WDXS31 PGTW 072100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 20S (MITCHELL) WARNING 
NR 008//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 20.3S 116.2E
   INITIAL INTENSITY: 80 KTS
   GEOGRAPHIC REFERENCE: 168 NM NORTHEAST OF LEARMONTH, AUSTRALIA
   MOVEMENT PAST 6 HOURS: SOUTHWESTWARD AT 06 KTS
   SIGNIFICANT WAVE HEIGHT: 24 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
CYCLONE 20S (MITCHELL) WITH A CORE OF VIGOROUS DEEP CONVECTION AND
CORRESPONDING CLOUD TOPS MEASURING COLDER THAN -90 C NEAR THE
CENTER. ANIMATED RADAR IMAGERY FROM KARRATHA SHOWS A DEFINED RADAR
EYE FEATURE WITH A SLIGHT WEAKNESS IN THE EASTERN SEMICIRCLE,
LIKELY DUE TO THE MODERATE NORTHEASTERLY SHEAR ACTING ON THE
SYSTEM. THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED
ON THE RADAR IMAGERY. THE INITIAL INTENSITY OF 80 KTS IS ASSESSED
WITH MEDIUM CONFIDENCE BASED ON THE AGENCY DVORAK FIXES AND CIMSS
OBJECTIVE INTENSITY ESTIMATES LISTED BELOW. ENVIRONMENTAL ANALYSIS
INDICATES THAT 20S REMAINS IN A FAVORABLE ENVIRONMENT CHARACTERIZED
BY ROBUST POLEWARD AND EQUATORWARD OUTFLOW ALOFT, MODERATE (15-20
KTS) NORTHEASTERLY VERTICAL WIND SHEAR, SLIGHT DRY AIR ENTRAINMENT,
AND WARM (29-30 C) SEA SURFACE TEMPERATURES.

INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK 

CURRENT STEERING MECHANISM: NORTHWESTERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) POSITIONED OVER EASTERN AUSTRALIA.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T4.5 - 77 KTS
   APRF: T4.5 - 77 KTS
   CIMSS SATCON: 78 KTS AT 071737Z
   CIMSS ADT: 82 KTS AT 071730Z
   CIMSS AIDT: 80 KTS AT 071730Z
   CIMSS D-MINT: 67 KTS AT 071737Z
   CIMSS D-PRINT: 72 KTS AT 071730Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 15-20 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG POLEWARD AND EQUATORWARD
   OTHER FACTORS: SLIGHT DRY AIR ENTRAINMENT FROM THE WEST AND
NORTH

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 TRACK SOUTHWESTWARD,
ALONG THE NORTHWESTERN PERIPHERY OF THE STR THROUGH TAU 24. 20S IS
NOW FORECAST TO MAKE LANDFALL WITHIN THE EXMOUTH GULF, SOUTH OF
LEARMONTH, RIGHT AROUND TAU 24. AFTER TAU 24, 20S WILL TURN
SOUTHWARD AS IT ROUNDS THE WESTERN EXTENT OF THE STR WITH
INCREASING TRACK SPEEDS. BY TAU 36, 20S WILL HAVE ROUNDED THE RIDGE
AXIS AND COMMENCE A SOUTHEASTWARD TURN AS IT TRACKS FURTHER INLAND
INTO WESTERN AUSTRALIA. IN TERMS OF INTENSITY, 20S IS FORECAST TO
MAINTAIN ITS CURRENT INTENSITY OVER THE NEXT 12 HOURS BEFORE
SLIGHTLY WEAKENING AS IT ENTERS THE EXMOUTH GULF DUE TO DRY AIR
ENTRAINMENT AND TERRAIN INTERACTION. LANDFALL INTENSITY IS FORECAST
TO BE AROUND 65-75 KTS. AFTER LANDFALL, 20S WILL BEGIN TO STEADILY
WEAKEN IN RESPONSE TO INCREASING NORTHERLY SHEAR, DRY AIR
ENTRAINMENT, AND FRICTIONAL EFFECTS. DISSIPATION IS EXPECTED TO
OCCUR NO LATER THAN TAU 72, OVER 200 NM EAST OF GERALDTON.

MODEL DISCUSSION: NUMERICAL MODEL TRACK GUIDANCE IS IN VERY GOOD
AGREEMENT THROUGH TAU 48 WITH THE EXCEPTION OF NAVGEM, WHICH
CONTINUES TO BE A FAR WESTWARD OUTLIER. DISCOUNTING NAVGEM, THERE
IS A MERE 15 NM CROSS-TRACK SPREAD AT TAU 24, NEAR THE TIMING OF
LANDFALL WHICH ONLY SLIGHTLY INCREASES TO 35 NM AT TAU 48. WHILE
CROSS-TRACK SPREAD REMAINS RELATIVELY SMALL, ALONG-TRACK SPREAD
INCREASES SIGNIFICANTLY AFTER TAU 48 WITH GFS BEING THE SLOWEST AND
THE EC-AIFS BEING THE FASTEST. THIS IS DUE TO INTERACTIONS WITH THE
STRONG UPPER-LEVEL WINDS TO THE SOUTH OF THE SYSTEM. THE JTWC TRACK
FORECAST IS PLACED CLOSE TO THE ALTERED CONSENSUS (REMOVING NAVGEM)
WITH HIGH CONFIDENCE. INTENSITY GUIDANCE IS SLIGHTLY SPLIT THROUGH
THE FIRST 12 HOURS, WITH SOME MODELS SHOWING A MORE RAPID WEAKENING
TREND STARTING AT TAU 0 (COAMPS-TC AND HWRF) WHILE OTHERS SHOW THE
SYSTEM MAINTAINING INTENSITY (HAFS-A, GDM, AND SHIPS). AFTER TAU 12,
MODELS ARE IN GOOD AGREEMENT ON STEADY WEAKENING THROUGH THE
REMAINDER OF THE FORECAST PERIOD. THE JTWC INTENSITY FORECAST IS
PLACED CLOSER TO THE HAFS-A THROUGH TAU 12 AND THEN NEAR THE
CONSENSUS AFTERWARD WITH MEDIUM CONFIDENCE.

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