WDXS31 PGTW 072100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 21S (OLGA) WARNING NR 
009//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 15.9S 118.9E
   INITIAL INTENSITY: 115 KTS
   GEOGRAPHIC REFERENCE: 471 NM NORTHEAST OF LEARMONTH, AUSTRALIA
   MOVEMENT PAST 6 HOURS: SOUTH-SOUTHWESTWARD AT 06 KTS
   SIGNIFICANT WAVE HEIGHT: 51 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY SHOWS TC 21S HAS 
BEGUN TO SIGNIFICANTLY DEGRADE AS EVIDENCED BY EXCESSIVE POLEWARD
ELONGATION, WARMING CLOUD TOPS, AND CONVECTIVE FRAGMENTATION AS THE 
SYSTEM DRIFTED INTO HIGH (30+ KNOTS) WESTERLY VWS. THE INITIAL 
POSITION IS PLACED WITH MEDIUM CONFIDENCE BASED ON A CLUSTER OF WARM 
PIXELS - REMNANT OF A RECENTLY OBSCURED EYE - IN THE EIR LOOP. THE 
INITIAL INTENSITY IS ALSO PLACED WITH MEDIUM CONFIDENCE BASED ON FOUR 
AGENCY DVORAK ESTIMATES ALL REPORTING T6.0 (115KTS) AND REFLECTS THE 
WEAKENING TREND. ANALYSIS INDICATES AN UNFAVORABLE ENVIRONMENT WITH 
THE STRONG POLEWARD OUTFLOW AND WARM SST OFFSET BY STRONG VWS AND DRY 
AIR INTRUSION AT THE LOWER LEVELS WRAPPING IN FROM THE SOUTHWEST 
PERIPHERIES.

INITIAL WIND RADII BASIS: ADJUSTED FROM RECENT SCATTEROMETER DATA.



CURRENT STEERING MECHANISM: DEEP-LAYERED SUBTROPICAL RIDGE (STR)
TO
THE EAST. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T6.0 - 115 KTS
   KNES: T6.0 - 115 KTS
   DEMS: T6.0 - 115 KTS
   APRF: T6.0 - 115 KTS

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

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   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 OLGA WILL CONTINUE SOUTHWESTWARD ALONG THE
NORTHWEST EDGE OF THE SOUTHWEST EXTENSION OF THE STEERING STR. THE
UNFAVORABLE ENVIRONMENT WILL ONLY GET WORSE WITH INCREASING VWS AND
COOL DRY AIR INTRUSION. THESE WILL PROMOTE RAPID EROSION LEADING TO
DISSIPATION BY TAU 96, POSSIBLY SOONER. 

MODEL DISCUSSION: NUMERICAL WEATHER PREDICTION MODELS ARE IN
OVERALL GOOD AGREEMENT WITH NAVGEM THE SOLE LEFT OUTLIER TRACKING
THE VORTEX INTO THE STR AND TOWARD LEARMONTH - AN UNLIKELY
SCENARIO. THE MAIN PACK GRADUALLY AND EVENLY SPREADS OUT TO A MERE
93NM BY TAU 72, LENDING HIGH CONFIDENCE TO THIS PORTION OF THE JTWC
TRACK FORECAST THAT IS LAID JUST TO THE RIGHT OF CONSENSUS TO
OFFSET NAVGEM. AFTERWARD, THE MODELS SIGNIFICANTLY DIVERGE ALONG-
AND ACROSS-TRACK, AN INDICATION OF A WEAK VORTEX UNDERGOING
DISSIPATION, LENDING LOW CONFIDENCE TO THE EXTENDED PORTION OF THE
JTWC TRACK FORECAST.

FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: HIGH
   TRACK 72-120 HR: LOW
   INTENSITY 0 - 72 HR: MEDIUM
   INTENSITY 72-120 HR: LOW//
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