WDPN31 PGTW 040300
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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