WDPN31 PGTW 121500 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 21W (TWENTYONE) WARNING NR 001// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 15.2N 151.3E INITIAL INTENSITY: 25 KTS GEOGRAPHIC REFERENCE: 318 NM EAST OF SAIPAN MOVEMENT PAST 6 HOURS: SOUTH-SOUTHEASTWARD AT 04 KTS SIGNIFICANT WAVE HEIGHT: 10 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A COMPACT, SYMMETRICAL AREA OF CONVECTION DISPLACED SLIGHTLY SOUTHEASTWARD OF A PARTIALLY EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC). THE LATEST AVAILABLE MICROWAVE IMAGE WAS A SSMIS 91GHZ PASS FROM 120705Z AND SHOWED WEAK LOW-LEVEL BANDING FEATURES TO THE NORTHWEST OF AN AREA OF DEEP, BUT RELATIVELY DISORGANIZED CONVECTION. RECENT SCATTEROMETER PASSES FROM THE 121100Z HOUR INDICATE A WELL-DEFINED LLCC, WHICH ALONG WITH MULTIPLE AGENCY FIX POSITIONS, PROVIDED HIGH CONFIDENCE TO THE INITIAL POSITION. THE INITIAL INTENSITY OF 25 KNOTS IS ASSESSED WITH HIGH CONFIDENCE BASED ON DVORAK CURRENT INTENSITY ESTIMATES OF T1.5 (25 KNOTS) FROM BOTH PGTW AND KNES AS WELL AS THE AFOREMENTIONED SCATTEROMETER PASSES WHICH SHOWED 25-28 KNOT WINDS IN THREE QUADRANTS OF THE CIRCULATION. THE OVERALL ENVIRONMENT IS MARGINALLY FAVORABLE, WITH THE SYSTEM MOVING SLOWLY OVER VERY WARM, HIGH OHC WATERS, PROVIDING AMPLE TCHP. A TROPICAL UPPER TROPOSPHERIC TROUGH (TUTT) CELL IS CURRENTLY SITTING DUE NORTH OF 21W, IMPARTING A MODERATE NORTHWESTERLY SHEAR OVER THE SYSTEM AS EVIDENCED BY THE SATELLITE DEPICTION. FINALLY, OUTFLOW IS MODERATE TOWARDS THE SOUTHWEST, BUT IS STRONGLY DIVERGENT RIGHT OVER THE SYSTEM, PROVIDING SUPPORT FOR THE CONVECTION, WHICH HAS PERSISTED FOR THE PAST SIX HOURS EVEN IN THE FACE OF THE MODERATE SHEAR. INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND RADII). CURRENT STEERING MECHANISM: WEAK STEERING PATTERN AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T1.5 - 25 KTS KNES: T1.5 - 25 KTS FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE VWS: 20-25 KTS SST: 29-30 CELSIUS OUTFLOW: MODERATE EQUATORWARD ANALYSIS CONFIDENCE: INITIAL POSITION: HIGH INITIAL INTENSITY: HIGH INITIAL WIND RADII: NOT APPLICABLE 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY. FORECAST DISCUSSION: TD 21W IS CURRENTLY DRIFTING SLOWLY TOWARDS THE EAST-SOUTHEAST IN A WEAK AND COMPETING STEERING PATTERN DEFINED BY A STRONG SUBTROPICAL RIDGE (STR) ENTRENCHED TO THE NORTHEAST AND AN EXTENSION OF A NEAR EQUATORIAL RIDGE (NER) TO THE SOUTH. OVER THE NEXT 24 HOURS THE STR SLIDES EAST AND EXTENDS TO SOUTHWARD TO NEAR THE EQUATOR, WITH ITS WESTERN PERIPHERY BECOMING ORIENTED TO A NORTH-SOUTH AXIS IN THE PROCESS. SIMULTANEOUSLY A MODERATELY STRONG UPPER-LEVEL TROUGH WILL MOVE ACROSS JAPAN AND INTO THE NORTHWEST PACIFIC AND STEADILY ERODE THE STR FROM THE WEST. THE COMBINED EFFECTS WILL PUSH 21W ONTO AN ACCELERATING NORTHWARD TRAJECTORY AFTER TAU 24. BY TAU 72 THE SYSTEM WILL BEGIN TO SLOW DOWN AS IT APPROACHES THE TAIL END OF A COLD FRONT TRAILING THE EXTRA-TROPICAL LOW THAT DEVELOPS AHEAD OF THE AFOREMENTIONED TROUGH. BY AROUND TAU 72, THE SYSTEM WILL TAKE A HARD RIGHT TURN, MOVING EASTWARD ALONG THE REMNANT FRONTAL BOUNDARY WHILE TRANSITIONING TO A EXTRATROPICAL CYCLONE AS IT BECOMES EMBEDDED WITHIN THE BAROCLINIC ZONE, IS ENGULFED BY MID-LEVEL DRY AIR AND MOVES INTO THE RIGHT ENTRANCE REGION OF A STRONG 200MB JET MAX. WHILE THE SYSTEM IS CURRENTLY IN A RATHER UNFAVORABLE POSITION RELATIVE TO THE TUTT CELL TO THE NORTH, THE TUTT IS EXPECTED TO MOVE OFF TO THE WEST BY TAU 24, WHICH WILL PUT TD 21W IN A MORE FAVORABLE POSITION TO TAKE ADVANTAGE OF THE DIFFLUENT POLEWARD OUTFLOW ON THE EAST SIDE OF THE TUTT. THE SYSTEM WILL BE SLOW TO INTENSIFY AT FIRST BUT ONCE THE TUTT MOVES TO THE WEST, THE RATE OF INTENSIFICATION WILL INCREASE, BUT THE SYSTEM WILL CONTINUE TO FIGHT AGAINST MODERATE SHEAR AND INCREASINGLY DRY AIR, PUTTING A CAP ON THE PEAK INTENSITY. STRONG DRY AIR ENTRAINMENT, COOLER SSTS AND INCREASED SHEAR WILL CONSPIRE TO WEAKEN THE SYSTEM AFTER TAU 72, AS IT UNDERGOES ETT. MODEL DISCUSSION: DETERMINISTIC AND ENSEMBLE GLOBAL MODEL FIELDS ARE IN GENERAL AGREEMENT ON THE OVERALL TRACK SCENARIO. THE CONSENSUS TRACKERS ARE CONFINED TO A 150NM ENVELOPE THROUGH TAU 48, BUT THEREAFTER BEGIN TO DIVERGE SIGNIFICANTLY BOTH IN TERMS OF ALONG AND ACROSS-TRACK. GLOBAL MODEL FIELDS SHOW DEVELOPMENT OF A COMPLEX LOW PRESSURE SYSTEM WITH MULTIPLE CENTERS OF CIRCULATION AROUND TD 21W, WHICH COMPLICATES THE OVERALL TRACK AND INTENSITY FORECAST. THE GFS, GEFS AND NAVGEM HOLD THE SYSTEM BACK FURTHER TO THE WEST THROUGH TAU 72, WHILE THE ECMWF, ECENS, GALWEM AND UKMET TAKE THE SYSTEM MUCH FURTHER TO THE EAST. THIS TREND CONTINUES THROUGH TAU 120, WITH THE GFS AND GEFS TURNING THE SYSTEM SOUTHWARD WHILE THE REMAINDER OF THE GUIDANCE RACES OFF TO THE EAST ALONG THE FRONTAL BOUNDARY. THE JTWC TRACK IS PLACED ON THE EAST SIDE OF THE GUIDANCE ENVELOPE THROUGH TAU 24 IN LIGHT OF THE CURRENT MOTION VECTOR, THEN LIES CLOSE TO THE CONSENSUS MEAN TO TAU 72 AND SLIGHTLY NORTH OF IT THROUGH TAU 120. CONFIDENCE IS MEDIUM TO TAU 72 AND LOW THEREAFTER. INTENSITY GUIDANCE IS MIXED, WITH THE COTC AND CTCX PEAKING THE SYSTEM NEAR 70 KNOTS DURING AND AFTER ETT, WHILE HWRF RAMPS UP FAST BUT PEAKS NEAR 50 KNOTS AROUND TAU 48. MEANWHILE THE GFS AND DECAY SHIPS NAVGEM DRAG THE CONSENSUS MEAN DOWN TO A PEAK NEAR 45 KNOTS. THE JTWC FORECAST IS BELOW ALL THE GUIDANCE IN THE FIRST 24 HOURS DUE TO THE FORECAST CONTINUANCE OF THE MODERATE SHEAR, THEN TRACKS THE CONSENSUS MEAN THEREAFTER, WITH LOW CONFIDENCE THROUGH THE FORECAST PERIOD. FORECAST CONFIDENCE: TRACK 0 - 72 HR: MEDIUM TRACK 72-120 HR: LOW INTENSITY 0 - 72 HR: LOW INTENSITY 72-120 HR: LOW// NNNN