Tropical Storm NAMTHEUN Advisory Sat Sep 03

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 15W (NAMTHEUN) WARNING NR 13//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TYPHOON (TY) 15W (NAMTHEUN), LOCATED APPROXIMATELY 144 NM SOUTH
OF SASEBO, JAPAN, HAS TRACKED NORTH-NORTHWESTWARD AT 07 KNOTS OVER
THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED (EIR) SATELLITE
IMAGERY AND A SERIES OF MICROWAVE IMAGES DEPICT A CONTINUED
WEAKENING OF THE SIGNIFICANT CONVECTION DUE TO HIGH VERTICAL WIND
SHEAR (VWS) THAT IS PUSHING THE CONVECTION TO THE NORTHEAST OF
CENTER. OVER THE LAST FEW FRAMES OF THE EIR LOOP, IT APPEARS THAT
THE LOW LEVEL CIRCULATION CENTER (LLCC) HAS BECOME PARTIALLY EXPOSED
JUST SOUTHWEST OF THE CONVECTION. THE CURRENT POSITION IS BASED ON
THE LLCC SEEN IN THE KYUSHU RADAR LOOP WITH GOOD CONFIDENCE. THE
INITIAL INTENSITY HAS BEEN LOWERED ONCE AGAIN DUE TO THE WEAKENED
STRUCTURE OF THE CYCLONE AND IS SUPPORTED BY DECREASING DVORAK
INTENSITY ESTIMATES AND SATCON INTENSITY ESTIMATES. TY NAMTHEUN IS
TRACKING IN A POOR ENVIRONMENT WITH HIGH (30-35 KNOT) VWS AND
INCREASING LAND INTERACTION. THE CYCLONE IS TRACKING ALONG THE
WESTERN PERIPHERY OF THE SUBTROPICAL RIDGE (STR) TO THE EAST.
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS
PROGNOSTIC REASONING MESSAGE.
   B. TY NAMTHEUN IS FORECAST TO CONTINUE TRACKING NORTH-
NORTHWESTWARD ALONG THE WESTERN PERIPHERY OF THE STR AND WEAKEN AS
IT GAINS LATITUDE DUE TO INCREASING VWS AND CONTINUED LAND
INTERACTION. JUST AFTER TAU 24, TY 15W WILL MAKE LANDFALL ALONG THE
WESTERN COAST OF KYUSHU, JUST SOUTH OF SASEBO. AFTERWARDS THE SYSTEM
WILL ROUND THE STR AXIS AND ACCELERATE TO THE NORTHEAST WITH THE MID-
LATITUDE WESTERLY FLOW. RAPID DECAY WILL BEGIN ONCE THE SYSTEM MOVES
OVER COLD WATER (LESS THAN 25C) IN THE SEA OF JAPAN. CONCURRENTLY,
TY 15W WILL BEGIN TO UNDERGO EXTRA-TROPICAL TRANSITION (ETT) NEAR
TAU 48. THE TYPHOON IS EXPECTED TO COMPLETE ETT AS A WEAK COLD-CORE
LOW BY THE END OF THE FORECAST PERIOD. DYNAMIC MODEL GUIDANCE
CONTINUES TO SHOW A SLIGHT SPLIT IN THE FIRST 24 HOURS WITH NAVGEM
AND EGRR TAKING A WIDER TURN OUT TO THE WEST BEFORE MAKING LANDFALL.
THE JTWC TRACK FORECAST IS LAID SLIGHTLY TO THE EAST OF CONSENSUS TO
OFFSET THE LESS LIKELY WIDE TURN PROPOSED BY NAVGEM AND EGRR. THERE
IS HIGH CONFIDENCE IN THE JTWC FORECAST TRACK; HOWEVER, ANY EASTWARD
WOBBLE IN THE STORM MOTION WILL RESULT IN A MUCH WEAKER SYSTEM
MAKING LANDFALL DUE TO INCREASED LAND INTERACTION.//
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