Tropical Storm WUTIP Advisory jue, 26-09

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 20W (TWENTY)
WARNING NR 01//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL DEPRESSION 20W (TWENTY), LOCATED APPROXIMATELY 728 NM
EAST-SOUTHEAST OF HANOI, VIETNAM, HAS TRACKED WEST-NORTHWESTWARD AT
03 KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED
SATELLITE IMAGERY INDICATES A CONSOLIDATING LOW-LEVEL CIRCULATION
CENTER (LLCC) WITH IMPROVED CONVECTIVE BANDING WRAPPING INTO THE
DEFINED LLCC. A 261543Z OSCAT PASS AND A 261820Z AMSU-B MICROWAVE
IMAGE SUPPORT THE CURRENT POSITION WITH FAIR CONFIDENCE AND SHOW
IMPROVED DEEP CONVECTIVE BANDING. ADDITIONALLY, THE DVORAK INTENSITY
ESTIMATES FROM PGTW, KNES, AND RJTD HAVE INCREASED TO T1.5 (25
KNOTS) AND IN ASSOCIATION WITH AVAILABLE SURFACE OBSERVATIONS,
SUPPORT THE CURRENT INTENSITY OF 25 KNOTS. OVER THE PAST 24 HOURS,
THE VORTICITY SIGNATURE HAS SIGNIFICANTLY STRENGTHENED AND BECOME
MORE SYMMETRIC. UPPER LEVEL ANALYSIS INDICATES A FAVORABLE
ENVIRONMENT OF LOW TO MODERATE (05 TO 15 KNOTS) VERTICAL WIND SHEAR
(VWS) AND FAVORABLE OUTFLOW. TD 20W IS SLOWLY TRACKING ALONG THE
SOUTHERN PERIPHERY OF A LOW- TO MID-LEVEL SUBTROPICAL RIDGE (STR) TO
THE NORTH.
3. FORECAST REASONING.
   A. THIS IS THE INITIAL PROGNOSTIC REASONING AND SETS THE FORECAST
PHILOSOPHY.
   B. TD 20W SHOULD CONTINUE TRACKING ALONG THE PERIPHERY OF THE STR
IN THE NEAR-TERM. AS THE SYSTEM INTENSIFIES, THE STEERING INFLUENCE
WILL SHIFT TO PREDOMINANTLY THE MID-LEVEL STR, CAUSING A SLIGHT TURN
TO THE WEST-SOUTHWEST BETWEEN TAU 24 AND 36. A MID-LATITUDE TROUGH
IS EXPECTED TO WEAKEN THE STR BETWEEN TAU 36 AND TAU 48, ALLOWING TD
20W TO TURN BACK TO A WEST-NORTHWESTWARD TRACK AS AN EXTENSION OF
THE SUBTROPICAL RIDGE REORIENTS TO THE NORTH OF THE SYSTEM. A STEADY
INTENSIFICATION IS EXPECTED THROUGH TAU 72 AS UPPER LEVEL REMAINS
FAVORABLE AND CONTINUES TO IMPROVE. SEA SURFACE TEMPERATURES (SST)
WILL REMAIN FAVORABLE ALONG TRACK THROUGH TAU 72 (28 TO 29 CELSIUS).
   C. BEYOND TAU 72, THE UPPER LEVEL ENVIRONMENT IS EXPECTED TO
REMAIN FAVORABLE THROUGH TAU 84, WHEN A MID-LATITUDE TROUGH TRANSITS
OVER CENTRAL ASIA, CREATING INCREASING LEVELS OF VWS OVER THE LLCC.
ADDITIONALLY, THE SST VALUES ARE EXPECTED TO DECREASE AS TD 20W
APPROACHES THE COAST OF VIETNAM. BEYOND TAU 96, INCREASING VWS,
DECREASING SST VALUES AND LAND INTERACTION WILL CAUSE THE SYSTEM TO
WEAKEN. AFTER LANDFALL, THE SYSTEM IS FORECASTED TO START
DISSIPATING DUE TO FRICTIONAL LAND EFFECTS THROUGH TAU 120. BASED ON
THE CURRENT SLOW TRACK SPEED AND VARIATIONS IN MODEL GUIDANCE, THE
CONFIDENCE IN THE FORECAST TRACK IS LOW.//
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