Tropical Storm MERANTI Advisory 12 Σεπ.

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 16W (MERANTI) WARNING
NR 13//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TYPHOON (TY) 16W (MERANTI), LOCATED APPROXIMATELY 576 NM SOUTH-
SOUTHEAST OF KADENA AB, HAS TRACKED WEST-NORTHWESTWARD AT 13 KNOTS
OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY
AND A 111837 AMSU-B 89 GHZ IMAGE DEPICT A RAPIDLY-CONSOLIDATING LOW-
LEVEL CIRCULATION CENTER (LLCC) WITH SPIRAL BANDS WRAPPING INTO A 4-NM
PINHOLE EYE. THEREFORE, THERE IS EXCELLENT CONFIDENCE IN THE CURRENT
POSITION AND RECENT TRACK MOTION. UPPER-LEVEL ANALYSIS AND ANIMATED
WATER VAPOR IMAGERY SHOW A FAVORABLE ENVIRONMENT WITH LOW VERTICAL
WIND SHEAR AND NEAR-RADIAL OUTFLOW. ADDITIONALLY, WARM SST AND HIGH
OCEAN HEAT CONTENT VALUES SUPPORT THE CURRENT RAPID INTENSIFICATION
TREND (TY 16W HAS INTENSIFIED 50 KNOTS OVER THE PAST 24 HOURS, FROM
50 TO THE CURRENT INTENSITY OF 100 KNOTS). THE CURRENT INTENSITY IS
BASED ON DVORAK INTENSITY ESTIMATES OF T5.5 (102 KNOTS) FROM BOTH
PGTW AND RJTD. TY 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, TY 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 TAIWAN WITH A MAJOR SHORTWAVE TROUGH DIGGING INTO EASTERN AND
SOUTHEASTERN CHINA. DYNAMIC MODEL GUIDANCE IS IN FAIR AGREEMENT
THROUGH TAU 48 WITH A 125-NM SPREAD IN SOLUTIONS AT TAU 48. AFTER
TAU 48, THE MODEL TRACKERS DIVERGE WITH SIGNIFICANT UNCERTAINTY IN
THE EXACT TRACK NEAR TAIWAN. TY 16W IS FORECAST TO CONTINUE
INTENSIFYING RAPIDLY TO A PEAK INTENSITY OF 135 KNOTS BY TAU 36.
AFTER TAU 36, THE SYSTEM WILL WEAKEN AS IT APPROACHES TAIWAN AND
CROSSES OVER ITS MOUNTAINOUS TERRAIN.
   C. AFTER TAU 72, DYNAMIC MODEL GUIDANCE DIVERGES WITH TWO
 DISTINCT CLUSTERS. THE FIRST CLUSTER OF MODELS (UKMET, ECMWF,
 GFDN AND NAVGEM) HAVE REMAINED QUITE ERRATIC AND HAVE NOW
 SHIFTED FROM A RE-CURVE TRACK BACK TO 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, HWRF, JGSM ENSEMBLE AND COAMPS-TC). TY 16W IS
 FORECAST TO MAKE LANDFALL ALONG THE EASTERN COAST OF TAIWAN
 BY TAU 60 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 48 HOURS,
 THERE IS LOW CONFIDENCE IN THE JTWC TRACK FORECAST.//
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