Tropical Storm NAMTHEUN Advisory Fri Sep 02

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 15W (NAMTHEUN) WARNING NR 09//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TYPHOON (TY) 15W (NAMTHEUN), LOCATED APPROXIMATELY 323 NM SOUTH OF
SASEBO, JAPAN, HAS TRACKED NORTHWARD AT 06 KNOTS OVER THE PAST
SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY AND RECENT
RADAR IMAGERY DEPICT A VERY SMALL SYSTEM WITH A 8-NM EYE, AND ALSO
INDICATE SLIGHT TROCHOIDAL MOTION AS THE SYSTEM TRACKS NORTHWARD.
A 021210Z METOP-B 89GHZ IMAGE SHOWS A SYMMETRIC CORE OF DEEP
CONVECTION OF APPROXIMATELY 60NM DIAMETER. BASED ON THE PLETHORA OF
HIGH RESOLUTION IMAGERY, THERE IS EXCELLENT CONFIDENCE IN THE
CURRENT POSITION. ANIMATED WATER VAPOR IMAGERY INDICATES NEAR-RADIAL
OUTFLOW ENHANCED BY THE MIDLATITUDE SHORTWAVE TROUGH POSITIONED TO
THE NORTH. CONSEQUENTLY, TY 15W HAS RAPIDLY INTENSIFIED AND IS
CURRENTLY ASSESSED AT 100 KNOTS BASED ON DVORAK CURRENT INTENSITY
ESTIMATES OF 5.5 (102 KNOTS) FROM BOTH KNES AND PGTW. TY 15W IS
TRACKING NORTHWARD ALONG THE WESTERN PERIPHERY OF THE STEERING RIDGE
POSITIONED TO THE EAST.
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS
PROGNOSTIC REASONING MESSAGE. THE JTWC TRACK FORECAST IS HIGHLY
CONSISTENT WITH THE PREVIOUS JTWC WARNING.
   B. TY 15W IS FORECAST TO CONTINUE TRACKING NORTHWARD THROUGH TAU
36 ALONG THE WESTERN PERIPHERY OF THE SUBTROPICAL RIDGE (STR). AFTER
TAU 36, THE SYSTEM WILL TURN NORTHEASTWARD AS IT ROUNDS THE STR AXIS
AND SHOULD MAKE LANDFALL ALONG THE WESTERN, CENTRAL COAST OF KYUSHU
NEAR TAU 36. ENVIRONMENTAL CONDITIONS WILL REMAIN FAVORABLE THROUGH
TAU 12 WITH SST NEAR 29C, HIGH OCEAN HEAT CONTENT AND VIGOROUS
POLEWARD OUTFLOW. INITIAL INTERACTION WITH LAND WILL BEGIN TO
GRADUALLY WEAKEN THE SYSTEM AFTER TAU 12 AND INCREASING VERTICAL
WIND SHEAR (VWS) WILL PRODUCE A MORE RAPID DECREASE IN INTENSITY AS
THE SYSTEM APPROACHES THE COAST AND MOVES INLAND OVER THE RUGGED
TERRAIN OF KYUSHU. WITH THE EXCEPTION OF NAVGEM, DYNAMIC MODEL
TRACKERS ARE IN GOOD AGREEMENT WITH A 100-NM SPREAD IN SOLUTIONS AT
TAU 36, NEAR SASEBO, THEREFORE, THERE REMAINS SOME UNCERTAINTY IN
THE EXACT TRACK AT CPA. AFTER TAU 48, TY 15W WILL TRACK OVER THE
EAST SEA AND BEGIN TO ACCELERATE INTO THE MIDLATITUDE WESTERLY FLOW,
WHICH WILL RAPIDLY WEAKEN THE SYSTEM.
    C. IN THE EXTENDED TAUS, TY 15W WILL BEGIN EXTRATROPICAL
TRANSITION (ETT) NEAR TAU 72 AND IS EXPECTED TO COMPLETE ETT BY TAU
96 AS IT BECOMES FULLY EMBEDDED WITHIN THE MIDLATITUDE SHORTWAVE
TROUGH. STRONG VWS AND COOLER SST WILL SERVE TO FURTHER WEAKEN THE
SYSTEM. BASED ON THE GOOD MODEL TRACKER AGREEMENT, THERE IS HIGH
CONFIDENCE IN THE JTWC FORECAST TRACK.//
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