20W CONTINUES TO TRACK ALONG THE SOUTHERN PERIPHERY OF A WESTERN ANNEX OF THE SUBTROPICAL RIDGE (STR), THE MAIN BODY OF WHICH IS TO THE NORTHEAST OF THE SYSTEM. 3. FORECAST REASONING. A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING MESSAGE. B. TS 20W WILL TAKE A MORE NORTHWESTWARD TRACK OVER THE NEXT 24 HOURS AS THE STR WEAKENS AND TRACKS TO THE EAST AND A MID-LATITUDE TROUGH FROM THE NORTHWEST APPROACHES. BEYOND TAU 24, TS NURI WILL TRACK NORTHWARD AS THE RIDGE RECEDES AND THE TROPICAL GOES INTO A WEAKER STEERING ENVIRONMENT PROVIDED BY A COL REGION. 20W WILL FOLLOW THE COL REGION IN A NORTHEAST PATTERN AS THE STR BUILDS BACK IN, NOW EAST OF THE SYSTEM. FAVORABLE UPPER LEVEL CONDITIONS WILL ALLOW THE SYSTEM TO PEAK AT 120 KNOTS BY TAU 72. THE NUMERIC MODEL GUIDANCE, ALONG WITH ENSEMBLE DATA, IS IN GOOD AGREEMENT WITH THIS PORTION OF THE FORECAST, LENDING TO HIGH CONFIDENCE THROUGH TAU 72. C. IN THE EXTENDED FORECAST, THERE IS A SLIGHT BIFURCATION IN THE MODELS WITH RESPECT TO THE EXTENT AND ORIENTATION OF THE STR TO THE EAST. GFS AND ECMWF ARE THE WESTERN OUTLIERS, WITH THE RIDGE REBUILDING IN STRONGLY WITH A SLIGHT BROAD BLOCKING PATTERN. THIS WILL FORCE TS NURI TO TEMPORARILY BACK TO A NORTH-NORTHWESTWARD TRACK AS THE STR REBUILDS BEFORE ACCELERATING NORTHEASTWARD, GETTING ABSORBED INTO THE MID-LATITUDE WESTERLIES. THE NAVGEM SOLUTION HAS A DIFFERENT, MORE ZONAL, ORIENTATION OF THE STR, HAVING THE SYSTEM SIMPLY TRACK TO THE NORTHEAST AS IT BEGINS TO BE ABSORBED IN THE WESTERLIES. BY TAU 96, THE CYCLONE WILL BEGIN EXTRA- TROPICAL TRANSITION. SINCE MODEL GUIDANCE IS WIDELY SPREAD ON THIS PORTION OF THE FORECAST TRACK DUE TO VARYING SOLUTIONS WITH THE REBUILDING STR, THERE IS LOW CONFIDENCE IN THE EXTENDED PORTION OF THE FORECAST.// NNNN NNNN