WDPN32 PGTW 220300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 17W (ATSANI) WARNING NR 32//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TYPHOON (TY) 17W (ATSANI), LOCATED APPROXIMATELY 238 NM NORTHEAST 
OF IWO TO, HAS TRACKED NORTH-NORTHWESTWARD AT 08 KNOTS OVER THE PAST 
SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY AND WATER VAPOR 
IMAGERY SHOWS DRY AIR WRAPPING INTO THE CORE OF THE TYPHOON WITH A 
LARGE 30-NM CLOUD FILLED EYE. A 212118Z SSMIS 91GHZ MICROWAVE IMAGE 
SHOWS LIMITED CONVECTION ALONG THE NORTHERN SEMI-CIRCLE DUE TO 
INCREASED SUBSIDENCE. UPPER LEVEL ANALYSIS SHOWS A MARGINAL 
ENVIRONMENT WITH 15 TO 20 KNOT NORTHEASTERLY VERTICAL WIND SHEAR AND 
GOOD EQUATORWARD OUTFLOW; HOWEVER, THE SIGNIFICANT DRY AIR 
ENTRAINMENT IS PROHIBITING INTENSIFICATION. THE INITIAL POSITION IS 
BASED ON THE LARGE EYE WITH HIGH CONFIDENCE. THE CURRENT INTENSITY 
HAS BEEN DECREASED TO 85 KNOTS BASED AN ASSESSMENT OF DVORAK 
INTENSITY ESTIMATES AND 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 
DEEP LAYERED STR AND TURN POLEWARD. BY TAU 36, 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 72, 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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