WDXS31 PGTW 092100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 21S (OLGA) 
WARNING NR 017//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 18.6S 116.1E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 249 NM NORTH-NORTHEAST OF LEARMONTH,
AUSTRALIA
   MOVEMENT PAST 6 HOURS: SOUTH-SOUTHWESTWARD AT 07 KTS
   SIGNIFICANT WAVE HEIGHT: 17 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
CYCLONE (TC) 21S (OLGA) WITH A TIGHTLY WRAPPED, EXPOSED LOW-LEVEL
CIRCULATION CENTER (LLCC) WITH PERIODIC CONVECTIVE BURSTS
TRIGGERING ALONG THE SOUTHERN SEMICIRCLE. SIGNIFICANT WESTERLY
VERTICAL WIND SHEAR (VWS) CAN BE SEEN BLOWING ALL CONVECTION
EASTWARD. ENVIRONMENTAL ANALYSIS INDICATES THAT TC 21S IS IN AN
UNFAVORABLE ENVIRONMENT FOR DEVELOPMENT CHARACTERIZED BY THE HIGH
VWS VALUES (35-40 KTS) AND EXTENSIVE DRY AIR ENTRAINMENT IN THE
NORTHERN AND WESTERN SECTORS OF THE VORTEX. WARM SEA SURFACE
TEMPERATURES AND MODERATE POLEWARD OUTFLOW ARE THE MAIN VARIABLES
IN KEEPING THE SYSTEM FROM MORE RAPID WEAKENING. THE INITIAL
POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE ANIMATED EIR
IMAGERY. THE INITIAL INTENSITY OF 40 KTS IS ASSESSED WITH MEDIUM
CONFIDENCE BASED ON THE SUBJECTIVE AND OBJECTIVE INTENISTY
ESTIMATES LISTED BELOW. ADDITIONALLY, A 1047Z SAR WINDSPEED PRODUCT
SHOWED A VMAX OF 40 KTS IN THE SOUTHWESTERN PERIPHERY.

INITIAL WIND RADII BASIS: SAR DATA 

CURRENT STEERING MECHANISM: WESTERN PERIPHERY OF A LOW-LEVEL
SUBTROPICAL RIDGE (STR) TO THE EAST AND SOUTH.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   KNES: T2.5 - 35 KTS
   APRF: T2.5 - 35 KTS
   CIMSS SATCON: 46 KTS AT 091800Z
   CIMSS ADT: 43 KTS AT 091800Z
   CIMSS AIDT: 44 KTS AT 091800Z
   CIMSS MW SOUNDERS: 47 KTS AT 091729Z
   

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 35-40 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE 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 LOW-LEVEL RIDGING, THROUGHOUT THE
FORECAST PERIOD. THE SYSTEM IS FORECAST TO PASS AROUND 45 NM
NORTHWEST OF LEARMONTH BETWEEN TAU 30-36. THIS JTWC TRACK FORECAST 
HAS SHIFTED 20 TO 30 NM SOUTHEAST OF THE PREVIOUS FORECAST AFTER TAU 
24, CLOSER TO THE AUSTRALIAN COASTLINE. THE 34 KNOT WIND RADII ARE 
EXPECTED TO QUICKLY SHRINK FROM THEIR CURRENT VALUES AS THE SYSTEM 
CONTINUES TO DEGRADE. VERY HIGH VWS VALUES ARE ANTICIPATED TO ONLY 
INCREASE FURTHER, WITH VALUES REACHING UPWARDS OF 70 KTS NEAR TAU 24. 
THIS, IN CONJUNCTION WITH DRY AIR CONTINUING TO INFRINGE ON THE 
VORTEX, WILL AID IN DISSIPATING TC 21S BY TAU 36. THE JTWC FORECAST 
CALLS FOR WEAKENING TO 35 KTS NEAR TAU 12 AND MAINTAINING 35 KTS 
THROUGH TAU 24 BEFORE DROPPING TO 30 KTS NEAR TAU 36 AS IT PASSES THE 
EXMOUTH PENINSULA. 

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN RELATIVELY GOOD
AGREEMENT CONCERNING TRACK FORECAST, WITH A 70 NM CROSS-TRACK
SPREAD IN JTWC CONSENSUS MEMBERS AT TAU 48. HAFS-A AND HWRF SLIGHTLY 
DIFFER FROM THE REMAINDER OF GUIDANCE, SUGGESTING A MORE SOUTHERLY 
TRACK AND PASSING CLOSER TO LEARMONTH. THE JTWC FORECAST IS SLIGHTLY 
SOUTHEAST OF THE MULTI-MODEL CONSENSUS, LEANING TOWARDS MODELS 
REPRESENTING THE SHORT-TERM MOTION MORE ACCURATELY. WITH RESPECT TO 
INTENSITY GUIDANCE, MODELS AGREE WITH A CONTINUED WEAKENING TREND AND 
DISSIPATION BY TAU 36. AS SUCH, THE JTWC TRACK AND INTENSITY
FORECAST ARE BOTH PLACED WITH MEDIUM CONFIDENCE. 

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