WDXS31 PGTW 082100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 20S (MITCHELL) WARNING 
NR 012//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 22.1S 113.8E
   INITIAL INTENSITY: 60 KTS
   GEOGRAPHIC REFERENCE: 21 NM NORTHWEST OF LEARMONTH, AUSTRALIA
   MOVEMENT PAST 6 HOURS: SOUTHWESTWARD AT 06 KTS
   SIGNIFICANT WAVE HEIGHT: 20 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
CYCLONE 20S (MITCHELL) WITH WARMING CLOUD TOPS AND AN INCREASINGLY
DISORGANIZED CONVECTIVE STRUCTURE. A 081102Z RCM-2 SAR IMAGE
REVEALED THAT THE SURFACE CIRCULATION HAS REMAINED RELATIVELY
SYMMETRICAL WITH 60-70 KNOT WINDS WITHIN ALL QUADRANTS. SURFACE
OBSERVATIONS FROM LEARMONTH REPORTED MAXIMUM SUSTAINED WINDS OF
AROUND 45 KTS WITH A PRESSURE MINIMUM OF 984.5 MB AS THE SYSTEM
PASSED BY ABOUT 20 NM TO THE WEST. THE INITIAL POSITION IS PLACED
WITH HIGH CONFIDENCE BASED ON THE ANIMATED RADAR IMAGERY FROM
LEARMONTH. THE INITIAL INTENSITY OF 60 KTS IS ASSESSED WITH MEDIUM
CONFIDENCE BASED ON A STEADY WEAKENING TREND FROM THE MENTIONED SAR
IMAGE, SUPPORTED BY THE AGENCY DVORAK FIXES AND OBJECTIVE INTENSITY
ESTIMATES LISTED BELOW. ENVIRONMENTAL ANALYSIS INDICATES THAT 20S
REMAINS IN A MARGINAL ENVIRONMENT CHARACTERIZED BY STRONG OUTFLOW
ALOFT GREATLY OFFSET BY MODERATE NORTHEASTERLY VERTICAL WIND SHEAR,
SIGNIFICANT DRY AIR ENTRAINMENT, AND BORDERLINE (26-27 C) SEA
SURFACE TEMPERATURES.

INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK 

CURRENT STEERING MECHANISM: WESTERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) POSITIONED OVER CENTRAL AUSTRALIA

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T4.0 - 65 KTS
   DEMS: T4.0 - 65 KTS
   APRF: T4.0 - 65 KTS
   CIMSS SATCON: 62 KTS AT 081800Z
   CIMSS ADT: 51 KTS AT 081800Z
   CIMSS AIDT: 43 KTS AT 081800Z
   CIMSS D-MINT: 60 KTS AT 081719Z
   CIMSS D-PRINT: 55 KTS AT 081800Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 15-20 KTS
   SST: 26-27 CELSIUS
   OUTFLOW: STRONG POLEWARD AND EQUATORWARD
   OTHER FACTORS: DRY AIR ENTRAINMENT AND TERRAIN INTERACTION

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: MEDIUM
   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 20S IS FORECAST TO TURN SOUTHWARD OVER THE
NEXT 12 HOURS AS THE SYSTEM ROUNDS THE WESTERN PERIPHERY OF THE STR
TO THE EAST. 20S IS EXPECTED TO SKIRT THE COAST WITH A LANDFALL
NORTH OF LAKE MACLEOD OCCURRING JUST BEFORE TAU 12. AFTER TAU 12,
20S IS FORECAST TO TURN SOUTHEASTWARD AS IT CONTINUES TO ROUND THE
RIDGE AXIS, PASSING EAST OF SHARK BAY. ENVIRONMENTAL CONDITIONS ARE
EXPECTED TO WORSEN THROUGH TAU 12 AS SEA SURFACE TEMPERATURES
CONTINUE TO COOL, WITH THE SYSTEM EXPECTED TO CROSS THE 26 DEGREE
ISOTHERM WITHIN THE NEXT 6 HOURS. AS A RESULT, 20S IS FORECAST TO
WEAKEN THROUGH THE TIME OF ITS FINAL LANDFALL. THE ENVIRONMENT WILL
ONLY WORSEN AFTER LANDFALL, WITH INCREASING NORTHWESTERLY SHEAR,
DRY AIR, AND FRICTIONAL EFFECTS. 20S IS EXPECTED TO WEAKEN BELOW TC
STRENGTH AROUND TAU 36 AS THE SHALLOW VORTEX TRACKS FURTHER INLAND
INTO WESTERN AUSTRALIA.    

MODEL DISCUSSION: NUMERICAL MODEL TRACK GUIDANCE REMAINS IN VERY
GOOD AGREEMENT WITH THE EXCEPTION OF NAVGEM, WHICH CONTINUES TO
TRACK THE SYSTEM WEST OF SHARK BAY. DISCOUNTING NAVGEM, CROSS-TRACK
SPREAD AT TAU 36 IS AROUND 30NM WITH MINIMAL ALONG-TRACK SPREAD,
LENDING HIGH CONFIDENCE TO THE JTWC TRACK FORECAST. INTENSITY
GUIDANCE AGREES ON STEADY WEAKENING THROUGHOUT THE FORECAST PERIOD
WITH NO NOTABLE OUTLIERS. THE JTWC INTENSITY FORECAST IS PLACED
NEAR THE MULTI-MODEL CONSENSUS WITH HIGH CONFIDENCE.

FORECAST CONFIDENCE:
   TRACK 00-72 HR: HIGH
   INTENSITY 00-72 HR: HIGH//
NNNN