Tropical Storm MERANTI Advisory Sun Sep 11

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 16W (MERANTI)
WARNING NR 10//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 16W (MERANTI), LOCATED APPROXIMATELY 431 NM
NORTH-NORTHWEST OF YAP, HAS TRACKED WEST-NORTHWESTWARD AT 14 KNOTS
OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY
DEPICTS A RAPIDLY-CONSOLIDATING LOW-LEVEL CIRCULATION CENTER (LLCC)
WITH TIGHTLY-CURVED BANDING WRAPPING INTO A WELL-DEFINED CENTER. A
102224Z SSMIS IMAGE DEPICTS DEEP CONVECTIVE BANDING WRAPPING INTO
THE WELL-DEFINED CENTER, WHICH SUPPORTS THE CURRENT POSITION WITH
HIGH CONFIDENCE. OVERALL, OUTFLOW HAS IMPROVED, ESPECIALLY OVER THE
NORTHWEST QUADRANT, WITH WARM SST AND HIGH OCEAN HEAT CONTENT
VALUES. THE CURRENT INTENSITY IS ASSESSED AT 60 KNOTS BASED ON AN
AVERAGE OF DVORAK INTENSITY ESTIMATES RANGING FROM 55 TO 65 KNOTS.
TS 16W IS TRACKING WEST-NORTHWESTWARD UNDER THE STEERING INFLUENCE
OF THE SUBTROPICAL RIDGE ENTRENCHED TO THE NORTH.
3. FORECAST REASONING.
   A. NO CHANGE IN THE FORECAST PHILOSOPHY SINCE THE PREVIOUS
PROGNOSTIC REASONING MESSAGE.
   B. THROUGH TAU 72, TS 16W WILL TRACK GENERALLY WEST-NORTHWESTWARD
UNDER THE STEERING INFLUENCE OF THE DEEP-LAYERED SUBTROPICAL RIDGE
(STR). THIS STR IS DEPICTED IN THE RECENT 500MB ANALYSES AS A BROAD,
STRONG STR EXTENDING WESTWARD, FROM A HIGH CENTERED NEAR 32N 158E,
TO HAINAN ISLAND, CHINA WITH A MAJOR SHORTWAVE TROUGH DIGGING INTO
EASTERN AND SOUTHEASTERN CHINA. DYNAMIC MODEL GUIDANCE IS IN FAIR
AGREEMENT THROUGH TAU 72 WITH A 210-NM SPREAD IN SOLUTIONS AT TAU
72, THEREFORE, SIGNIFICANT UNCERTAINTY REMAINS. TS 16W IS FORECAST
TO CONTINUE INTENSIFYING AT A NEAR CLIMATOLOGICAL RATE TO A PEAK
INTENSITY OF 120 KNOTS BY TAU 48.
   C. AFTER TAU 72, DYNAMIC MODEL GUIDANCE DIVERGES WITH TWO
DISTINCT CLUSTERS. THE FIRST CLUSTER OF MODELS (UKMET, GFDN, COAMPS-
TC, JGSM, NAVGEM) DEPICTS A MORE WESTWARD TRACK INTO SOUTHEAST
CHINA. ANALYSIS OF AVAILABLE MODEL FIELDS INDICATES THAT
THESE MODELS ARE DRIVING THE SYSTEM INTO THE MAJOR TROUGH RATHER
THAN TRACKING IT POLEWARD TOWARD THE WEAKNESS IN THE STR AS THE
SECOND CLUSTER DEPICTS. THIS SCENARIO IS CURRENTLY ASSESSED AS
UNLIKELY / ERRONEOUS, THEREFORE, THE JTWC FORECAST FAVORS THE RE-
CURVE SCENARIO AS DEPICTED BY THE SECOND CLUSTER OF MODELS (GFS, GFS
ENSEMBLE, ECMWF, ECMWF ENSEMBLE AND HWRF, JGSM ENSEMBLE). TS 16W IS
FORECAST TO MAKE LANDFALL ALONG THE EASTERN COAST OF TAIWAN BY TAU
78 AND WILL WEAKEN RAPIDLY AS IT CROSSES THE MOUNTAINOUS TERRAIN.
THE SYSTEM WILL WEAKEN FURTHER AS IT TRACKS OVER EASTERN CHINA. DUE
TO THE LARGE MODEL SPREAD BEYOND 72 HOURS, THERE IS LOW CONFIDENCE
IN THE JTWC TRACK FORECAST.//
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