WDXS31 PGTW 081500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 20S (MITCHELL) WARNING 
NR 011//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 21.6S 114.0E
   INITIAL INTENSITY: 60 KTS
   GEOGRAPHIC REFERENCE: 42 NM NORTH OF LEARMONTH, AUSTRALIA
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 08 KTS
   SIGNIFICANT WAVE HEIGHT: 24 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL CYCLONE (TC) 20S (MITCHELL) EXHIBITS A CONTINUING WEAKENING 
TREND AS IT TRAVERSES SOUTHWESTWARD, CURRENTLY PASSING JUST NORTH OF 
THE NORTHWEST CAPE OF THE EXMOUTH PENINSULA. ANIMATED ENHANCED 
INFRARED (EIR) SATELLITE IMAGERY DEPICTS A DEGRADED CENTRAL DENSE 
OVERCAST (CDO) CHARACTERIZED BY WARMING CLOUD TOPS AND DIMINISHING 
CONVECTION, RESULTING IN AN OBSCURED LOW-LEVEL CIRCULATION CENTER 
(LLCC). LAND-BASED RADAR DATA FROM LEARMONTH HOWEVER, PROVIDE A 
GLIMPSE UNDER THE CDO AND DEPICT A RAGGED AND BROAD LLCC POSITIONED 
APPROXIMATELY 42NM NNW OF LEARMONTH. ANIMATED MULTI-CHANNEL WATER 
VAPOR IMAGERY AND MODEL SKEW-T DATA SUPPORT THE FACT THAT WHILE DEEP-
LAYER SHEAR IS IN THE LOW TO MODERATE RANGE, MID-LEVEL, SUB-OUTFLOW 
LEVEL, SHEAR IS A FEW KNOTS HIGHER, WHICH IS DEGRADING THE SYSTEM. 
THE INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE USING THE LAND-
BASED RADAR DATA. THE INITIAL INTENSITY IS ASSESSED WITH MEDIUM 
CONFIDENCE PRIMARILY BASED ON THE OBJECTIVE INTENSITY ESTIMATES NOTED 
BELOW, WHICH ARE RUNNING LOWER THAN THE SUBJECTIVE DVORAK CURRENT 
INTENSITY (CI) ESTIMATES FROM BOTH PGTW AND APRF. ENVIRONMENTAL 
CONDITIONS ARE MARGINAL, WITH MODERATE TO HIGH MID-LEVEL SHEAR 
OFFSETTING WARM SSTS AND STRONG POLEWARD, DIVERGENT OUTFLOW AHEAD OF 
AN APPROACHING TROUGH TO THE SOUTHWEST.

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: T4.5 - 77 KTS
   DEMS: T4.0 - 65 KTS
   APRF: T4.0 - 65 KTS
   CIMSS ADT: 61 KTS AT 081130Z
   CIMSS AIDT: 53 KTS AT 081130Z
   CIMSS D-PRINT: 58 KTS AT 081200Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 15-20 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: DRY, LOW ENTROPY AIR ENTRAINING INTO THE CORE OF 
THE
SYSTEM FROM THE CONTINENT; HIGH MID-LEVEL NORTHEASTERLY SHEAR. 

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 HAS OSCILLATED SLIGHTLY WESTWARD OVER THE 
PAST FEW HOURS BASED ON RADAR DERIVED FIXES, CONTINUING THE WESTWARD 
BIAS SEEN OVER THE LAST 24 HOURS. A LANDFALL ALONG THE EXMOUTH 
PENINSULA IS NO LONGER ANTICIPATED; THE SYSTEM IS PROJECTED TO PASS 
TO THE WEST OF THE PENINSULA AS IT UNDERGOES A GRADUAL SOUTHERLY 
RECURVATURE, PARALLELING THE COASTLINE. THE SYSTEM WILL ROUND THE 
RIDGE AXIS BY TAU 12, AND BEGIN TO ACCELERATE ON A SOUTHEASTWARD 
TRAJECTORY THROUGH THE REMAINDER OF THE FORECAST, AS IT TRACKS ALONG 
THE GRADIENT BETWEEN THE TROUGH MOVING IN FROM THE SOUTHWEST AND THE 
STR OVER AUSTRALIA. LANDFALL IS FORECAST TO OCCUR ON THE FAR WESTERN 
COAST OF WESTERN AUSTRALIA, ROUGHLY NEAR MCLEOD, THEN TRACK 
SOUTHEASTWARD ALONG THE EASTERN SHORE OF SHARK BAY BEFORE MOVING 
ASHORE FOR THE FINAL TIME IN THE SOUTHERN EXTENT OF SHARK BAY SHORTLY 
AFTER TAU 24. THE SYSTEM WILL THEN CONTINUE INLAND THROUGH THE 
REMAINDER OF THE FORECAST PERIOD. REGARDING INTENSITY, SHEAR IS 
FORECAST TO MODERATE IN THE NEAR-TERM; HOWEVER, GIVEN THE EXTENDED 
MARITIME TRACK, TC 20S WILL CONTINUE TO UNDERGO CYCLOLYSIS DRIVEN BY 
COOLING SSTS AND DRY AIR ENTRAINMENT, ALBEIT AT A REDUCED DECAY RATE 
THROUGH TAU 24. AFTER MAKING FINAL LANDFALL SOUTHEAST OF SHARK BAY, 
TC 20S WILL RAPIDLY DISSIPATE DUE TO FRICTIONAL EFFECTS AND INCREASED 
STABILITY AS IT TRAVERSES INLAND. 

MODEL DISCUSSION: MODEL GUIDANCE REMAINS TIGHTLY CLUSTERED WITHIN A 
CONSTRAINED ENVELOPE BUT EXHIBITS A PERSISTENT WESTWARD SHIFT WITH 
SUCCESSIVE INITIALIZATIONS. THE CURRENT MODEL CONSENSUS DEPICTS AN 
EASTERN BOUNDARY, DEFINED BY THE GFS TRACKER, POSITIONED OVER THE 
WESTERN EDGE OF THE EXMOUTH PENINSULA. THE WESTERN BOUNDARY IS 
DEFINED BY NAVGEM, WHICH MAINTAINS AN OFFSHORE TRACK THROUGH THE 
WESTERN SECTOR OF SHARK BAY. THE REMAINDER OF THE MODEL MEMBERS ARE 
SITUATED WITHIN 25NM WEST OF THE GFS SOLUTIONS. THE JTWC FORECAST IS 
ALIGNED WITH THE WESTERN EDGE OF THE MULTI-MODEL CONSENSUS WITH HIGH 
CONFIDENCE. INTENSITY GUIDANCE IS IN GOOD AGREEMENT, SUPPORTING
SLOW WEAKENING THROUGH TAU 24 FOLLOWED BY MORE RAPID WEAKENING
THEREAFTER AND THE JTWC FORECAST IS PLACED NEAR THE HWRF TRACKER
WITH HIGH CONFIDENCE. 

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