WDXS31 PGTW 091500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 21S (OLGA) WARNING NR 
016//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 18.1S 116.4E
   INITIAL INTENSITY: 45 KTS
   GEOGRAPHIC REFERENCE: 283 NM NORTH-NORTHEAST OF LEARMONTH,
AUSTRALIA
   MOVEMENT PAST 6 HOURS: SOUTH-SOUTHWESTWARD AT 05 KTS
   SIGNIFICANT WAVE HEIGHT: 19 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
CYCLONE (TC) 21S (OLGA) WITH A FULLY EXPOSED LOW-LEVEL CIRCULATION
CENTER AS STRONG VERTICAL WIND SHEAR (VWS) OF 30-35KTS CONTINUES TO
BEAR DOWN ON THE VORTEX. THE RECENT FLARE-UP OF CONVECTION NEAR THE
CENTER HAS ALREADY BEEN PUSHED OFF TO THE SOUTHEAST ALONG THE
STRONG SHEAR VECTOR. MODERATE POLEWARD OUTFLOW IS DAMPENED AS THE
SYSTEM APPROACHES THE SUBTROPICAL JET TO THE SOUTH, MITIGATING
UPPER-LEVEL DIVERGENCE. SEA SURFACE TEMPERATURES REMAIN FAVORABLE
IN 29-30C. THE CYCLONE APPEARS TO HAVE BEGUN TO STALL AND DRIFT
OVER THE LAST SIX HOURS AS THE LOW-LEVEL STEERING MECHANISM HAS
WEAKENED AS THE STRUCTURE OF THE CYCLONE WEAKENS. THE INITIAL
POSITION WAS ASSESSED WITH HIGH CONFIDENCE BASED ON ANIMATED EIR
DEPICTING AN EXPOSED LLCC. THE INITIAL INTENSITY WAS ASSESSED WITH
LOW CONFIDENCE BASED ON AGENCY AND OBJECTIVE FIXES LISTED BELOW.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: ON THE WESTERN PERIPHERY OF A LOW-LEVEL
SUBTROPICAL RIDGE TO THE EAST.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   KNES: T2.5 - 35 KTS
   DEMS: T2.5 - 35 KTS
   APRF: T3.0 - 45 KTS
   CIMSS SATCON: 38 KTS AT 090830Z
   CIMSS ADT: 47 KTS AT 091230Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 30-35 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE POLEWARD

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

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 OVER THE NEXT 48 HOURS WHILE DRIVEN ALONG THE
WESTWARD PERIPHERY OF A LOW-LEVEL STR OVER THE WESTERN PORTION OF
THE AUSTRALIAN CONTINENT. HOWEVER, MINOR DEVIATIONS IN THE
ORIENTATION OF THE RIDGE COULD RESULT IN A TRACK CLOSER TO THE
EXMOUTH PENINSULA OR FURTHER OUT TO SEA. THE INTENSITY IS FORECAST
TO CONTINUE TO WEAKEN TO FULL DISSIPATION THROUGH TAU 48 WHILE
OVERRUN BY INCREASINGLY STRONG VWS, WHICH IS EXPECTED TO REACH OVER
75KTS. DESPITE THE DISSIPATION, THE STRONGEST WINDS WILL PERSIST IN
THE SOUTHERN HEMISPHERE OF THE SYSTEM, SKIRTING THE COAST OF
NORTHWEST AUSTRALIA AS THE SYSTEM PASSES OFFSHORE.

MODEL DISCUSSION: DETERMINISTIC MODEL GUIDANCE HAS BEGUN TO EDGE
SLIGHTLY EAST OVER THE LAST SIX HOURS, INDICATING THAT THE STR TO
THE EAST MAY NOT BE AS STRONG AS ORIGINALLY SUSPECTED. THE 48 HOUR
CROSS-TRACK ERROR HAS OPENED TO 200NM WHICH MAY RESULT FROM CHANGES
IN HOW STRONGLY THE RIDGE TO THE EAST IS FORMED AS WELL AS
DIFFERENCES IN THE ORIENTATION OF THE RIDGE BETWEEN THE VARIOUS
MODELS. THE INTENSITY GUIDANCE REMAINS IN GOOD AGREEMENT,
FORECASTING GRADUAL WEAKENING AS THE SYSTEM BECOMES EMBEDDED IN THE
SUBTROPICAL JET AND VWS INCREASES. THE JTWC CONSENSUS GUIDANCE
FORECASTS FULL DISSIPATION BETWEEN 36-60 HOURS. 

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