美帝JTWC:
3. FORECAST REASONING.
SIGNIFICANT FORECAST CHANGES: THE TRACK HAS SLOWED SIGNIFICANTLY IN
THE EXTENDED FORECAST, AND REMAINS SOUTH OF 27N LATITUDE FOR THE
ENTIRE FORECAST PERIOD.
FORECAST DISCUSSION: SUPER TYPHOON 20W (KRATHON) CONTINUES TO
SLOWLY TRACK WESTWARD AT ABOUT 5 KNOTS OVER THE PAST SIX HOURS IN A
GENERALLY WEAK STEERING PATTERN. THE LATEST RADAR ANIMATION
SUGGESTS THE FORWARD MOTION HAS SLOWED EVEN FURTHER IN JUST THE
LAST COUPLE OF HOURS AS THE TWO COMPETING STEERING INFLUENCES
COMPETE FOR DOMINANCE. GLOBAL MODELS INCLUDING ECWMF AND GFS ARE
FOR THE MOST PART IN AGREEANCE WITH THE STR TO THE EAST BUILDING
AND SLIDING NORTHWEST OVER THE NEXT 48 HOURS, WHILE THE RIDGE OVER
CHINA ALSO LOOKS TO STRENGTHEN AND MOVE SOUTH INTO NORTHERN VIETNAM
OVER THE SAME TIME PERIOD. HOWEVER THE STR TO THE EAST WILL BE
CLOSER TO THE STY 20W WHICH WILL LEAD TO AN STRONGER STEERING
GRADIENT ON THE EAST SIDE OF THE SYSTEM, WHICH SHOULD RESULT IN A
TURN NORTHWARD WITHIN THE NEXT 12 TO 24 HOURS. AFTER THIS POINT,
THE ULTIMATE TRACK OF STY 20W WILL BE HIGHLY INFLUENCED BY THE
INTENSITY AS DEPICTED BY THE MODELS. THE GFS SHOWS THE SYSTEM
REMAINING OFFSHORE FOR LONGER, UNDERGOING AN ERC OVER THE NEXT 24
TO 48 HOURS, HINTS OF THIS ARE ALREADY SEEN IN THE RADAR DATA, AND
THE ONSET OF EXTREMELY RAPID WEAKENING DUE TO THE INFLUENCE OF THE
ERC AND LAND INTERACTIONS. THE RAPID WEAKENING IN THE GFS MEANS THE
SYSTEM NEVER MAKES LANDFALL, PERFORMS A 180 AND TRACKS
SOUTHWESTWARD AFTER TAU 72 ALONG THE LOW-LEVEL FLOW LINES. THE
ECMWF ON THE OTHER HAND TRACKS THE SYSTEM AT A FASTER PACE TOWARDS
LANDFALL, AND KEEPS IT STRONGER THROUGH LANDFALL. THIS ALLOWS THE
STEERING RIDGE TO THE EAST TO MAINTAIN A MORE PRONOUNCED INFLUENCE
ON THE TRACK. AT THE SAME TIME, THE ECMWF SHOWS A SLIGHTLY DEEPER
MID-LATITUDE TROUGH PASSING THROUGH THE YELLOW SEA AROUND TAU 60,
WHICH PICKS UP STY 20W AND ALLOWS IT TO TRACK OVER TAIWAN. THUS WE
ARE PRESENTED WITH AN EXTREME BIFURCATION SCENARIO, COMPLICATED BY
THE COMPLEX NATURE OF TROPICAL CYCLONE INTERACTION WITH TAIWAN AND
THE POTENTIAL FOR AN ERC. THE JTWC TRACK HEDGES MORE TOWARDS THE
ECMWF SOLUTION, WITH THE SYSTEM MAKING LANDFALL AROUND TAU 48, WITH
A LEE-SIDE JUMP OCCURRING AROUND BY TAU 72. POST RE-EMERGENCE, THE
SIGNIFICANTLY WEAKENED REMNANTS OF STY 20W WILL CONTINUE TO WEAKEN
AND WILL NOT LIKELY BE PICKED UP BY THE UPPER-LEVEL TROUGH WHICH BY
THIS TIME WILL BE EXITING THE AREA, WITH THE RESULT THAT THE SYSTEM
WILL MEANDER NORTHEAST OF TAIWAN THROUGH THE END OF THE FORECAST.
MODEL DISCUSSION: DETERMINISTIC AND ENSEMBLE TRACK GUIDANCE IS FOR
THE MOST PART IN DECENT AGREEMENT IN THE SHORT-TERM, THROUGH TAU 48
WITH ALL OF THE GUIDANCE EXCEPT THE GFS, SHOWING A SLOW TURN
NORTHWARD OVER THE NEXT 12 TO 24 HOURS, FOLLOWED BY A TURN
NORTHEASTWARD WITH LANDFALL AROUND TAU 48 IN THE VICINITY OF
KAOHSIUNG, TAIWAN. AFTER THIS POINT, SUFFICE TO SAY THERE IS LITTLE
IN THE WAY OF AGREEMENT IN THE MODEL GUIDANCE. AS DISCUSSED ABOVE,
THE GFS WEAKENS THE SYSTEM RAPIDLY AND IT SHIFTS THE TRACK
SOUTHWESTWARD THROUGH TAU 120. THE REMAINDER OF THE GUIDANCE KEEPS
MOVING THE SYSTEM NORTHEASTWARD OVER TAIWAN, THOUGH SEVERAL
(INCLUDING NAVGEM) BECOME HIGHLY ERRATIC AND DISSIPATE THE SYSTEM
OVER NORTHERN TAIWAN. THE ECMWF JUMPS OVER TAIWAN BUT THEN KEEPS
THE SYSTEM QUASI-STATIONARY WEST OF ISHIKAGIJIMA THROUGH TAU 120.
MEANWHILE THE GEFS ENSEMBLE MEAN TURNS THE SYSTEM NORTHWESTWARD
INTO EASTERN CHINA BY TAU 120 AND THE GALWEM TRACKS THE SYSTEM INTO
CHEJU DO. SUFFICE TO SAY THAT SPREAD IS SIGNIFICANT AND THE TERM IS
NOT REPRESENTATIVE OF THE SIGNIFICANT AMOUNT OF UNCERTAINTY PRESENT
IN THE FORECAST. THE JTWC FORECAST REMAINS CONSISTENT WITH THE
PREVIOUS FORECAST, BUT SLOWS SIGNIFICANTLY IN THE LONG-TERM
FORECAST POINTS AS WE WAIT TO SEE WHICH SCENARIO WINS OUT IN THE
NEAR-TERM. INTENSITY FORECAST IS IN SOLID AGREEMENT THAT THE SYSTEM
WILL WEAKEN STEADILY TO RAPIDLY AS IT CROSSES TAIWAN. 
