Tropical Storm GAEMI Advisory Thu Oct 04

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (GAEMI) WARNING NR
11//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 21W (GAEMI), LOCATED APPROXIMATELY 630 NM
EAST OF HUE, VIETNAM, HAS TRACKED EASTWARD AT 04 KNOTS OVER THE PAST
SIX HOURS. ANIMATED MULISPECTRAL SATELLITE IMAGERY (MSI) REVEALS
THAT THE LOW LEVEL CIRCULATION CENTER (LLCC) HAS BECOME FULLY
EXPOSED, WITH THE DEEPEST CONVECTION LOCATED ABOUT 90 NM TO THE WEST
OF THE LLCC. THE INITIAL POSITION IS BASED ON THE ABOVE MSI WITH
HIGH CONFIDENCE, HOWEVER IT IS NOTED THAT THIS POSITION IS
APPROXIMATELY 50 NM TO THE EAST OF THE PREVIOUS BEST TRACK POSITION
WHICH HAD BEEN ESTIMATED FROM ENHANCED INFRARED IMAGERY WITH POOR
CONFIDENCE. THE INITIAL INTENSITY IS AVERAGED FROM DVORAK ESTIMATES
OF 35 TO 45 KNOTS FROM MULTIPLE AGENCIES. UPPER LEVEL ANALYSIS
INDICATES THE SYSTEM IS 08 DEGREES SOUTH OF THE RIDGE AXIS IN AN
AREA OF MODERATE (15-20 KNOT) EASTERLY VERTICAL WIND SHEAR (VWS).
THE SYSTEM HAS SLOWLY DRIFTED EASTWARD OVER THE PAST 24 HOURS AS THE
STEERING MECHANISM SWITCHES FROM THE NEAR-EQUATORIAL RIDGE TO THE
SOUTH TO A BUILDING SUBTROPICAL RIDGE (STR) TO THE NORTH.
3. FORECAST REASONING.
   A. THERE IS NO MAJOR CHANGE TO THE FORECAST PHILOSOPHY SINCE THE
PREVIOUS PROGNOSTIC REASONING MESSAGE.
   B. TS 21W IS FORECAST TO SLOWLY DRIFT WEST-SOUTHWESTWARD OVER THE
NEXT 24 HOURS AS IT TRANSITIONS TO THE STEERING INFLUENCE OF THE
STR. AFTER TAU 24, WITH THE STR FIRMLY IN PLACE, TS 21W WILL PICK UP
SPEED AND TRACK DECISIVELY WESTWARD MAKING LANDFALL SOUTH OF HUE
BEFORE TAU 72. AS THE STORM MOTION BECOMES MORE IN-PHASE WITH UPPER
LEVEL WIND FLOW, THE VWS WILL DECREASE AND ALLOW TS 21W TO INTENSIFY
SLIGHTLY, PEAKING AT 50 KNOTS BEFORE LANDFALL. THE INTENSIFICATION
WILL BE TEMPERED BY UPPER LEVEL SUBSIDENCE FROM THE STR.
   C. AFTER TAU 72, TS GAEMI WILL QUICKLY ERODE OVER LAND DUE TO
FRICTIONAL EFFECTS FROM THE TOPOGRAPHY, DISSIPATING AS A SIGNIFICANT
TROPICAL CYCLONE OVER LAND BY TAU 96. THE AVAILABLE NUMERICAL
GUIDANCE IS IN TIGHT AGREEMENT WITH THE WESTWARD TRACK, HOWEVER,
THERE ARE DISTINCT DIFFERENCES IN THE TRACK SPEED. THE FORECAST
TRACK IS SLOWER THAN CONSENSUS TAKING INTO ACCOUNT THE QUASI-
STATIONARY MOVEMENT WHICH IS FORECAST TO CONTINUE FOR 12-24 HOURS.
DUE TO THE LARGE DIFFERENCES IN MODEL TRACK SPEEDS AND UNCERTAINTIES
ASSOCIATED WITH QUASI-STATIONARY MOTION, THERE IS LOW CONFIDENCE IN
THE TRACK FORECAST.//
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