Tropical Storm ATSANI Advisory Sun Aug 23

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 17W (ATSANI) WARNING NR 33//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TYPHOON (TY) 17W (ATSANI), LOCATED APPROXIMATELY 528 NM
SOUTH-SOUTHEAST OF YOKOSUKA, JAPAN, HAS TRACKED NORTHWARD AT 06 KNOTS
OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY
SHOWS SLIGHT WARMING OF CONVECTIVE TOPS AS DRY AIR WRAPPED INTO THE
CORE OF THE TYPHOON WITH A LARGE 40-NM EYE. THE INITIAL POSITION IS
BASED ON THE EYE FEATURE WITH FAIR CONFIDENCE. UPPER LEVEL ANALYSIS
SHOWS A MARGINAL ENVIRONMENT WITH 15 TO 20 KNOT NORTHEASTERLY
VERTICAL WIND SHEAR AND GOOD EQUATORWARD OUTFLOW; HOWEVER, THE DRY
AIR ENTRAINMENT IS PROHIBITING INTENSIFICATION. THE INITIAL INTENSITY
OF 80 KNOTS IS BASED ON AN ASSESSMENT OF DVORAK INTENSITY ESTIMATES
AND REFLECTS A WEAKENING IN THE CONVECTIVE CORE STRUCTURE. TY 17W IS
TRACKING ALONG THE WESTERN 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 CONTINUE TO ROUND THE WESTERN PERIPHERY OF THE STR
AND TURN POLEWARD. BY TAU 24, TY ATSANI WILL BEGIN TO ACCELERATE
NORTHEASTWARD IN FRONT OF AN APPROACHING MID-LATITUDE TROUGH.
ENVIRONMENTAL CONDITIONS WILL REMAIN MOSTLY UNCHANGED THROUGH TAU 72,
CREATING A SLOW WEAKENING TREND. BY TAU 48, SEA SURFACE TEMPERATURES
WILL DROP BELOW 26 CELSIUS AND THE SYSTEM WILL BEGIN EXTRA-TROPICAL
TRANSITION (ETT).
   C. IN THE EXTENDED FORECAST, TY 17W WILL CONTINUE TO ACCELERATE
NORTHEASTWARD AND EMBED FURTHER INTO THE WEAK BAROCLINIC ZONE. BY
THE END OF THE FORECAST PERIOD, TY ATSANI WILL SLOW AS IT GETS
ENVELOPED BY THE MID-LATITUDE TROUGH AND IS FORECAST TO COMPLETE ETT
AS A STRONG COLD-CORE LOW. DYNAMIC MODEL GUIDANCE REMAINS IN TIGHT
AGREEMENT LENDING HIGH CONFIDENCE IN THE JTWC FORECAST TRACK.//
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