Tropical Storm ATSANI Advisory lun, 24-08

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 17W (ATSANI) WARNING NR 40//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TYPHOON (TY) 17W (ATSANI), LOCATED APPROXIMATELY 426 NM NORTHEAST
OF CHICHI JIMA, HAS TRACKED NORTHEASTWARD AT 10 KNOTS OVER THE PAST
SIX HOURS. ANIMATED MULSPECTRAL SATELLITE IMAGERY (MSI) INDICATES A
CONTINUED WEAKENING TREND, WITH SPIRAL BANDING ALONG THE WESTERN
PERIPHERY BECOMING SHALLOWER IN RESPONSE TO COOL DRY AIR WRAPPING IN
FROM THE NORTH. THE INITIAL POSITION IS BASED ON THE ANIMATED MSI AS
WELL AS CURVED BANDING IN A 240012Z METOP-B MICROWAVE IMAGE WITH
GOOD CONFIDENCE. THE INITIAL INTENSITY OF 65 KNOTS IS CONSISTENT
WITH DVORAK ESTIMATES FROM ALL REPORTING AGENCIES. A 240011Z ASCAT
PARTIAL PASS INDICATES THE 50 KT WIND FIELD HAS SIGNIFICANTLY
CONTRACTED OVER THE PAST 12 HOURS. UPPER LEVEL ANALYSIS INDICATES
LOW 5-10 KNOT VERTICAL WIND SHEAR (VWS) OVER THE CENTER AND GOOD
RADIAL OUTFLOW. THE NORTHERN EXTENT OF THIS EXPANSIVE SYSTEM HOWEVER
IS UNDER CONSIDERABLY HIGHER VWS AND IS INTERACTING WITH THE MID-
LATITUDE BAROCLINIC ZONE. TY 17W IS TRACKING ALONG THE NORTHWESTERN
PERIPHERY OF THE DEEP-LAYERED SUB-TROPICAL RIDGE (STR) LOCATED TO
THE EAST.
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS
PROGNOSTIC REASONING MESSAGE.
   B. TY 17W WILL ACCELERATE NORTHEASTWARD IN THE MID-LATITUDE
WESTERLY FLOW. SEA SURFACE TEMPERATURES WILL DROP BELOW 26 CELSIUS
OVER THE NEXT 12-24 HOURS AND THE SYSTEM WILL BEGIN EXTRA-TROPICAL
TRANSITION (ETT). THE ETT WILL BE COMPLETE BY TAU 48. GOOD OUTFLOW
AND LOW VWS WILL SOMEWHAT OFFSET COLDER SSTS, RESULTING IN A GRADUAL
WEAKENING TREND. TY 17W IS THEREFORE EXPECTED TO BECOME A STRONG
GALE-FORCE COLD CORE LOW BY THE END OF THE FORECAST PERIOD.//
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