Tropical Storm TWENTY Advisory Wed Oct 21

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 20W (TWENTY)
WARNING NR 003//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL DEPRESSION (TD) 20W (TWENTY), LOCATED APPROXIMATELY
77 NM WEST OF IWO TO, HAS TRACKED SOUTHEASTWARD AT 03 KNOTS OVER
THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY DEPICTS
A FULLY EXPOSED, WELL DEFINED LOW LEVEL CIRCULATION CENTER WITH
ISOLATED DEEP CONVECTION DISPLACED TO THE SOUTHEAST, WHICH SUPPORTS
THE INITIAL POSITION WITH GOOD CONFIDENCE. A 202259Z ASCAT-A
BULLSEYE IMAGE REVEALS A DEFINED, SYMMETRIC CIRCULATION WITH 25-30
KNOT WINDS OVER THE NORTHWEST QUADRANT, WHICH SUPPORTS THE INITIAL
INTENSITY ASSESSMENT OF 30 KNOTS. ENVIRONMENTAL ANALYSIS REVEALS TD
20W IS CURRENTLY IN A MARGINAL ENVIRONMENT FOR DEVELOPMENT WITH
STRONG (35-40 KNOTS) WESTERLY VERTICAL WIND SHEAR (VWS) OFFSET BY
WARM (28C) SEA SURFACE TEMPERATURES AND VIGOROUS EQUATORWARD OUTFLOW
ALOFT. TD 20W IS TRACKING SLOWLY EAST-NORTHEASTWARD ALONG THE
NORTHWESTERN PERIPHERY OF A WEAKENED LOW LEVEL SUBTROPICAL RIDGE
(STR) POSITIONED TO THE EAST.
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS
PROGNOSTIC REASONING MESSAGE.
   B. TD 20W IS EXPECTED TO CONTINUE TRACKING SLOWLY AND ERRATICALLY
WITHIN THIS WEAK STEERING ENVIRONMENT THROUGH TAU 12. AFTER TAU 12,
A SHORTWAVE TROUGH POSITIONED TO THE NORTH WILL PROPAGATE EASTWARD
ALLOWING THE STR TO BUILD AND REORIENT POLEWARD. CONSEQUENTLY THE
SYSTEM SHOULD ACCELERATE NORTHWARD ALONG THE WESTERN PERIPHERY OF
THE STR THROUGH TAU 36. TD 20W WILL GRADUALLY WEAKEN THROUGH THE
FORECAST PERIOD WITH DISSIPATION ANTICIPATED BY TAU 36 DUE TO
PERSISTENT STRONG (40-45 KNOTS) VWS. THE REMNANTS OF THE SYSTEM WILL
BE ABSORBED INTO A DEVELOPING FRONT WELL SOUTHEAST OF THE KANTO
PLAIN. NUMERICAL MODEL GUIDANCE REMAINS IN GOOD AGREEMENT, HOWEVER,
DUE TO THE INITIAL ERRATIC TRACK THERE IS OVERALL FAIR CONFIDENCE IN
THE JTWC FORECAST TRACK WHICH LIES NEAR THE MULTI-MODEL CONSENSUS.//
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