Tropical Storm SEPAT Advisory Sa, 01.10.

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 21W (SEPAT)
WARNING NR 04//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL DEPRESSION (TD) 21W (SEPAT), LOCATED APPROXIMATELY 84 NM
NORTH OF CHICHI JIMA, JAPAN, HAS TRACKED WEST-NORTHWESTWARD AT 15
KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE
IMAGERY SHOWS A FULLY-EXPOSED AND WELL-DEFINED LOW-LEVEL CIRCULATION
CENTER (LLCC) WITH SHALLOW CONVECTION WRAPPING ALONG THE NORTHEAST
QUADRANT AND LONG STREAKS OF COLD AIR STRATOCUMULUS CLOUDS ADVECTING
IN FROM THE NORTHWEST. THESE FEATURES ARE CORROBORATED BY A 301758Z
SSMI/S MICROWAVE PASS. THERE IS HIGH CONFIDENCE IN THE INITIAL
POSITION IN VIEW OF THE ABOVE IMAGERY. THE INITIAL INTENSITY OF 30
KNOTS IS BASED ON CONGRUENT DVORAK ESTIMATES FROM ALL REPORTING
AGENCIES. UPPER-LEVEL ANALYSIS INDICATES A POOR TO MARGINAL
ENVIRONMENT AS AN UPPER-LEVEL LOW OVER THE SYSTEM IS CAUSING
MODERATE (15 TO 20 KNOT) VERTICAL WIND SHEAR AND IS ALSO HINDERING
OUTFLOW. TD 21W IS TRACKING ALONG THE SOUTHERN PERIPHERY OF THE
SUBTROPICAL RIDGE (STR) TO THE NORTHEAST.
3. FORECAST REASONING.
   A. NO CHANGE TO FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC
REASONING MESSAGE.
   B. TD 21W WILL BEGIN TO ROUND THE WESTERN EDGE OF THE STR AFTER
TAU 12 AND RECURVE NORTHEASTWARD. AFTER TAU 24, THE CYCLONE IS
FORECAST TO ACCELERATE AHEAD OF A MIDLATITUDE SHORTWAVE TROUGH,
WHICH IS EXPECTED TO DEEPEN OVER JAPAN. TD 21W IS FORECAST TO REMAIN
IN A MARGINAL ENVIRONMENT THROUGHOUT THE FORECAST WITH A SLIGHT
INCREASE AT TAU 24 AS POLEWARD OUTFLOW IMPROVES PEAKING THE
INTENSITY TO 35 KNOTS. ADDITIONALLY, SEA SURFACE TEMPERATURES ARE
EXPECTED TO GET COLDER AFTER TAU 06 WHICH WILL ALSO IMPEDE
DEVELOPMENT. TD 21W IS FORECAST TO BEGIN EXTRA-TROPICAL TRANSITION
(ETT) BEFORE TAU 36 AS IT BEGINS TO INTERACT WITH THE MIDLATITUDE
WESTERLIES AHEAD OF THE DEEPENING TROUGH. COMPLETE TRANSITION IS
EXPECTED BY TAU 48 AS THE SYSTEM BECOMES EMBEDDED IN THE COLD
BAROCLINIC AIR MASS. THE AVAILABLE DYNAMIC MODELS, AS WELL AS THE
ENSEMBLE MODELS ARE IN EXCELLENT AGREEMENT, LENDING HIGH CONFIDENCE
TO THE JTWC FORECAST TRACK, WHICH IS POSITIONED SLIGHTLY FASTER AND
TO THE RIGHT OF THE MULTI-MODEL CONSENSUS.   //
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