WDPN31 PGTW 202100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 13W (KOMPASU)
WARNING NR 05//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL DEPRESSION 13W (KOMPASU), LOCATED APPROXIMATELY 160 NM
SOUTH-SOUTHEAST OF MISAWA, JAPAN, HAS TRACKED NORTHWESTWARD AT 17
KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE
IMAGERY SHOWS TIGHTLY CURVED BANDING WRAPPING INTO AN EXPOSED LOW
LEVEL CIRCULATION CENTER (LLCC). THE INITIAL POSITION IS BASED WITH
HIGH CONFIDENCE ON A COMBINATION OF THE AFOREMENTIONED SATELLITE
IMAGERY, COMPOSITE RADAR IMAGERY FROM JAPAN AND ANALYSIS OF A
201736Z GPM MICROWAVE IMAGER SHOT DEPICTING A WEAK MICROWAVE EYE
FEATURE. DEEP CONVECTION HAS DECREASED SIGNIFICANTLY OVER THE PAST
SIX HOURS, WITH WHAT WEAK CONVECTION REMAINS CONFINED TO THE
SOUTHERN SEMICIRCLE. THE INITIAL INTENSITY HAS BEEN REDUCED TO 30
KNOTS, BUT REMAINS SLIGHTLY HIGHER THAN DVORAK INTENSITY ESTIMATES
FROM ALL AGENCIES, DUE TO THE PRESENCE OF A BROAD SWATCH OF 30-35
KNOT WINDS VISIBLE IN A 201047Z ASCAT PASS. UPPER LEVEL ANALYSIS
INDICATES THE SYSTEM CONTINUES TO TRACK UNDER AN AREA OF LOW (5-10
KNOT) VERTICAL WIND SHEAR (VWS) AND IS TAPPING INTO A RELATIVELY
WEAK POLEWARD OUTFLOW CHANNEL. WHILE THE UPPER LEVEL ENVIRONMENT IS
FAVORABLE, THIS IS BEING OFFSET BY SEA SURFACE TEMPERATURES (SSTS)
WHICH ARE ONLY MARGINALLY SUPPORTIVE AT 26 DEGREES CELSIUS. THE
FORWARD SPEED SLOWED TO 17 KNOTS OVER THE PAST 6 HOURS AND THE TRACK
MAINTAINED A SLIGHTLY STRAIGHTER TRACK DUE TO THE DEVELOPMENT OF A
SMALL EXTENSION OF THE STR TO THE NORTH OF THE SYSTEM, BUT OVERALL
CONTINUES TO TRACK NORTHWESTWARD AROUND THE WESTERN PERIPHERY OF A
SUBTROPICAL STEERING RIDGE (STR).
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS
PROGNOSTIC REASONING MESSAGE.
   B. TD KOMPASU IS CURRENTLY SHOWING SIGNS THAT IT IS BEGINNING TO
UNDERGO EXTRA-TROPICAL TRANSITION AS IT CONTINUES TRACKING ALONG THE
WESTERN PERIPHERY OF THE STR. THE EXTENSION OF THE STR WHICH NOSED
IN FROM THE EAST WILL WEAKEN AND MOVE OUT OF THE AREA OVER THE NEXT
6 HOURS, ALLOWING TD 13W TO BEGIN TO RECURVE TO THE NORTHEAST IN
RESPONSE TO AN APPROACHING MID-LATITUDE TROUGH. AS THE TROUGH
APPROACHES, TD 13W IS EXPECTED TO INTENSIFY SLIGHTLY FOR A SHORT
PERIOD AS IT TAPS INTO THE STRONG OUTFLOW PROVIDED BY THE DIVERGENT
QUADRANT OF THE TROUGH AND BEFORE MOVING OVER COOLER WATERS. AFTER
THIS SHORT BURST OF INTENSIFICATION, TD 13W WILL TRANSITION BY TAU
24 TO A 30-KNOT COLD-CORE EXTRA-TROPICAL LOW AS THE SYSTEM BECOMES
FULLY ENGULFED IN THE BAROCLINIC ZONE. DYNAMIC MODEL GUIDANCE
REMAINS IN TIGHT AGREEMENT, LENDING HIGH CONFIDENCE TO THE JTWC
TRACK FORECAST./=