WDPN31 PGTW 122100 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 28W (YAMANEKO) WARNING NR 002// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 21.8N 165.7E INITIAL INTENSITY: 35 KTS GEOGRAPHIC REFERENCE: 158 NM NORTH-NORTHWEST OF WAKE ISLAND MOVEMENT PAST 6 HOURS: NORTH-NORTHEASTWARD AT 06 KTS SIGNIFICANT WAVE HEIGHT: 14 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS AN ASYMMETRIC SYSTEM, WITH AN ELONGATED AREA OF MODERATE CONVECTION OBSCURING THE LOW LEVEL CIRCULATION CENTER (LLCC). A 121755Z SSMIS COLOR ENHANCED 91GHZ MICROWAVE IMAGE SHOWED AN AREA OF MODERATE CONVECTION WHICH IS ELONGATED DOWN THE SHEAR VECTOR TOWARDS THE NORTHEAST, FLARING CONVECTION EXTENDING IN A CURVED BAND TO THE SOUTHWEST OVER WAKE ISLAND AND SHALLOW BANDING FEATURES WHICH ARE WRAPPING INTO A WEAK LLCC DISPLACED TO THE SOUTHWEST. THE INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE MICROWAVE IMAGE DESCRIBED ABOVE. THE INITIAL INTENSITY IS ASSESSED WITH MEDIUM CONFIDENCE, HEDGED TOWARDS THE HIGHER END OF THE AGENCY DVORAK CURRENT INTENSITY ESTIMATES FROM PGTW, PHFO AND KNES. THE ENVIRONMENT IS MARGINAL AT BEST, WITH MODERATE TO HIGH VWS IMPINGING THE SYSTEM FROM THE WEST. CIMSS ANALYSIS SUGGESTS 30 KNOT OR HIGHER SHEAR, BUT THE SATELLITE DEPICTION WOULD SUGGEST A SLIGHTLY LOWER VALUE BUT REGARDLESS IT IS RELATIVELY HIGH. THE OTHER ENVIRONMENTAL FACTORS ARE SUPPORTIVE FOR DEVELOPMENT, WITH SSTS REMAINING ABOVE 26C AND ROBUST POLEWARD OUTFLOW PROVIDED BY A SMALL POINT SOURCE CENTERED JUST EAST OF THE SYSTEM. TS 28W IS MOVING NORTHWARD ALONG THE WESTERN SIDE OF A DEEP-LAYER SUBTROPICAL RIDGE (STR) TO THE EAST. INITIAL WIND RADII BASIS: PERSISTENCE CURRENT STEERING MECHANISM: WESTERN PERIPHERY OF A DEEP-LAYER SUBTROPICAL RIDGE (STR) CENTERED NEAR THE DATELINE. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T2.5 - 35 KTS RJTD: T2.0 - 30 KTS KNES: T2.5 - 35 KTS PHFO: T2.5 - 35 KTS CIMSS SATCON: 30 KTS AT 121458Z CIMSS ADT: 32 KTS AT 121740Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL VWS: 25-30 KTS SST: 27-28 CELSIUS OUTFLOW: STRONG POLEWARD ANALYSIS CONFIDENCE: INITIAL POSITION: HIGH INITIAL INTENSITY: MEDIUM INITIAL WIND RADII: LOW 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. THE FORECAST HAS BEEN TRUNCATED TO 48 HOURS, AT WHICH POINT THE SYSTEM WILL TRANSITION TO AN EXTRA-TROPICAL CYCLONE. FORECAST DISCUSSION: HAVING QUICKLY FORMED OVERNIGHT, TROPICAL STORM (TS) 28W HAS BEEN ABLE TO CONSOLIDATE A LOW LEVEL CORE, EVEN IN THE FACE OF A MARGINAL ENVIRONMENT WITH RELATIVELY HIGH SHEAR. CONVECTION HSA BEEN ABLE TO PERSIST, AND THIS IS PUSHING BACK ON THE SHEAR JUST ENOUGH TO ALLOW FOR A SMALL AMOUNT OF INTENSIFICATION OVER THE PAST FEW HOURS. THE SYSTEM IS MOVING NORTH AT A RELATIVELY SLOW PACE, ALONG THE WESTERN SIDE OF THE DEEP RIDGE CENTERED NEAR THE DATELINE. HOWEVER, TRACK MOTION IS EXPECTED TO SLOW AND TURN A BIT TO THE NORTHWEST IN THE NEAR-TERM AS THE LOW-LEVEL FLOW IS DOMINATED BY A TRANSITORY RIDGE PASSING BY TO THE NORTH. AROUND TAU 24, THE RIDGE TO THE NORTH WILL MOVE TO A POSITION EAST OF DUE NORTH OF TS 28W, AND THE SYSTEM WILL QUICKLY TURN NORTHEAST AND ACCELERATE ALONG THE TIGHT GRADIENT BETWEEN THIS TRANSITORY RIDGE AND AN APPROACHING LOW PRESSURE SYSTEM AND ASSOCIATED LOW-LEVEL FRONT AND UPPER-LEVEL TROUGH. SOME ADDITIONAL NEAR-TERM INTENSIFICATION IS POSSIBLE AS THE SYSTEM CONTINUES TO TAP INTO THE STRONGLY DIVERGENT UPPER-LEVEL POLEWARD OUTFLOW. AFTER TAU 24 THE SYSTEM WILL START TO FEEL THE EFFECTS OF SHARPLY INCREASED SHEAR BUT WILL STILL HAVE ACCESS TO VERY STRONG POLEWARD OUTFLOW AND THE TWO WILL OFFSET FOR TIME, ALLOWING FOR THE SYSTEM TO MAINTAIN TS STRENGTH THROUGH THE END OF THE FORECAST. A DEEP TROUGH CURRENTLY MOVING OFF THE COAST OF JAPAN WILL BECOME NEGATIVELY TILTED OVER THE NEXT 24 HOURS AS IT MOVES FURTHER EAST. TS 28W WILL MOVE INTO PHASE WITH THIS NEGATIVELY TILTED TROUGH AROUND TAU 36, MARKING THE ONSET OF A VERY RAPID EXTRA-TROPICAL TRANSITION (ETT). THE SYSTEM IS FORECAST TO COMPLETE ETT NO LATER THAN TAU 48 AS A WARM SECLUSION TYPE SYSTEM. ADDITIONAL INTENSIFICATION IS POSSIBLE AFTER ETT AND BEYOND THE FORECAST PERIOD AS THE SYSTEM RACES OFF THE NORTHEAST. MODEL DISCUSSION: DETERMINISTIC AND ENSEMBLE TRACK GUIDANCE IS IN GOOD AGREEMENT THROUGH THE FORECAST PERIOD, WITH MINIMAL CROSS-TRACK DEVIATION BETWEEN THE VARIOUS MODELS. ALONG-TRACK SPREAD INCREASES MARGINALLY BY TAU 48 AS THE SYSTEM UNDERGOES ETT AND SOME OF THE MODELS LOSE THE VORTEX. THE JTWC TRACK LIES AMONGST THE MODEL PACK, NEAR THE MULTI-MODEL CONSENSUS WITH HIGH CONFIDENCE. INTENSITY GUIDANCE IS ALSO IN GOOD AGREEMENT, WITH THE HWRF PEAKING THE SYSTEM AT 50 KNOTS IN 12 HOURS, WHILE THE DECAY-SHIPS KEEP THE SYSTEM AT OR BELOW 40 KNOTS, WHILE ALL MODELS SHOW A POST-FORECAST INTENSIFICATION AFTER ETT. THE JTWC FORECAST LIES JUST BELOW THE CONSENSUS MEAN WITH MEDIUM CONFIDENCE. FORECAST CONFIDENCE: TRACK 0 - 72 HR: HIGH INTENSITY 0 - 72 HR: MEDIUM// NNNN