WDXS31 PGTW 100900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 21S (OLGA) WARNING NR 
019//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 19.5S 115.6E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 188 NM NORTH-NORTHEAST OF LEARMONTH,
AUSTRALIA
   MOVEMENT PAST 6 HOURS: SOUTHWESTWARD AT 08 KTS
   SIGNIFICANT WAVE HEIGHT: 14 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A FULLY
EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC) WITH SCANT AMOUNTS OF
CONVECTION BLOWN TO THE SOUTHEAST AS TROPICAL CYCLONE (TC) 21S
(OLGA) IS BESIEGED BY STRONG 45-50KT VERTICAL WIND SHEAR FROM THE
NORTHWEST. THE SYSTEM STRUCTURE CONTINUES TO BROADEN AS IT HAS
BECOME NEARLY COMPLETELY ENGULFED WITH DRY AIR. TC 21S CONTINUES TO
TRACK SOUTHWEST AS THE STORM IS GUIDED ON THE WESTERN PERIPHERY OF
A LOW-LEVEL SUBTROPICAL RIDGE (STR) TO THE EAST. UPPER-LEVEL
OUTFLOW IS NONEXISTENT WHILE THE SYSTEM IS EMBEDDED IN THE
SUBTROPICAL JET. THE ONLY FAVORABLE ENVIRONMENTAL FACTOR IS SEA
SURFACE TEMPERATURES, WHICH CONTINUE TO BE 29-30C. THE INITIAL
POSITION WAS ASSESSED WITH HIGH CONFIDENCE BASED ON ANIMATED MSI
DEPICTING A BROADENING LLCC. THE INITIAL INTENSITY WAS ASSESSED
WITH HIGH CONFIDENCE BASED ON AN EARLIER 100149Z METOP-B IMAGE
DEPICTING WINDS UP TO 40KTS IN THE SOUTHERN HEMISPHERE, AND THE
AGENCY AND OBJECTIVE FIXES LISTED BELOW.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: THE WESTERN PERIPHERY OF A LOW-LEVEL
STR TO THE EAST.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   KNES: T1.5 - 25 KTS
   DEMS: T2.5 - 35 KTS
   APRF: T2.0 - 30 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: HIGHLY UNFAVORABLE
   VWS: 30+ KTS
   SST: 29-30 CELSIUS
   OUTFLOW: WEAK EASTWARD

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

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 TO TRACK
SOUTHWEST OVER THE NEXT 36 HOURS CONTINUING ON THE WESTERN
PERIPHERY OF A STR TO THE EAST. IT IS POSSIBLE THAT DEVIATIONS IN
THE ORIENTATION OF THE RIDGE COULD RESULT IN THE TRACK CLOSER TO OR
EVEN OVER THE EXMOUTH PENINSULA. THE INTENSITY IS FORECAST TO
CONTINUE TO WIND DOWN GRADUALLY TO 30KTS BETWEEN TAU 12-24. THE
SYSTEM WILL CONTINUE TO LOSE MOMENTUM WHILE IN AN UNFAVORABLE
ENVIRONMENT DUE TO ELEVATED VWS, AN UPPER-LEVEL SUBTROPICAL JET,
AND DRY AIR ENGULFING THE CIRCULATION. 

MODEL DISCUSSION: DETERMINISTIC MODEL GUIDANCE IS IN AGREEMENT THAT
THE SYSTEM WILL GENERALLY TRACK SOUTHWEST OVER THE NEXT 36 HOURS
BUT DOES VARY WHETHER THAT TRACK IS MORE WEST-SOUTHWESTERLY
(NAVGEM-BASED COAMPS-TC) OR SOUTH-SOUTHWESTERLY (GFS-BASED
COAMPS-TC). THE JTWC TRACK FORECAST LIES ON THE EASTERN EDGE OF THE
GUIDANCE ENVELOPE, THOUGH THE HAFS-A AND THE COAMPS-TC (GFS) ARE
FURTHER EAST, AND FAVORS THE DIRECTLY SOUTHWESTERLY ROUTE GIVEN
THAT THE MODEL GUIDANCE DOES NOT APPEAR TO CAPTURE THE RECENT
MOTION AS DEPICTED IN THE MSI. THE INTENSITY GUIDANCE IS IN GOOD
AGREEMENT THAT TC 21S WILL CONTINUE TO GRADUALLY DISSIPATE BELOW
35KTS BETWEEN TAU 12-24. THE JTWC INTENSITY GUIDANCE FAVORS A
SLOWER WEAKENING BASED ON HIGHER-THAN-EXPECTED WIND SPEED DATA
EARLIER TODAY, SPECIFICALLY A 100149Z METOP-B ASCAT IMAGE DEPICTING
UP TO 40KTS IN THE SOUTHWESTERN PORTION OF THE SYSTEM.

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