Typhoon 28W(RAI): extremely rapid intensification: now a powerful CAT 4, forecast to hit Surigao City as a formidable cyclone in 12/15hours,15/21utc

JTWC IS ISSUING 6HOURLY WARNINGS AND 3HOURLY SATELLITE BULLETINS ON 28W(RAI).

15/1950UTC.

FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THE FORECASTED PEAK INTENSITY HAS BEEN INCREASED BY 55 KNOTS, FROM 85 KNOTS/CAT 2 TO 140 KNOTS/CAT 5. FORECAST DISCUSSION: AS DISCUSSED EARLIER, TY 28W HAS UNDERGONE A PERIOD OF EXTREMELY RAPID INTENSIFICATION, WHICH WAS NOT PREVIOUSLY FORECAST AND WHILE TRACK HAS NOT SIGNIFICANTLY CHANGED, THE INTENSITY FORECAST HAS BEEN INCREASED DRAMATICALLY. THE SYSTEM IS TRACKING JUST NORTH OF DUE WEST ALONG THE SOUTHERN SIDE OF A DEEP SUBTROPICAL RIDGE (STR) TO THE NORTH. THE SYSTEM IS FORECAST TO MAKE LANDFALL ALONG THE NORTHEASTERN TIP OF MINDANAO, NEAR SURIGAO CITY, WITHIN THE NEXT 12 TO 15 HOURS. THE SYSTEM WILL THEN CROSS THE SOUTHERN PORTION OF THE PHILIPPINE ISLANDS AND INTO THE SULU SEA BY 36H. AS THE SYSTEM MOVES INTO THE SOUTH CHINA SEA, A WEAKNESS IS FORECAST TO DEVELOP IN THE RIDGE TO THE NORTH, AND THE SYSTEM WILL GRADUALLY TURN POLEWARD IN RESPONSE, THROUGH THE REMAINDER OF THE FORECAST PERIOD. THE SYSTEM HAS ANOTHER TWELVE HOURS OVER VERY WARM, HIGH OHC WATERS, AND NOW THAT THE CORE OF THE SYSTEM HAS VERY OBVIOUSLY BECOME ESTABLISHED AND IS FIGHTING OFF THE MODERATE EASTERLY SHEAR, IT IS EXPECTED TO CONTINUE TO RAPIDLY INTENSIFY TO A PEAK OF AT LEAST 140 KNOTS/CAT 5 PRIOR TO LANDFALL. LATE RECEIPT OF A 151930Z SSMIS 89GHZ MICROWAVE IMAGE SUGGESTS THE DEVELOPMENT OF A SECONDARY EYEWALL ON THE SOUTHERN SIDE OF THE SYSTEM. IF THIS CONTINUES TO DEVELOP, IT MAY LEAD TO A LOWER INTENSITY AT LANDFALL, BUT THE PEAK MAY STILL OCCUR PRIOR TO THIS POINT.VERTICAL WIND SHEAR (VWS) IS FORECAST TO STRENGTHEN A BIT AFTER LANDFALL, AND COMBINED WITH THE DISRUPTION OF THE CORE DUE TO INTERACTION WITH THE COMPLEX TERRAIN OF THE SOUTHERN PHILIPPINE ISLANDS, WILL LEAD TO A SIGNIFICANT WEAKENING THROUGH 36H. HOWEVER, ONCE THE SYSTEM EMERGES BACK OVER THE SOUTH CHINA SEA, INCREASED UPPER-LEVEL OUTFLOW AND DECREASED SHEAR WILL ALLOW FOR A PERIOD OF INTENSIFICATION TO A SECOND PEAK OF 110 KNOTS/CAT 3 BY 72H. THEREAFTER A STRONG COLD SURGE WILL BRING COOL, RELATIVELY DRY-AIR DOWN FROM THE NORTH, AND SOUTHWESTERLY SHEAR WILL INCREASE SHARPLY AS THE SYSTEM INTERACTS WITH A MID-LATITUDE TROUGH TO THE WEST. THE COMBINED EFFECTS OF THESE UNFAVORABLE CONDITIONS WILL BE FAIRLY RAPID WEAKENING AS THE SYSTEM TRACKS NORTHWARD TO THE EAST OF HAINAN ISLAND.

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: TY 28W HAS UNDERGONE A PERIOD OF EXTREMELY RAPID INTENSIFICATION OVER THE PAST SIX HOURS, WITH THE INITIAL INTENSITY INCREASING OVER 30 KNOTS IN THE SIX HOUR PERIOD BETWEEN 1200Z AND 1800Z. ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY OVER THE PREVIOUS FEW HOURS SHOWS THE RAPID DEVELOPMENT OF A PINHOLE EYE APPROXIMATELY 13KM IN DIAMETER WITH DEEP CONVECTIVE TOWERS ROTATING UPSHEAR TO THE EAST OF THE EYE. 151626Z AMSR2 89GHZ AND 37GHZ MICROWAVE IMAGES SHOW THE VERY WELL DEFINED EYE, WHICH IS VERTICALLY STACKED WITH LITTLE TO NO TILT, INDICATING A VAST IMPROVEMENT FROM THE PREVIOUSLY TILTED CORE. THE INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE PINHOLE EYE IN BOTH THE EIR AND MICROWAVE DATA. MODERATE EASTERLY SHEAR, OF AROUND 10-15 KNOTS IS INDICATED BY THE SHARP UPSHEAR EDGE OF THE CONVECTIVE ENVELOPE AND CIRRUS SHIELD SEEN IN THE INFRARED AND MICROWAVE IMAGERY. HOWEVER THE CONVECTIVE TOWERS HAVING MOVED UPSHEAR HAVE SHELTERED THE CORE FROM THE EASTERLY SHEAR AND ENABLED THE RAPID INTENSIFICATION. THE INITIAL INTENSITY HAS BEEN INCREASED DRAMATICALLY, TO A CONSERVATIVE 115 KNOTS, CONSERVATIVE DUE TO THE EXTREMELY RAPID RATE OF INTENSIFICATION. AGENCY FIXES ARE BEING HELD BY CONSTRAINTS, BUT DATA-T NUMBERS ARE CLIMBING RAPIDLY, NOW EXCEEDING T7.0 IN SOME CASES. DUE TO THE SMALL EYE AND RAPIDLY DEVELOPING SITUATION, THE ADT IS SIGNIFICANTLY BEHIND, THOUGH SUBSEQUENT TO THE 2000Z HOUR IS BEGINNING TO PICK UP THE EYE, WITH RAW-T NUMBERS NOW ABOVE T7.0. THE ENVIRONMENT IS VERY FAVORABLE FOR CONTINUED DEVELOPMENT, WITH WARM (29-30C) SSTS AND ROBUST WESTWARD AND POLEWARD OUTFLOW. THE SYSTEM WILL ALSO PASS OVER A DEEP POOL OF WARM WATERS WITH HIGH OHC FROM NOW THROUGH LANDFALL IN THE PHILIPPINES.

MODEL DISCUSSION: TRACK GUIDANCE IS IN GOOD AGREEMENT THROUGH 72H, WITH ONLY MINIMAL SPREAD THROUGH THE FIRST 36 HOURS OF THE FORECAST AND INCREASES MODESTLY TO 165KM AT 72H. SPREAD INCREASES AFTER 72H AS THE SYSTEM RECURVES POLEWARD, INCREASING TO 325KM BETWEEN THE JGSM ON THE LEFT AND THE GFS ON THE RIGHT OF THE ENVELOPE. THE JTWC FORECAST LIES AMONGST THE TIGHT GROUPING THROUGH 72H, THEN JUST SLIGHTER LEFT OF THE MEAN THROUGH 120H. DUE TO TECHNICAL DIFFICULTIES, AVAILABLE INTENSITY GUIDANCE IS VERY LIMITED, WITH ONLY THE COAMPS-TC BEING AVAILABLE TO SUPPORT THIS FORECAST. THUS THE INTENSITY FORECAST IS BEING GENERATED USING ENVIRONMENTAL ANALYSIS AND TRENDS THROUGH THE FIRST 24 HOURS, THEN GENERALLY FOLLOWS THE COAMPS-TC GUIDANCE THEREAFTER. CONFIDENCE IN THE INTENSITY FORECAST IS LOW DUE TO THE LACK OF GUIDANCE AND THE RAPID DEVELOPMENT OF THE SYSTEM IN THE NEAR-TERM.

THE AREA OF CONVECTION (INVEST 97W) PREVIOUSLY LOCATED NEAR 5.9N 106.9E IS NOW LOCATED NEAR 4.8N 107.1E, APPROXIMATELY 660 KM SOUTH OF HO CHI MINH, VIETNAM. ANIMATED ENHANCED MULTISPECTRAL SATELLITE IMAGERY (MSI) AND A PARTIAL 151106Z SSMIS 91GHZ MICROWAVE IMAGE DEPICT FLARING CONVECTION DISPLACED TO THE NORTHWEST OF A FULLY EXPOSED LOW LEVEL CIRCULATION CENTER (LLC). ENVIRONMENTAL ANALYSIS INDICATES A MARGINALLY FAVORABLE ENVIRONMENT WITH GOOD POLEWARD OUTFLOW, WARM (27-28C) SEA SURFACE TEMPERATURES, OFFSET BY MODERATE (15-20KT) VERTICAL WIND SHEAR. GLOBAL MODELS ARE IN AGREEMENT THAT INVEST 97W WILL PERSIST AND SLOWLY TRACK WESTWARD OVER THE NEXT 72 HOURS. MAXIMUM SUSTAINED SURFACE WINDS ARE ESTIMATED AT 15 TO 20 KNOTS. MINIMUM SEA LEVEL PRESSURE IS ESTIMATED TO BE NEAR 1005 MB. THE POTENTIAL FOR THE DEVELOPMENT OF A SIGNIFICANT TROPICAL CYCLONE WITHIN THE NEXT 24 HOURS IS UPGRADED TO MEDIUM.