Tropical Storm NAMTHEUN Advisory Dom, 04-09

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 15W (NAMTHEUN) WARNING NR 15//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TYPHOON (TY) 15W (NAMTHEUN), LOCATED APPROXIMATELY 96 NM SOUTH OF
SASEBO, JAPAN, HAS TRACKED NORTH-NORTHWESTWARD AT 04 KNOTS OVER THE
PAST SIX HOURS. ANIMATED ENHANCED IR AND VISIBLE SATELLITE IMAGERY
SHOW THE SYSTEM HAS MAINTAINED CENTRAL CONVECTIVE SIGNATURE EVEN AS
STRONG NORTHWARD VERTICAL WIND SHEAR (VWS) INUNDATE THE CYCLONE,
PARTLY EXPOSING THE LOW LEVEL CIRCULATION THAT REMAINS TIGHTLY WOUND.
THE INITIAL POSITION IS BASED ON A COMPOSITE RADAR LOOP FROM JMA AND
PLACED WITHIN A TIGHT CLUSTER OF SATELLITE FIXES. THE INITIAL
INTENSITY OF 65 KNOTS IS BASED ON CONGRUENT DVORAK ESTIMATES FROM
PGTW, KNES, AND RJTD. UPPER LEVEL ANALYSIS INDICATES THE SYSTEM IS IN
AN AREA OF HIGH (30-35 KNOT) VWS AND INCREASING LAND INTERACTION.
HOWEVER, A STRONG POLEWARD OUTFLOW, AS EVIDENT ON ANIMATED WATER
VAPOR IMAGERY IS SUSTAINING THE DISPLACED CENTRAL CONVECTION. 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 NORTHWARD ALONG
THE WESTERN PERIPHERY OF THE STR AND WEAKEN AS IT GAINS LATITUDE DUE
TO INCREASING VWS AND LAND INTERACTION. TY 15W WILL MAKE LANDFALL
ALONG THE WESTERN COAST OF KYUSHU, JUST SOUTH OF SASEBO, NEAR TAU 12.
AFTERWARDS THE SYSTEM WILL ROUND THE STR AXIS AND ACCELERATE
NORTHEASTWARD 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 (SOJ). TY 15W WILL BEGIN EXTRA-TROPICAL TRANSITION (ETT)
NEAR TAU 24 AND COMPLETE ETT AS A WEAK COLD-CORE LOW BY TAU 48. THERE
IS A DISTINCT POSSIBILITY THAT THE SYSTEM WILL DISSIPATE IN THE SOJ
DURING ETT. DYNAMIC MODEL GUIDANCE REMAINS IN TIGHT AGREEMENT,
LENDING 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 WITH HIGHER
TERRAIN.//
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