Tropical Storm CHABA Advisory вт, 04.10.

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR SUPER TYPHOON 21W (CHABA) WARNING NR
24//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   SUPER TYPHOON (STY) 21W (CHABA), LOCATED APPROXIMATELY 413 NM
SOUTH-SOUTHWEST OF SASEBO, JAPAN, HAS TRACKED NORTHWARD AT 09 KNOTS
OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE
IMAGERY DEPICTS A SLIGHT WEAKENING OF STY CHABA OVER THE PAST SIX
HOURS, HOWEVER, THE SYSTEM HAS RETAINED A 13-NM EYE, WHICH PROVIDES
GOOD CONFIDENCE IN THE CURRENT POSITION. A 031812Z SSMI 85GHZ IMAGE
AND RADAR IMAGERY SHOW A COMPACT CORE OF DEEP CONVECTION WITH A 90-
NM DIAMETER EYEWALL AND DEEP CONVECTIVE BANDING OVER THE NORTHWEST
QUADRANT. A ROBUST POLEWARD OUTFLOW AND VERY WARM SEA SURFACE
TEMPERATURES (SSTS) ARE COMBINING TO FUEL THE STORM AND SUSTAIN THE
CURRENT INTENSITY. THE CURRENT INTENSITY IS ASSESSED AT 135 KNOTS
BASED ON DVORAK ESTIMATES RANGING FROM 6.5 (127 KNOTS) TO 7.5 (155
KNOTS), AND A SATCON ESTIMATE OF 136 KNOTS. SOUTHWESTERLY VERTICAL
WIND SHEAR (VWS) HAS INCREASED TO 25-30 KNOTS AND IS RESPONSIBLE FOR
THE RECENT WEAKENING OF THE CONVECTIVE STRUCTURE. STY 21W IS
TRACKING NORTHWARD ALONG THE WESTERN PERIPHERY OF A DEEP-LAYERED
SUBTROPICAL RIDGE (STR) TO THE EAST.
3. FORECAST REASONING.
   A. DECREASED THE INTENSITY FORECAST SIGNIFICANTLY AFTER TAU 12
DUE TO VERY STRONG VERTICAL WIND SHEAR EXPECTED NORTH OF 33N. THE 34-
KNOT WIND RADII HAVE BEEN ADJUSTED BASED ON DETAILED ANALYSIS OF
RECENT ASCAT IMAGERY.
   B. STY CHABA WILL CONTINUE TRACKING NORTHWARD OVER THE NEXT 12
HOURS AND THEN NORTHEASTWARD AS IT ROUNDS THE STR AXIS. THE SYSTEM
WILL ENCOUNTER INCREASING VWS AS IT MOVES CLOSER TO THE STRONG ZONAL
WESTERLY FLOW SITUATED JUST SOUTH OF THE KOREAN PENINSULA. THIS WILL
FORCE THE SYSTEM TO FURTHER ELONGATE TO THE NORTHEAST AND RAPIDLY
WEAKEN. STY 21W WILL BEGIN EXTRA-TROPICAL TRANSITION (ETT) NEAR TAU
24 AS IT EMBEDS WITHIN THE MID-LATITUDE WESTERLY FLOW ALONG
THE WESTERN EDGE OF MAINLAND JAPAN, AND WILL COMPLETE ETT BY TAU 48.
IT WILL CONTINUE TO WEAKEN AS IT TRACKS OVER THE COOL WATERS IN THE
SEA OF JAPAN AND THEN ACROSS THE RUGGED JAPANESE TERRAIN. DYNAMIC
MODEL GUIDANCE IS IN TIGHT AGREEMENT THROUGHOUT THE FORECAST.
ADDITIONALLY, GLOBAL MODEL FIELDS SHOW NO CHANGE TO THE LOCATION OF
THE MID-LATITUDE WESTERLIES; THEREFORE, ONLY MINIMAL CHANGES TO THE
FORECAST TRACK ARE EXPECTED. GIVEN THESE FACTORS, THERE IS HIGH
CONFIDENCE IN THE JTWC FORECAST.//
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