Tropical Storm ATSANI Advisory vie, 06-11

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 23W (ATSANI) WARNING
NR 028//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 23W (ATSANI), LOCATED APPROXIMATELY 399 NM
SOUTH-SOUTHWEST OF KADENA AB, OKINAWA, HAS TRACKED WESTWARD AT 09
KNOTS (KTS) OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL
SATELLITE IMAGERY (MSI) REVEALS A PARTIALLY-EXPOSED LOW LEVEL
CIRCULATION CENTER (LLCC) OBSCURED BY THINNING CONVECTION LOCATED
OVERHEAD. A 050521Z ATMS 88.2 GHZ MICROWAVE IMAGE DEPICTS BANDING
WRAPPING INTO THE DISCRETE LLCC, LENDING GOOD CONFIDENCE TO THE
INITIAL POSITION. HEDGED SLIGHTLY HIGHER THAN MULTI-AGENCY DVORAK
CURRENT INTENSITY ESTIMATES OF T3.0 (45 KTS, PGTW/RJTD), THE
INITIAL INTENSITY REMAINS AT 55 KTS BASED ON A 050429Z SATELLITE
CONSENSUS ESTIMATE OF 55 KTS. THE 34- AND 50-KT WIND RADII HAVE
BEEN UPDATED BASED ON THE 050245Z DIRECT ASCAT-B PASS.
ENVIRONMENTAL ANALYSIS REVEALS AN OVERALL CONDUCIVE ENVIRONMENT FOR
DEVELOPMENT CHARACTERIZED BY LOW (5-10 KT) VERTICAL WIND SHEAR
(VWS), ESTABLISHED DUAL CHANNEL OUTFLOW, AND WARM (28-29 CELSIUS)
SEA SURFACE TEMPERATURES (SST). TS 23W IS TRACKING GENERALLY
WESTWARD ALONG THE SOUTHERN PERIPHERY OF A SUBTROPICAL RIDGE (STR).
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS
PROGNOSTIC REASONING MESSAGE.
   B. TS 23W IS EXPECTED TO TRACK GENERALLY WESTWARD THROUGH TAU 12
THEN TURN WEST-NORTHWESTWARD THROUGH TAU 36 AS IT BEGINS TO TRACK
AROUND THE SOUTHEASTERN PERIPHERY OF THE AFOREMENTIONED STR. DURING
THIS TIME, THE SYSTEM WILL STEADILY INTENSIFY TO 70 KTS DUE TO
PERSISTENT FAVORABLE ENVIRONMENTAL CONDITIONS. AFTERWARDS, THE
SYSTEM IS EXPECTED TO SLOWLY TURN TO A WEST-SOUTHWESTWARD TRACK AS
A STR POSITIONED OVER CENTRAL CHINA TAKES OVER AS THE PRIMARY
STEERING MECHANISM. THIS STEERING MECHANISM SHIFT WILL CAUSE THE
SYSTEM TO TURN TO A SOUTHWESTWARD TRACK AFTER TAU 48. AT THIS TIME,
THE SYSTEM WILL BEGIN TO EXPERIENCE INCREASING (>25 KTS) VWS WHICH
WILL BEGIN TO WEAKEN THE SYSTEM TO 35 KTS BY TAU 72. NUMERICAL
MODEL GUIDANCE IS IN TIGHT AGREEMENT THROUGH TAU 36. HOWEVER,
NAVGEM AND GALWEM DIVERGE SIGNIFICANTLY FROM THE MULTI-MODEL
CONSENSUS WITH NAVGEM TRACKING THE VORTEX GENERALLY NORTHWARD WHILE
GALWEM TURNS THE SYSTEM SOUTH TOWARDS LUZON AT TAU 48. IN ORDER TO
OFFSET THE EFFECTS OF THESE UNLIKELY SOLUTIONS, THE JTWC FORECAST
TRACK IS POSITIONED SLIGHTLY SOUTH OF THE MULTI-MODEL CONSENSUS
WITH GOOD CONFIDENCE.
   C. IN THE EXTENDED FORECAST, TS 23W SHOULD CONTINUE TO TRACK
SOUTHWESTWARD TOWARDS SOUTHERN VIETNAM ALONG THE SOUTHEASTERN
PERIPHERY OF THE STR. AS IT WEAKENS, A MID-LEVEL RIDGE WILL
CONTRIBUTE TO THE STEERING OF THE SYSTEM. CONTINUED HIGH VWS AND
COOLING SST WILL STEADILY WEAKEN THE SYSTEM PRIOR TO LANDFALL OVER
SOUTH CENTRAL VIETNAM. COMBINED WITH THE UNFAVORABLE ENVIRONMENT,
TERRAIN INTERACTION AFTER LANDFALL WILL RAPIDLY WEAKEN THE SYSTEM
UNTIL FULL DISSIPATION OVER LAND BY TAU 96. WITH THE EXCEPTION OF
THE AFOREMENTIONED OUTLIERS, THERE REMAINS GOOD AGREEMENT IN THE
EXTENDED RANG NUMERICAL MODEL GUIDANCE. THEREFORE, THERE IS GOOD
CONFIDENCE IN THE JTWC FORECAST TRACK WHICH LIES ALONG THE MULTI-
MODEL CONSENSUS.//
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