Tropical Storm KROVANH Advisory za, 19-09

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 20W (KROVANH) WARNING NR 18//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TYPHOON (TY) 20W (KROVANH), LOCATED APPROXIMATELY 60 NM NORTHEAST
OF CHICHI JIMA, HAS TRACKED NORTH-NORTHEASTWARD AT 11 KNOTS OVER THE
PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY SHOWS CURVED
CONVECTIVE BANDING WRAPPING TIGHTLY INTO A PARTIALLY-EXPOSED LOW-
LEVEL CIRCULATION CENTER (LLCC). A 182215Z SSMIS 91GHZ IMAGE DEPICTS
A MICROWAVE EYE FEATURE, WHICH SUPPORTS THE CURRENT POSITION AND
TRACK MOTION WITH GOOD CONFIDENCE. UPPER-LEVEL ANALYSIS SHOWS A
MARGINALLY-FAVORABLE ENVIRONMENT WITH STRONG SOUTHWESTERLY VERTICAL
WIND SHEAR OFFSET BY VIGOROUS POLEWARD OUTFLOW. THE INITIAL
INTENSITY IS ASSESSED AT 75 KNOTS BASED ON DVORAK CURRENT INTENSITY
ESTIMATES OF 77 KNOTS FROM PGTW AND RJTD. TY 20W IS TRACKING ALONG
THE NORTHWESTERN PERIPHERY OF A DEEP-LAYERED STR.
3. FORECAST REASONING.
   A. THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE
PREVIOUS PROGNOSTIC REASONING MESSAGE.
   B. TY 20W WILL RE-CURVE NORTHEASTWARD THROUGH THE FORECAST PERIOD
AS IT TRACKS POLEWARD OF THE STR AND BEGINS TO ENCOUNTER MIDLATITUDE
WESTERLIES. TY 20W IS FORECAST TO BEGIN EXTRA-TROPICAL TRANSITION
(ETT) BY TAU 48 AS IT BECOMES EMBEDDED WITHIN THE MIDLATITUDE
WESTERLIES. THE SYSTEM SHOULD COMPLETE ETT BY TAU 72 AS IT GAINS
FRONTAL CHARACTERISTICS. DYNAMIC GUIDANCE IS IN GOOD AGREEMENT
THROUGH TAU 48 AND SUPPORTS THE JTWC FORECAST TRACK WITH HIGH
CONFIDENCE. TY 20W IS FORECAST TO CONTINUE TO WEAKEN DUE TO THE
STRONG VWS AND COOLER SST AFTER TAU 48. AFTER TAU 48, DYNAMIC
GUIDANCE DIVERGES DUE TO AN ATYPICAL SYNOPTIC PATTERN, WHICH WILL
SERVE TO WEAKEN THE MIDLATITUDE WESTERLIES, PRODUCING A SLOWER RE-
CURVE SCENARIO. BASICALLY, A MIDLATITUDE HIGH TO THE NORTH WILL
RESULT IN A WEAKER WESTERLY STEERING ENVIRONMENT BUT SHOULD STILL
STEER THE SYSTEM EASTWARD. OVERALL, THERE IS HIGH CONFIDENCE IN THE
JTWC FORECAST TRACK.//
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