Tropical Storm ETAU Advisory lun, 09-11

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 24W (TWENTYFOUR)
WARNING NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL DEPRESSION 24W (TWENTYFOUR), LOCATED APPROXIMATELY 548
NM EAST-SOUTHEAST OF DA NANG, VIETNAM, HAS TRACKED WESTWARD AT 16
KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE
IMAGERY DEPICTS A RELATIVELY DISORGANIZED, BUT CONSOLIDATING SYSTEM,
WITH FLARING CONVECTION OBSCURING THE LOW LEVEL CIRCULATION CENTER
(LLCC). THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON
LATE RECEIPT OF A PARTIAL 081218Z ASCAT-B PASS, WHICH SHOWS THAT THE
WELL DEFINED LLCC IS TUCKED UNDER THE FLARING DEEP CONVECTION. THE
INITIAL INTENSITY OF 25 KTS IS ASSESSED WITH HIGH CONFIDENCE BASED
ON MULTI-AGENCY DVORAK CURRENT INTENSITY ESTIMATES OF T1.5 (25 KTS)
FROM PGTW AND RJTD, WITH CONFIRMATION PROVIDED BY THE 081218Z ASCAT-
B PASS THAT DEPICTS 20-25 KTS IN THE NORTHWEST QUADRANT OF THE
SYSTEM. TD 24W IS TRACKING ALONG THE SOUTHERN PERIPHERY OF A
SUBTROPICAL RIDGE (STR) POSITIONED TO THE NORTH AND THROUGH A WEAKLY
FAVORABLE ENVIRONMENT FOR FURTHER DEVELOPMENT WITH WARM (29-30
CELSIUS) SEA SURFACE TEMPERATURES (SST), LOW TO MODERATE (10-15 KTS)
VERTICAL WIND SHEAR (VWS), AND SINGLE CHANNEL WESTWARD OUTFLOW.
3. FORECAST REASONING.
   A. THIS IS THE FIRST PROGNOSTIC REASONING MESSAGE AND SETS
THE FORECAST PHILOSOPHY.
   B. TD 24W WILL TRACK WESTWARD THROUGH THE ENTIRE FORECAST PERIOD
ALONG THE SOUTHERN PERIPHERY OF THE DEEP-LAYER STR TO THE NORTH.
FORWARD TRACK SPEED IS FORECAST TO DECREASE AFTER TAU 24 AS THE STR
WEAKENS AND THE GRADIENT SLACKENS SLIGHTLY. AS THE SYSTEM TRACKS
WEST, ENVIRONMENTAL CONDITIONS REMAIN UNCHANGED THROUGH THE FIRST 36
HOURS, BUT VWS WILL SLACKEN SOMEWHAT, ALLOWING FOR A BURST OF
INTENSIFICATION, PEAKING AT 40 KNOTS BY TAU 24, JUST BEFORE THE
SYSTEM IS FORECAST TO MAKE LANDFALL ALONG THE SOUTH-CENTRAL VIETNAM
COAST AT OR NEAR TAU 36. AFTER THIS POINT, 24W WILL RAPIDLY WEAKEN
DUE TO INTERACTION WITH THE RUGGED TERRAIN, DISSIPATING BY TAU 72
OVER CENTRAL CAMBODIA. NUMERICAL MODEL TRACK GUIDANCE IS IN TIGHT
AGREEMENT WITH NAVGEM REPRESENTING THE POLEWARD EDGE OF THE
ENVELOPE, AND GFS THE EQUATORWARD, BUT OVERALL SPREAD IS LESS THAN
100 NM, LENDING HIGH CONFIDENCE TO THE JTWC TRACK WHICH LIES CLOSE
TO THE MULTI-MODEL CONSENSUS.//
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