MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TYPHOON 21W (NEOGURI) WARNING NR 015// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TYPHOON 21W (NEOGURI), LOCATED APPROXIMATELY 186 NM SOUTH OF KADENA AB, OKINAWA, JAPAN HAS TRACKED NORTH-NORTHEASTWARD AT 10 KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY DEPICTS A COMPACT SYSTEM THAT HAS DETERIORATED SLIGHTLY OVER THE PAST SIX HOURS AS THE CONVECTION IS OFFSET TO THE NORTH OF THE LOW-LEVEL CIRCULATION CENTER (LLCC) CAUSED BY THE MODERATE TO STRONG (25-30 KTS) VERTICAL WIND SHEAR (VWS). THERE IS NO VISIBLE EYE, HOWEVER CONFIDENCE IN THE INITIAL POSITION REMAINS GOOD BASED ON THE EXTRAPOLATION FROM A WELL-DEFINED LOW BRIGHTNESS CIRCULATION FEATURE IN THE 202233Z SSMIS 91GHZ IMAGE THAT LINED UP WELL WITH THE 20/00Z AGENCY FIX CLUSTER. THE CURRENT INTENSITY IS ASSESSED AT 90 KTS BASED ON A DECREASING TREND IN THE DVORAK INTENSITY ESTIMATES BY RJTD AND PGTW AND CONSISTENT WITH A DETERIORATION IN THE STORM STRUCTURE. HOWEVER, THE SIGNIFICANT VWS IS STILL BEING OFFSET BY ROBUST POLEWARD OUTFLOW INTO A DEEP UPPER-LEVEL TROUGH TO THE NORTH THAT IS PROVIDING VENTILATION TO THE CENTRAL CONVECTION. SST VALUES OF 27-28C REMAIN CONDUCIVE. THE SYSTEM HAS CRESTED THE SUBTROPICAL RIDGE (STR) AXIS AND IS TRACKING ALONG THE WESTERN/POLEWARD PERIPHERY OF THE STR POSITIONED TO THE EAST. 3. FORECAST REASONING. A. THERE IS NO SIGNIFICANT CHANGE TO THE FORECAST PHILOSOPHY FROM THE PREVIOUS PROGNOSTIC REASONING MESSAGE. B. TY 21W IS FORECAST TO CONTINUE TRACKING NORTH-NORTHEASTWARD THROUGH TAU 24, TRANSITIONING TO A NORTHEASTWARD TRACK THROUGH TAU 72. BY TAU 24, THE SYSTEM WILL BEGIN INTERACTING WITH A DEEP SHORTWAVE TROUGH POSITIONED OVER THE EAST CHINA SEA AND BEGIN EXTRA-TROPICAL TRANSITION (ETT). THE SYSTEM HAS WEAKENED SLIGHTLY OVER THE PAST SIX HOURS AND IS EXPECTED TO CONTINUE WEAKENING STEADILY THROUGH TAU 72. AFTER TAU 24, VWS IS EXPECTED TO INCREASE SIGNIFICANTLY AND MAINTAIN BETWEEN 45-60 KNOTS THROUGH TAU 72, WHICH CONTRIBUTES TO THE SIGNIFICANT WEAKENING OF TY 21W. AFTER TAU 36, TY 21W BECOMES EMBEDDED WITHIN THE MIDLATITUDE WESTERLIES AND SHOULD COMPLETE ETT BY TAU 48. BY TAU 72, TY 21W WILL BE REDUCED TO 35KTS. THE MODELS ARE IN TIGHT AGREEMENT THROUGH TAU 36, LENDING FAIR CONFIDENCE TO THIS PORTION OF THE JTWC TRACK FORECAST. HOWEVER, AFTER TAU 36 THE GUIDANCE BEGINS TO EXCESSIVELY DIVERGE, WITH GFS NOTABLY REVERSING THE TRACK SOUTHWESTWARD AND A NUMBER OF THE MODELS STOP TRACKING THE SYSTEM AFTER TAU 60, LIKELY BECAUSE THE VORTICES BECOME LOST IN THE BAROCLINIC ZONE AS TY 21W BECOMES EXTRA-TROPICAL AT THIS TIME. DUE TO LARGE INCONSISTENCIES IN THE MODEL GUIDANCE BEYOND TAU 36 THE OFFICIAL TRACK FORECAST IS PRIMARILY EXTRAPOLATED OFF OF THE 192100Z JTWC FORECAST. THEREFORE, THERE IS LOW CONFIDENCE IN THIS PORTION OF THE TRACK FORECAST.// NNNN NNNN