Tapah(18W) intensifying, typhoon intensity forecast in 48h south of Jeju

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WESTERN NORTH PACIFIC

TS TAPAH(18W)
As of 00:00 UTC Sep 20, 2019:
Location: 23.0°N 126.7°E
Maximum Winds: 45 kt ( 85km/h)
Gusts: 55 kt ( 100km/h)
Minimum Central Pressure: 991 mb
WDPN31 PGTW 200300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (TAPAH) WARNING NR
005//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 18W (TAPAH), LOCATED APPROXIMATELY 218 NM
SOUTH OF KADENA AB, OKINAWA, JAPAN HAS TRACKED NORTHWESTWARD AT 06
KNOTS (KTS) OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL
SATELLITE IMAGERY (MSI) SHOWS DEEP CONVECTIVE BANDING WRAPPING INTO
A BROAD, PARTIALLY EXPOSED LOW LEVEL CIRCULATION (LLC). THE SYSTEM
HAS EXTENSIVE DEEP CONVECTION IN THE SOUTH AND SOUTHEAST THAT AT
TIMES APPEARS TO WRAP INTO THE EASTERN AND NORTHERN QUADRANTS BUT
FAILS TO PERSIST. OVER THE LAST SIX HOURS, THE SYSTEM HAS SHOWN
MODEST IMPROVEMENTS IN BOTH CONVECTION AND STRUCTURE. THE INITIAL
POSITION IS PLACED WITH FAIR CONFIDENCE BASED ON A 192229Z 37GHZ
SSMIS MICROWAVE IMAGE THAT SHOWS CONVECTIVE BANDING WRAPPING INTO A
BROAD (90NM) CIRCULATION CENTER. THE INITIAL INTENSITY OF 45 KNOTS
IS BASED ON EARLIER ASCAT IMAGES AND ADJUSTED FOR THE IMPROVEMENTS
IN STRUCTURE. THE INTENSITY IS ABOVE THE PGTW AND RJTD DVORAK FIX
ESTIMATES OF T2.0 (30KTS) DUE TO THE BROAD NATURE OF THE SYSTEM
CAUSING ERRONEOUSLY LOW DVORAK ESTIMATES. UPPER LEVEL ANALYSIS
INDICATES THAT THE WESTERN SIDE OF THE SYSTEM IS IN AN AREA OF LOW
TO MODERATE (10-20 KTS) VERTICAL WIND SHEAR WHILE THE EASTERN SIDE
OF THE SYSTEM IS IN AN AREA OF MODERATE TO HIGH (15-30 KTS). THE
GENERALLY UNFAVORABLE VERTICAL WIND SHEAR IS OFFSET BY STRONG
EQUATORWARD OUTFLOW AND HIGH (29-30 CELSIUS) SEA SURFACE
TEMPERATURES (SST) IN THE REGION. TS 18W IS TRACKING WEST-NORTHWEST
BETWEEN A SUBTROPICAL RIDGE (STR) TO THE NORTHWEST AND A STR TO THE
NORTHEAST. THE STR TO THE NORTHEAST HAS BEGUN TO BUILD AND IS TAKING
OVER AS THE PRIMARY STEERING MECHANISM FOR THE SYSTEM.
3. FORECAST REASONING.
   A. THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE
PREVIOUS PROGNOSTIC REASONING MESSAGE.
   B. TS 18W IS FORECAST TO TURN MORE NORTHWESTWARD AS IT BEGINS TO
RECURVE AROUND THE STR TO THE NORTHEAST. THE SYSTEM WILL STEADILY
INTENSIFY AS IT RECURVES; REACHING 60 KNOTS AS IT ROUNDS THE RIDGE
AXIS BY TAU 36. THE PEAK INTENSITY WILL OCCUR SHORTLY AFTER AS AN
APPROACHING MID-LATITUDE TROUGH BRIEFLY ENHANCES POLEWARD OUTFLOW,
ALLOWING ANOTHER BRIEF PERIOD OF INTENSIFICATION TO A PEAK OF 65
KNOTS. THEREAFTER, INCREASING VERTICAL WIND SHEAR FROM THE TROUGH
WILL BEGIN TO WEAKEN THE SYSTEM AND CAUSE THE SYSTEM TO BEGIN
EXTRATROPICAL TRANSITION. BY TAU 48, 18W WILL BE TRACKING TO THE
NORTHEAST, PASSING BETWEEN CHEJU ISLAND AND SASEBO, JAPAN BEFORE TAU
72. AS TAPAH TRACKS THROUGH THE KOREAN STRAIT, LAND INTERACTION WILL
CONTINUE TO WEAKEN THE SYSTEM. THE WEAKENING TREND WILL PERSIST FOR
THE DURATION OF THE FORECAST. NUMERICAL GUIDANCE IS IN STRONG
AGREEMENT THROUGH THIS PORTION OF THE FORECAST LENDING HIGH
CONFIDENCE IN THE JTWC FORECAST TRACK.
   C. AFTER TAU 72, TS 18W BEGINS TO ENCOUNTER AN INCREASINGLY
UNFAVORABLE ENVIRONMENT AS VERTICAL WIND SHEAR REMAINS STRONG AND
SSTS CONTINUE TO DECREASE. THE SYSTEM WILL CONTINUE TO WEAKEN AS IT
TRACKS THROUGH THIS UNFAVORABLE ENVIRONMENT IN THE SEA OF JAPAN AND
BECOMES EMBEDDED IN THE PASSING MID-LATITUDE TROUGH. TS 18W WILL
MAKE LANDFALL AS AN EXTRATROPICAL CYCLONE OVER SOUTHERN HOKKAIDO
AFTER TAU 84 BEFORE EXITING BACK INTO THE PACIFIC OCEAN AROUND TAU
96. MODEL GUIDANCE BEGINS TO SPREAD AFTER TAU 72; HOWEVER, IT
REMAINS IN GENERALLY GOOD AGREEMENT THROUGH TAU 96. DUE TO THE GOOD
AGREEMENT IN MODEL GUIDANCE THERE IS OVERALL HIGH CONFIDENCE IN THE
JTWC FORECAST TRACK.//
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