Tropical Storm CHABA Advisory Sat Oct 01

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (CHABA)
WARNING NR 10//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 21W (CHABA), LOCATED APPROXIMATELY 934 NM
SOUTHEAST OF KADENA AB, HAS TRACKED NORTHWESTWARD AT 07 KNOTS OVER
THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI)
SHOWS THAT TS 21W IS SLOWLY CONSOLIDATING WITH IMPROVED CONVECTIVE
BANDING WRAPPING INTO A MORE DEFINED BUT PARTIALLY-EXPOSED LOW-LEVEL
CIRCULATION CENTER (LLCC). A 300546Z AMSU 89GHZ IMAGE INDICATES THAT
THE CURVED CONVECTIVE BANDING REMAINS SOMEWHAT FRAGMENTED AS THE
LLCC CONTINUES TO ORGANIZE. ANIMATED WATER VAPOR IMAGERY AND CIMSS
UPPER-LEVEL WIND ANALYSES CONTINUE TO REVEAL GOOD OVERALL OUTFLOW
AIDED BY THE TUTT TO THE NORTH AND LOW VERTICAL WIND SHEAR. THE
CURRENT POSITION AND RECENT TRACK MOTION IS BASED ON THE MSI WITH
GOOD CONFIDENCE. RECENT MSI UP TO 300800Z CONTINUES TO CONFIRM THE
NORTHWESTWARD TRACK MOTION, BOLSTERING CONFIDENCE THAT THE SYSTEM
HAS CLEARLY MADE THE POLEWARD TRACK CHANGE. THE CURRENT INTENSITY
REMAINS ASSESSED AT 50 KNOTS BASED ON AN AVERAGE OF DVORAK CURRENT
INTENSITY ESTIMATES RANGING FROM T3.0 (45 KNOTS) TO T3.5 (55 KNOTS).
ADDITIONALLY A 300545Z SATCON ESTIMATE OF 47 KNOTS SUPPORTS THE
CURRENT INTENSITY ASSESSMENT. TS 21W IS TRACKING NORTHWESTWARD ALONG
THE SOUTHWEST PERIPHERY OF THE DEEP-LAYERED SUBTROPICAL RIDGE (STR).
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS
PROGNOSTIC REASONING BULLETIN.
   B. TS CHABA WILL CONTINUE TO TRACK NORTHWESTWARD ALONG THE
SOUTHWEST PERIPHERY OF THE DEEP-LAYERED SUBTROPICAL RIDGE WHILE
INTENSIFYING STEADILY DUE TO FAVORABLE ENVIRONMENTAL CONDITIONS.
WITH THE EXCEPTION OF NAVGEM AND GFDN (TWO PRIMARY OUTLIERS), THE
NUMERICAL MODEL GUIDANCE IS IN FAIR AGREEMENT WITH A SPREAD OF 170-
NM AT TAU 72, JUST SOUTH OF OKINAWA. THE MAIN DIFFERENCE BETWEEN
NAVGEM AND THE OTHER MODELS IS THAT NAVGEM MAINTAINS AN EXTENSION OF
THE STR TO THE NORTH WHILE THE MIDLATITUDE SHORTWAVE TROUGH DIGS
OVER THE EAST CHINA SEA EFFECTIVELY DELAYING THE RE-CURVE. THE BULK
OF THE GUIDANCE DEPICTS A MORE REALISTIC SCENARIO WITH A CLEAR BREAK
IN THE STR THAT ALLOWS THE SYSTEM TO RE-CURVE. THE NAVGEM (AND GFDN)
SOLUTIONS APPEAR TO BE UNLIKELY AND ARE CURRENTLY DISCOUNTED. TS 21W
IS EXPECTED TO REACH A PEAK INTENSITY OF 100 KNOTS BY TAU 72.
   C. AFTER TAU 72, THE DYNAMIC MODELS DIVERGE SIGNIFICANTLY WITH
NAVGEM AND GFDN REMAINING THE PRIMARY OUTLIERS WEST OF THE BULK OF
THE NUMERICAL MODEL GUIDANCE, WHICH IS CLUSTERED NEAR OKINAWA AND
WESTERN JAPAN. THE JTWC FORECAST CONTINUES TO FAVOR THE RE-CURVE
SCENARIO OVER WESTERN JAPAN AS THE HIGHEST PROBABILITY SCENARIO,
HOWEVER, THERE IS, OF COURSE, UNCERTAINTY IN THE EXACT TRACK NEAR
OKINAWA THUS THE CURRENT FORECAST TRACK REMAINS HIGHLY CONSISTENT
WITH THE PREVIOUS JTWC FORECAST TRACK. TS 21W SHOULD ACCELERATE
NORTHEASTWARD AFTER TAU 96 AND IS FORECAST TO BEGIN EXTRA-TROPICAL
TRANSITION AS THE SYSTEM BEGINS TO INTERACT WITH THE MIDLATITUDE
WESTERLIES AND ENCOUNTER INCREASING VERTICAL WIND SHEAR. LAND
INTERACTION WILL ALSO SERVE TO WEAKEN THE SYSTEM RAPIDLY. OVERALL,
THERE IS LOW CONFIDENCE (HIGH UNCERTAINTY) IN THE JTWC FORECAST DUE
PRIMARILY TO THE LARGE MODEL SPREAD AFTER TAU 72, WHICH IS TYPICAL
IN RE-CURVE SCENARIOS.//
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