Western Pacific: TD 20W forecast to reach Typhoon/CAT 3 in 72hours/TD 21W forecast to make landfall over Vietnam in 24hours, 22/15utc updates

FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY. FORECAST DISCUSSION: TD 20W IS FORECAST TO TRACK TO THE NORTHWEST THROUGH 72H BEFORE SLOWLY AND STEADILY TURNING NORTHWARD AS IT ROUNDS THE SUBTROPICAL RIDGE AXIS. AFTER ROUNDING THE RIDGE AXIS BY 96H, TD 20W WILL ACCELERATE NORTHEASTWARD INTO A WEAKNESS IN THE RIDGE PATTERN WHICH DEVELOPS IN RESPONSE TO PASSAGE OF AN EXCEPTIONALLY STRONG MID-LATITUDE EXTRA-TROPICAL LOW PRESSURE SYSTEM AND ACCOMPANYING DEEP UPPER-LEVEL TROUGH. THE SYSTEM IS FORECAST TO TRACK OVER OR VERY CLOSE TO THE ISLAND OF GUAM BY 24H. OVER THE NEXT 24 HOURS, TD 20W IS FORECAST TO STEADILY INTENSIFY TO 45 KNOTS PRIOR TO CLOSEST POINT OF APPROACH(CPA) TO GUAM UNDER FAVORABLE CONDITIONS. AS THE SYSTEM MOVES NORTHWESTWARD THE TUTT CURRENTLY ENTRENCHED TO THE NORTH ALONG THE 20TH PARALLEL WILL MOVE NORTH AND WEST WHILE FILLING, ALLOWING FOR MORE ROBUST POLEWARD OUTFLOW ALOFT AFTER 24H. COMBINED WITH THE EXPECTED DEVELOPMENT OF A POINT SOURCE OR ANTICYCLONE OVER TOP OF THE SYSTEM BY 24H, THE VERY WARM SSTS AND PASSAGE OVER A POOL OF VERY HIGH OCEAN HEAT CONTENT(OHC) TO THE NORTHWEST OF GUAM, THE SYSTEM IS EXPECTED TO UNDERGO A PERIOD OF RAPID INTENSIFICATION ONCE IT MOVES PAST THE MARIANAS, REACHING A PEAK OF 120 KNOTS/CAT 4 BY 96H. MOVEMENT OVER LOWER OHC WATERS, INCREASED MID-LEVEL SHEAR AND SLIGHTLY LESS OPTIMUM OUTFLOW ALOFT WILL RESULT IN STEADY INTENSITY THROUGH 120H.

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A BROAD AND STILL SOMEWHAT ELONGATED, BUT QUICKLY CONSOLIDATING LOW LEVEL CIRCULATION CENTER (LLCC). AN AREA OF SYMMETRICAL DEEP CONVECTIVE ACTIVITY HAS RAPIDLY DEVELOPED OVER THE PAST SIX HOURS OVER THE ASSESSED LLCC, THOUGH AS OF YET, THERE ARE NO SIGNS IN THE EIR OF THE DEVELOPMENT OF DISTINCT BANDING FEATURES. A PARTIAL 220758Z SSMIS 89GHZ MICROWAVE IMAGE INDICATED FORMATIVE AND FRAGMENTED LOW LEVEL BANDS ABOUT 90KM OUT FROM THE CENTER, PARTICULARLY TO THE NORTHEAST. THE INITIAL POSITION IS ASSESSED TO THE NORTH OF ALL AGENCY FIXES EXCEPT RJTD, WITH LOW CONFIDENCE DUE TO THE OBSCURED LLCC IN THE EIR AND THE LACK OF RECENT MICROWAVE OR SCATTEROMETER DATA. THE INITIAL INTENSITY IS ASSESSED AT 25 KNOTS WITH MEDIUM CONFIDENCE, ON THE LOWER END OF THE PGTW AND KNES INTENSITY ESTIMATES BETWEEN T1.5-T2.0 (25-30 KNOTS). TD 20W IS IN A HIGHLY FAVORABLE ENVIRONMENT CHARACTERIZED BY VERY WARM (30-31C), HIGH OHC WATERS, AND LOW (5 KNOTS OR LESS) VWS. WHILE UPPER-LEVEL OUTFLOW IS ASSESSED AS STRONG BOTH POLEWARD AND EQUATORWARD, IT IS SOMEWHAT CONSTRAINED TO POLEWARD BY THE PRESENCE OF A SHARP, EAST-WEST ORIENTED TUTT THAT LIES ROUGHLY ALONG THE 20TH PARALLEL.

MODEL DISCUSSION: TRACK GUIDANCE IS IN GOOD AGREEMENT OVERALL WITH A SPREAD OF 120 KM AT 48H INCREASING TO 340 KM BY 72H. CROSS-TRACK SPREAD INCREASES TO 945 KM BY 120H WITH NAVGEM REPRESENTING THE LEFT EXTENT OF THE ENVELOPE AND THE ECMWF THE RIGHT. THE REMAINDER OF THE GUIDANCE IS FAIRLY WELL CLUSTERED ABOUT THE CONSENSUS MEAN. THE JTWC FORECAST LIES TO THE NORTH SIDE OF THE ENVELOPE THROUGH 72H, THEN SHIFTS CLOSER TO THE MEAN THROUGH 120H. WHILE TRACK GUIDANCE IS TIGHTLY CLUSTERED THROUGH 48H, OVERALL CONFIDENCE THROUGH 72H IS ONLY MEDIUM DUE TO UNCERTAINTY IN THE INITIAL POSITION. CONFIDENCE DECREASES TO LOW AFTER 72H DUE TO UNCERTAINTIES IN THE STRENGTH OF THE WEAKNESS IN THE RIDGE AND THE AMOUNT OF INTERACTION BETWEEN THE WEAKNESS AND TD 20W. INTENSITY GUIDANCE IS IN GOOD AGREEMENT AND IS TIGHTLY CLUSTERED, WITH ALL MODELS IN AGREEMENT ON AGGRESSIVE INTENSIFICATION, PARTICULARLY AFTER 24H. THE JTWC FORECAST LIES BELOW THE CONSENSUS MEAN THROUGH 24H, THEN SLIGHTLY ABOVE IT THROUGH 120H, WITH MEDIUM CONFIDENCE FOR THE DURATION OF THE FORECAST.

FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY. FORECAST DISCUSSION: TD 21W IS FORECASTED TO TRACK STEADILY WEST-NORTHWESTWARD THROUGH THE DURATION OF THE FORECAST PERIOD ALONG THE SOUTHERN PERIPHERY OF THE LOW- TO MID-LEVEL SUBTROPICAL RIDGE CENTERED TO THE NORTH. THE SYSTEM IS EXPECTED TO MAKE LANDFALL ALONG THE CENTRAL VIETNAM COASTLINE AROUND 24H, TO THE SOUTH OF DA NANG. CONDITIONS ARE GENERALLY FAVORABLE FOR INTENSIFICATION WITH THE ONLY LIMITING FACTORS BEING THE MODEST, SINGLE-CHANNEL OUTFLOW AND LIMITED TIME AVAILABLE PRIOR TO LANDFALL. TD 21W IS EXPECTED TO INTENSIFY AT A CLIMATOLOGICAL RATE TO A PEAK OF 35 KNOTS AT OR JUST PRIOR TO LANDFALL NEAR 24H. ONCE INLAND THE SYSTEM WILL RAPIDLY DISSIPATE BY TAU 48 OVER EASTERN THAILAND DUE TO TERRAIN INTERACTION.

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A RAPIDLY CONSOLIDATING SYSTEM WITH FLARING DEEP CONVECTIVE BLOOMING OVER THE ASSESSED LOW LEVEL CIRCULATION CENTER (LLCC) OVER THE PAST SIX HOURS. A 221121Z SSMIS 89GHZ MICROWAVE IMAGE SHOWED INTENSE CONVECTIVE BANDS WRAPPING ALONG THE WESTERN PERIPHERY OF THE DEFINED LLCC. THE INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE AFOREMENTIONED MICROWAVE IMAGE AND A 221248Z ASCAT-B SCATTEROMETER BULLSEYE. THE INITIAL INTENSITY IS ASSESSED WITH LOW CONFIDENCE BASED ON THE T1.5 PGTW CURRENT INTENSITY ESTIMATE AND THE RAPIDLY IMPROVED STRUCTURE. WHILE THE AFOREMENTIONED ASCAT-B PASS ONLY SHOWED 20 KNOT WIND BARBS MOSTLY FAR TO THE NORTHEAST, AN EARLIER SCATTEROMETER PASS INDICATED 25-30 KNOT WINDS SURROUNDING THE LLCC. ADDITIONALLY, A SHIP OBSERVATION FROM 0800Z WITHIN 75 KM OF THE ASSESSED CENTER AT THAT TIME INDICATED A PRESSURE OF 1004 MB EQUATING TO 24-25 KNOTS USING THE WIND PRESSURE RELATIONSHIP. SUBSEQUENT IMAGERY FROM THE 1400Z HOUR SHOWS WHAT COULD POTENTIALLY BE THE INITIAL INDICATIONS OF AN EYE IN THE EIR, THOUGH WITHOUT MICROWAVE IMAGERY IT IS HARD TO CONFIRM THIS AS A NASCENT EYE FEATURE OR JUST AN AREA OF WARMER CLOUD TOPS. THE SYSTEM LIES IN A FAVORABLE ENVIRONMENT CHARACTERIZED BY LOW VWS, WARM SSTS AND MODERATE EQUATORWARD AND WEAK POLEWARD OUTFLOW.

MODEL DISCUSSION: DUE TO THE RAPID DEVELOPMENT OF TD 21W, THERE IS LITTLE AVAILABLE DETERMINISTIC TRACK GUIDANCE AVAILABLE, WITH THE NAVGEM AND GFS THE ONLY AVAILABLE TRACKERS AT 1200Z. THESE TWO MODELS ARE TIGHTLY GROUPED ON A NORTHWESTWARD TRACK, MAKING LANDFALL BY 24H, HOWEVER THE NAVGEM INDICATES A LANDFALL ABOUT 140 KM FURTHER NORTH UP THE COAST THAN THE GFS. THE GFS ENSEMBLE AND ECMWF ENSEMBLE MEMBERS ALSO AGREE ON A NORTHWESTWARD TRACK, BUT SHOW A LANDFALL A BIT FURTHER NORTH, AT OR JUST SOUTH OF DA NANG. IN LIGHT OF THE LACK OF DISCREET TRACK GUIDANCE, THERE IS ONLY MEDIUM CONFIDENCE IN THE JTWC FORECAST TRACK, WHICH CLOSELY TRACKS THE GFS FORECAST. SIMILARLY, THERE IS LITTLE TO NO DISCREET INTENSITY GUIDANCE FOR THIS FORECAST, AND THE INTENSITY FORECAST IS THUS BASED SOLELY ON ENVIRONMENTAL ANALYSIS, GLOBAL MODEL FIELDS AND ENSEMBLE ESTIMATES, AND THUS THERE IS LOW CONFIDENCE IN THE JTWC INTENSITY FORECAST.

JTWC IS ISSUING 6HOURLY WARNINGS AND 3HOURLY SATELLITE BULLETINS ON 20W AND 21W.