Tropical Storm CHABA Advisory Fri Sep 30

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (CHABA)
WARNING NR 06//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 21W (CHABA), LOCATED APPROXIMATELY 198 NM
WEST-NORTHWEST OF NAVSTA GUAM, HAS TRACKED WESTWARD AT 14 KNOTS OVER
THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY DEPICTS
IMPROVED OVERALL CONVECTIVE STRUCTURE, WITH BANDING BEGINNING TO
WRAP INTO THE LOW-LEVEL CIRCULATION CENTER (LLCC). A 290603Z SSMIS
91 GHZ IMAGE REVEALS THE LLCC IS POSITIONED ALONG THE NORTHERN
PERIPHERY OF THE DEEPEST CONVECTION, AND SUPPORTS THE CURRENT BEST
TRACK POSITION WITH FAIR CONFIDENCE. THE INTENSITY IS ASSESSED AT 35
KNOTS BASED ON DVORAK CURRENT INTENSITY ESTIMATES OF T2.5 (35 KNOTS)
FROM PGTW AND RJTD. UPPER-LEVEL ANALYSIS INDICATES A MORE FAVORABLE
ENVIRONMENT FOR DEVELOPMENT WITH IMPROVING OUTFLOW AND LOW (10 TO 15
KNOTS) VERTICAL WIND SHEAR (VWS). TS CHABA IS TRACKING GENERALLY
WESTWARD ALONG THE SOUTHERN PERIPHERY OF A DEEP LAYERED SUB-TROPICAL
RIDGE (STR) TO THE NORTH.
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS
PROGNOSTIC REASONING BULLETIN.
   B. THE STR WILL STEER TS CHABA TOWARD THE WEST-NORTHWEST WITH A
GRADUAL TURN TOWARD THE NORTHWEST, AS THE UPPER-LEVEL FEATURE RE-
ORIENTS. STEADY INTENSIFICATION WILL COMMENCE AS POLEWARD OUTFLFOW
CONTINUES TO IMPROVE IN RESPONSE TO A WEAKENING TUTT CELL TO THE
WEST OF THE SYSTEM. TS CHABA IS FORECAST TO REACH TYPHOON INTENSITY
BETWEEN TAU 36 AND 48. DYNAMIC MODEL GUIDANCE REMAIN IN GOOD
AGREEMENT ON THE TRACK OF THE SYSTEM IN THE NEAR TERM AND THE
OFFICIAL JTWC FORECAST TRACK REMAINS NEAR THE MULTI-MODEL CONSENSUS.
   C. IN THE EXTENDED PERIOD, DYNAMIC MODEL GUIDANCE DIVERGES
GREATLY BEYOND TAU 72, WITH THE GFS AND JGSM REMAINING ON THE RIGHT
SIDE OF THE MODEL CONSENSUS, WITH THE NAVGEM AND ECMFW TO THE LEFT
OF CONSENSUS. AT THIS TIME, THE OFFICIAL FORECAST TRACK REMAINS TO
THE LEFT OF THE MULTI-MODEL CONSENSUS FROM TAU 72 THROUGH 120 BASED
ON RUN-TO-RUN CONSISTENCY FROM THE NAVGEM AND GFS GLOBAL MODELS. THE
TIMING OF THE TURN TO THE NORTH AND EVENTUAL RECURVE TOWARD THE
NORTHEAST WILL DEPEND ON THE STRENGTH OF THE MID-LATITUDE TROUGH
THAT IS FORECAST TO ERODE THE WESTERN PORTION OF THE STR. VWS IS
FORECAST TO REMAIN LOW AND ACCOMPANIED BY FAVORABLE OUTFLOW THROUGH
MOST OF THE FORECAST PERIOD, THEREFORE CONTINUED INTENSIFICATION IS
FORECAST, WITH A PEAK INTENSITY OF 95 KNOTS AT TAU 96. DUE TO THE
MODEL UNCERTAINTY BEYOND TAU 72, THERE IS LOW CONFIDENCE IN THE JTWC
FORECAST TRACK DURING THIS PORTION OF THE FORECAST.//
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