Tropical Storm KROVANH Advisory ven, 18.09.

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 20W (KROVANH) WARNING NR 14//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TYPHOON (TY) 20W (KROVANH), LOCATED APPROXIMATELY 93 NM SOUTHEAST
OF IWO TO, JAPAN HAS TRACKED NORTH-NORTHWESTWARD AT 08 KNOTS OVER THE
PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY DEPICTS A 17NM
CLOUD FILLED EYE SURROUNDED BY AN ELONGATED CONVECTIVE CORE THAT HAS
BEEN FLARING OVER THE PAST SIX HOURS. WATER VAPOR IMAGERY SHOWS
SIGNIFICANT DRY AIR ALONG THE WESTERN PERIPHERY ASSOCIATED WITH A DEEP
MID-LATITUDE TROUGH LOCATED TO THE WEST. HOWEVER, THE TROUGH IS ALSO
PROVIDING AN ENHANCED POLEWARD OUTFLOW CHANNEL THAT IS SUSTAINING THE
CURRENT CONVECTION. A 172228Z SSMIS 91GHZ MICROWAVE IMAGE SHOWS A WELL
DEFINED 20NM MICROWAVE EYE WITH THE BULK OF CONVECTION ALONG THE NORTHERN
PERIPHERY DUE TO PERSISTENT 20 KNOT SOUTHWESTERLY VERTICAL WIND SHEAR
(VWS). THE CURRENT POSITION IS BASED ON THE EYE FEATURE SEEN IN THE ABOVE
IMAGERY WITH HIGH CONFIDENCE. THE INITIAL INTENSITY HAS BEEN KEPT AT 105
KNOTS BASED ON AN ASSESSMENT OF DVORAK INTENSITY ESTIMATES FROM ALL
AGENCIES AND THE UNCHANGED STRUCTURE OF THE TYPHOON. ENVIRONMENTAL ANALYSIS
INDICATES A DETERIORATING ENVIRONMENT AS VWS HAS INCREASED AND PRESSURE
ALONG THE WESTERN PERIPHERY HAS BEGUN TO ELONGATE AND TILT THE SYSTEM.
TY 20W IS TRACKING ALONG THE SOUTHWESTERN PERIPHERY OF A DEEP-LAYERED
SUB-TROPICAL RIDGE (STR).
3. FORECAST REASONING.
   A. THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE
PREVIOUS PROGNOSTIC REASONING MESSAGE.
   B. TY 20W WILL BEGIN TO TURN POLEWARD OVER THE NEXT SIX HOURS AND ROUND
THE STR AXIS BEFORE TURNING TO A NORTHEASTERN TRAJECTORY. MARGINAL CONDITIONS
WILL PERSIST FOR THE NEXT 12 HOURS ALLOWING TY KROVANH TO MAINTAIN ITS
CURRENT INTENSITY. BEYOND TAU 12, CONTINUED DRY AIR ENTRAINMENT AND INCREASING
VWS WILL BEGIN TO WEAKEN THE TYPHOON. SSTS WILL DROP BELOW 26 CELSIUS JUST
AFTER TAU 48 WHICH WILL INCREASE THE RATE OF WEAKENING. DYNAMIC MODEL GUIDANCE
IS IN VERY TIGHT AGREEMENT THROUGH THIS PORTION OF THE FORECAST PERIOD.
   C. IN THE EXTENDED TAUS, TY KROVANH WILL CONTINUE TO TRACK TO THE NORTHEAST
AROUND THE STR. INCREASING VWS AND COOLER SSTS WILL FURTHER WEAKEN THE SYSTEM
AS IT ACCELERATES INTO THE WEAK MID-LATITUDE BAROCLINIC ZONE. THE SYSTEM IS
FORECAST TO BEGIN EXTRA-TROPICAL TRANSITION AT TAU 96 AND CONTINUE THROUGH THE
END OF THE FORECAST PERIOD. DYNAMIC MODEL GUIDANCE DIVERGES IN THIS PORTION OF
THE FORECAST DUE TO A COMPLEX STEERING ENVIRONMENT WHICH COULD CAUSE THE SYSTEM
TO BECOME QUASI-STATIONARY. ALTHOUGH THE MODEL GUIDANCE CONTINUES TO STRUGGLE
IN THE EXTENDED FORECAST, THE TIGHT AGREEMENT IN THE FIRST 72 HOURS, LENDS TO
HIGH CONFIDENCE IN THE JTWC TRACK FORECAST.//
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