Tropical Storm CHAN-HOM Advisory Thu Oct 08

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 16W (CHAN-HOM) WARNING NR 011//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TYPHOON (TY) 16W (CHAN-HOM), LOCATED APPROXIMATELY 378 NM EAST OF
KADENA AB, OKINAWA, HAS TRACKED NORTHWESTWARD AT 11 KNOTS OVER THE
PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY DEPICTS
A CONSOLIDATING SYSTEM WITH A RAGGED EYE, WHICH SUPPORTS THE INITIAL
POSITION AND RECENT TRACK MOTION WITH GOOD CONFIDENCE. A 070435Z
AMSR2 36GHZ MICROWAVE IMAGE INDICATES IMPROVED CONVECTIVE BANDING
WRAPPING TIGHTLY INTO A MICROWAVE EYE FEATURE. THE INITIAL
INTENSITY IS ASSESSED AT 65 KNOTS BASED ON PGTW/KNES DVORAK
INTENSITY ESTIMATES OF T4.0 (65 KNOTS). ENVIRONMENTAL CONDITIONS
REMAIN FAVORABLE WITH ENHANCED POLEWARD OUTFLOW, GOOD EQUATORWARD
OUTFLOW, LOW (5-10 KT) VERTICAL WIND SHEAR (VWS), AND WARM (28-29
CELSIUS) SEA SURFACE TEMPERATURES. THE SYSTEM IS TRACKING ALONG THE
SOUTHWESTERN PERIPHERY OF A SUBTROPICAL RIDGE (STR) POSITIONED TO
THE NORTHEAST.
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS
PROGNOSTIC REASONING MESSAGE. IMPORTANT NOTE: TY 16W HAS MAINTAINED
EXTENSIVE GALE-FORCE WINDS AS LARGE AS 180NM RADIUS. TO FURTHER
COMPLICATE THIS SCENARIO, THE SYSTEM IS TRACKING INTO A STRONG
BAROCLINIC ZONE WITH ITS OWN TIGHT GRADIENT AS EVIDENCED BY 25 TO 35
KNOT SURFACE WIND REPORTS ACROSS WESTERN JAPAN AND THE RYUKYU
ISLANDS. THUS GALE-FORCE WINDS ARE POSSIBLE ALONG AND OUTSIDE THE
PERIPHERY OF THE SYSTEM AS IT TRACKS CLOSER TO JAPAN.
   B. TY 16W IS FORECAST TO TRACK NORTHWESTWARD TO NORTHWARD THROUGH
TAU 48 AS IT RECURVES. NUMERICAL MODEL GUIDANCE IS IN BETTER
AGREEMENT BUT STILL INDICATES SIGNIFICANT UNCERTAINTY IN THE TIMING
AND POINT OF THE RECURVE WITH A 130NM SPREAD IN SOLUTIONS AT TAU 48.
AFTER TAU 48, THE SYSTEM WILL ACCELERATE NORTHEASTWARD AS IT BEGINS
TO INTERACT WITH THE MIDLATITUDE WESTERLIES. THE UNCERTAINTY IN THE
NUMERICAL MODEL SOLUTIONS IS DUE TO THE PRESENCE OF A LARGE BREAK IN
THE WESTERN AND EASTERN SUBTROPICAL RIDGES AND THE LACK OF A DYNAMIC
MIDLATITUDE SHORTWAVE TROUGH, WHICH WOULD TYPICALLY ACCELERATE THE
SYSTEM POLEWARD. IN FACT, THE GLOBAL MODELS DEPICT A ZONAL PATTERN
WITH A SHORTWAVE RIDGE PROPAGATING OVER THE KOREAN PENINSULA NEAR
TAU 36-48. TY 16W IS FORECAST TO INTENSIFY TO A PEAK INTENSITY OF 80
KNOTS BY TAU 24-36 UNDER FAVORABLE CONDITIONS. BASED ON THE COMPLEX
STEERING PATTERN AND SPREAD IN SOLUTIONS, THERE IS FAIR CONFIDENCE
IN THIS PORTION OF THE JTWC FORECAST TRACK.
   C. AFTER TAU 72, THE SYSTEM WILL ROUND THE STR AXIS AND TURN TO A
GENERALLY EAST-NORTHEASTWARD TRACK ALONG THE SOUTHERN COAST OF
JAPAN. DUE TO HIGH (>35 KTS) VWS, TY 16W IS EXPECTED TO WEAKEN
STEADILY FOR THE REMAINDER OF THE FORECAST PERIOD TO AN INTENSITY OF
45 KTS BY TAU 120. TY 16W WILL BEGIN EXTRATROPICAL TRANSITION (ETT)
BY TAU 72 AND WILL COMPLETE ETT BETWEEN TAU 96 AND TAU 120 AS IT
GAINS FRONTAL CHARACTERISTICS. THE JTWC FORECAST TRACK IS POSITIONED
ON THE SOUTHERN EDGE OF THE AIDS ENVELOPE CLOSE TO THE ECMWF
SOLUTION AND THE OFFICIAL JMA FORECAST TRACK. DUE TO UNCERTAINTY IN
THE TRACK AS THE SYSTEM UNDERGOES ETT, THERE IS FAIR CONFIDENCE
(MODERATE UNCERTAINTY) IN THE JTWC FORECAST TRACK.//
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