WDXS31 PGTW 082100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 21S (OLGA) 
WARNING NR 013//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 16.9S 118.1E
   INITIAL INTENSITY: 45 KTS
   GEOGRAPHIC REFERENCE: 395 NM NORTHEAST OF LEARMONTH, AUSTRALIA
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 05 KTS
   SIGNIFICANT WAVE HEIGHT: 21 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
CYCLONE (TC) 21S (OLGA) WITH AN EXPOSED LOW-LEVEL CIRCULATION
CENTER (LLCC) AND CONVECTION BEING DISPLACED TO THE SOUTHEAST DUE
TO VERY APPARENT NORTHWESTERLY SHEAR ACTING ON THE SYSTEM.
ENVIRONMENTAL ANALYSIS INDICATES THAT 21S IS IN AN UNFAVORABLE
ENVIRONMENT FOR DEVELOPMENT CHARACTERIZED BY VERY HIGH (30-35 KTS) 
VERTICAL WIND SHEAR AND SIGNIFICANT DRY AIR ENTRAINMENT, SLIGHTLY 
OFFSET BY STRONG POLEWARD OUTFLOW AND WARM (29-30 C) SEA SURFACE
TEMPERATURES. THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE
BASED ON THE ANIMATED EIR AND A 081750Z AMSR2 36 GHZ MICROWAVE
IMAGE REVEALING A BROADLY DEFINED CENTER. THE INITIAL INTENSITY OF 45 
KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE SUBJECTIVE AND
OBJECTIVE INTENSITY ESTIMATES LISTED BELOW.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: SUBTROPICAL RIDGE (STR) POSITIONED TO
THE EAST AND SOUTH.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS
   KNES: T4.0 - 65 KTS
   APRF: T3.5 - 55 KTS
   CIMSS SATCON: 50 KTS AT 081800Z
   CIMSS MICROWAVE SOUNDERS: 41 KTS AT 081657Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 30+ KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG POLEWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: MEDIUM

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: TC 21S IS FORECAST TO CONTINUE ON A
SOUTHWESTWARD TRACK, ALONG THE STR, THROUGHOUT THE FORECAST PERIOD.
THE SYSTEM IS EXPECTED TO CONTINUE WEAKENING IN RESPONSE TO
SIGNIFICANT DRY AIR ENTRAINMENT IN THE NORTHERN AND WESTERN
QUADRANTS. ADDITIONALLY, HIGH VERTICAL WIND SHEAR VALUES WILL AID
IN DISSIPATION WITH VALUES REACHING OVER 50 KTS NEAR TAU 36 AS 21S
CLOSES IN ON THE SUBTROPICAL JET TO THE SOUTH. AS A RESULT, THE
JTWC FORECAST HAS THE SYSTEM WEAKENING TO 35 KTS AT TAU 24 AND
DISSIPATING BELOW 35 KTS NEAR TAU 48, ABOUT 100 NM NORTH-NORTHWEST
OF LEARMONTH. 

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN FAIR AGREEMENT
WITH A 145 NM CROSS-TRACK SPREAD AT TAU 72. THE 080600Z ECMWF
ENSEMBLE AND 081200Z GFS ENSEMBLE BOTH SUPPORT THE JTWC TRACK
FORECAST WITH MEMBERS DEPICTING A SIMILAR SOUTHWESTWARD TRACK. AS
SUCH, THE JTWC FORECAST TRACK IS PLACED WITH MEDIUM CONFIDENCE.
INTENSITY GUIDANCE IS IN AGREEMENT THAT TC 21S WILL WEAKEN TO BELOW
35 KTS BY TAU 48. HOWEVER, HWRF AND GFS BOTH SUGGEST A SLIGHTLY MORE 
GRADUAL WEAKENING TREND. OVERALL, THE JTWC INTENSITY FORECAST IS 
ASSESSED WITH MEDIUM CONFIDENCE. 

FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: MEDIUM
   INTENSITY 0 - 72 HR: MEDIUM//
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