Tropical Storm GAEMI Advisory Sa, 02.10.

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (GAEMI) WARNING NR
02//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 21W (GAEMI), LOCATED APPROXIMATELY 435 NM
EAST OF HUE, VIETNAM, HAS TRACKED QUASI-STATIONARY OVER THE PAST SIX
HOURS. ANIMATED INFRARED SATELLITE IMAGERY DEPICTS A SLOWLY-
CONSOLIDATING, LOW-LEVEL CIRCULATION CENTER (LLCC) WITH FRAGMENTED
DEEP CONVECTIVE BANDING OVER THE SOUTHERN SEMI-CIRCLE. ANIMATED
WATER VAPOR IMAGERY INDICATES THAT THE LACK OF ORGANIZED DEEP
CONVECTION OVER THE NORTHERN SEMI-CIRCLE IS LIKELY A RESULT OF
INCREASING PRESSURE DUE TO NORTHERLY FLOW ASSOCIATED WITH A DEEP
MIDLATITUDE SHORTWAVE TROUGH DIGGING INTO EASTERN CHINA. A 011728Z
AMSU IMAGE AS WELL AS A 011324Z ASCAT BULLS-EYE IMAGE AND A 011153Z
SSMIS IMAGE PROVIDE EVIDENCE THAT THE SYSTEM HAS BOTH INTENSIFIED
AND BECOME QUASI-STATIONARY OVER THE PAST SIX HOURS. THE ASCAT IMAGE
DEPICTED 30-35 KNOT WINDS WHILE THE MICROWAVE IMAGERY SHOWS IMPROVED
CONVECTIVE BANDING. THE INITIAL POSITION IS BASED ON THIS IMAGERY
WITH FAIR CONFIDENCE AND REFLECTS A 30-NM MINOR RELOCATION AT
01/12Z. THE INITIAL INTENSITY IS ASSESSED AT 35 KNOTS BASED ON THE
ASCAT DATA AND DVORAK ESTIMATES OF 35 KNOTS. THE 01/12Z 500MB
ANALYSIS DEPICTS A LARGE BREAK IN THE SUBTROPICAL RIDGE NORTH OF TS
21W CAUSED BY THE DEEP, BROAD MIDLATITUDE SHORTWAVE TROUGH AND
STRONG WESTERLIES EXTENDING AS FAR SOUTH AS 25N.
3. FORECAST REASONING.
   A. THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE
PREVIOUS PROGNOSTIC REASONING MESSAGE; HOWEVER, THE TRACK WAS
ADJUSTED DUE TO THE RECENT QUASI-STATIONARY MOTION AND MINOR
RELOCATION.
   B. TS 21W IS FORECAST TO REMAIN EMBEDDED WITHIN A WEAK STEERING
ENVIRONMENT THROUGH THE NEXT 36 TO 48 HOURS AND WILL LIKELY DRIFT
SOUTHWARD TO SOUTHEASTWARD UNDER THE STEERING INFLUENCE OF THE NEAR-
EQUATORIAL RIDGE POSITIONED TO THE SOUTH. AFTER TAU 36, THE
SUBTROPICAL RIDGE (STR) IS EXPECTED TO BUILD OVER SOUTHEAST CHINA
PROVIDING A STRONG WESTWARD STEERING INFLUENCE. IN GENERAL, DYNAMIC
MODEL GUIDANCE IS IN FAIR AGREEMENT WITH ALL MODELS INDICATING THE
SOUTHWARD DRIFT THEN ACCELERATION WESTWARD. TS 21W IS FORECAST TO
INTENSIFY SLOWLY TO A PEAK INTENSITY OF 55 KNOTS BY TAU 72. DUE TO
THE ERRATIC MOTION AND WEAK STEERING INFLUENCE, THERE IS LOW
CONFIDENCE IN THE FORECAST TRACK, WHICH IS POSITIONED CLOSE TO THE
MULTI-MODEL CONSENSUS.
   C. IN THE EXTENDED TAUS, TS 21W IS FORECAST TO ACCELERATE
WESTWARD AND SHOULD MAKE LANDFALL NEAR TAU 120 ALONG THE CENTRAL
COAST OF VIETNAM. DYNAMIC MODEL GUIDANCE IS IN FAIR AGREEMENT WITH
SOME MINOR DIFFERENCES IN THE TIMING OF THE WESTWARD TURN AND THE
EXACT WESTWARD TRACK. THERE REMAINS LOW CONFIDENCE IN THE FORECAST
TRACK THROUGH TAU 120.//
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