WDXS31 PGTW 090300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 20S (MITCHELL) WARNING 
NR 013//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 23.1S 113.6E
   INITIAL INTENSITY: 55 KTS
   GEOGRAPHIC REFERENCE: 55 NM SOUTH-SOUTHWEST OF LEARMONTH,
AUSTRALIA
   MOVEMENT PAST 6 HOURS: SOUTHWARD AT 10 KTS
   SIGNIFICANT WAVE HEIGHT: 18 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
CYCLONE 20S (MITCHELL) WITH CONVECTION NOW SLIGHTLY OFFSET TO THE
SOUTH OF THE LOW-LEVEL CIRCULATION CENTER (LLCC) DUE TO THE
MODERATE NORTHERLY SHEAR ACTING ON THE SYSTEM. ANIMATED RADAR
IMAGERY FROM LEARMONTH SHOWS THE RADAR EYE HAS BECOME CONSIDERABLY
MORE RAGGED OVER THE PAST 6 HOURS. ADDITIONALLY, A 082242Z WSFM 37
GHZ MICROWAVE IMAGE SHOWED A BREAK IN THE MICROWAVE EYE ALONG THE
NORTHERN SEMICIRCLE. THE INITIAL POSITION IS PLACED WITH HIGH
CONFIDENCE BASED ON THE ANIMATED RADAR IMAGERY AND EXTRAPOLATION
FROM THE WSFM MICROWAVE IMAGE. THE INITIAL INTENSITY OF 55 KTS IS
ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE AGENCY DVORAK FIXES
AND OBJECTIVE INTENSITY ESTIMATES LISTED BELOW. ENVIRONMENTAL
ANALYSIS INDICATES THAT 20S REMAINS IN A MARGINAL ENVIRONMENT
CHARACTERIZED BY MODERATE OUTFLOW ALOFT GREATLY OFFSET BY MODERATE
NORTHERLY VERTICAL WIND SHEAR, SIGNIFICANT DRY AIR ENTRAINMENT, AND
BORDERLINE (26-27 C) SEA SURFACE TEMPERATURES.

INITIAL WIND RADII BASIS: PERSISTENCE WITH SLIGHT ADJUSTMENTS BASED
ON THE 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: T3.5 - 55 KTS
   CIMSS ADT: 51 KTS AT 090000Z
   CIMSS AIDT: 45 KTS AT 090000Z
   CIMSS D-MINT: 55 KTS AT 082242Z
   CIMSS D-PRINT: 47 KTS AT 090000Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 15-20 KTS
   SST: 25-26 CELSIUS
   OUTFLOW: MODERATE POLEWARD AND EQUATORWARD
   OTHER FACTORS: TERRAIN INTERACTION AND SIGNIFICANT DRY AIR
ENTRAINMENT

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 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 MAKE LANDFALL NORTH OF LAKE MACLEOD
WITHIN THE NEXT 6 HOURS. AFTER TAU 12, 20S IS FORECAST TO TURN
SOUTHEASTWARD AS IT CONTINUES TO ROUND THE RIDGE AXIS, PASSING JUST
EAST OF SHARK BAY. ENVIRONMENTAL CONDITIONS ARE EXPECTED TO ONLY
WORSEN AFTER LANDFALL WITH INCREASING NORTHWESTERLY SHEAR, DRY AIR,
AND FRICTIONAL EFFECTS. 20S IS EXPECTED TO WEAKEN BELOW TC STRENGTH
NO LATER THAN TAU 36 AS THE SHALLOW VORTEX TRACKS FURTHER INLAND
INTO WESTERN AUSTRALIA.    


MODEL DISCUSSION: NUMERICAL MODEL TRACK GUIDANCE REMAINS IN VERY
GOOD AGREEMENT WITH A CROSS-TRACK SPREAD OF ONLY 30 NM AND AN
ALONG-TRACK SPREAD OF 90 NM AT TAU 36. THE JTWC TRACK FORECAST IS
PLACED CLOSE TO THE MULTI-MODEL CONSENSUS WITH HIGH CONFIDENCE.
INTENSITY GUIDANCE AGREES ON STEADY WEAKENING THROUGHOUT THE
FORECAST PERIOD WITH NO NOTABLE OUTLIERS. THE JTWC INTENSITY
FORECAST IS ALSO PLACED NEAR THE CONSENSUS WITH HIGH CONFIDENCE.

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