WDXS31 PGTW 060300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 20S (TWENTY) WARNING 
NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.8S 121.9E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 18 NM WEST-NORTHWEST OF BROOME, AUSTRALIA
   MOVEMENT PAST 6 HOURS: WESTWARD AT 07 KTS
   SIGNIFICANT WAVE HEIGHT: 14 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
THE LOW-LEVEL CIRCULATION CENTER (LLCC) OF TROPICAL CYCLONE 20S HAS
EMERGED OVER SOUTH INDIAN OCEAN WATERS JUST WEST OF BROOME,
AUSTRALIA WITH IMPRESSIVE BANDS OF DEEP CONVECTION AND CONSISTENTLY
COOL CLOUD TOP TEMPERATURES AS DEPICTED BY ANIMATED MULTISPECTRAL
SATELLITE IMAGERY (MSI). ENVIRONMENTAL CONDITIONS ARE RIPE FOR
RAPID DEVELOPMENT IF THE CENTER REMAINS OVER WATER WITH SEA SURFACE
TEMPERATURES AT 29 C TO 30 C, LOW TO MODERATE VERTICAL WIND SHEAR
IN THE AREA, AND MODERATE POLEWARD AND EQUATORWARD OUTFLOW. THE
INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON RADAR
FIXES. THE INITIAL INTENSITY OF 30 KTS IS ASSESSED WITH MEDIUM
CONFIDENCE BASED ON LOCAL OBSERVATIONS FROM BROOME, AUSTRALIA ALONG
WITH THE OBJECTIVE AND SUBJECTIVE ESTIMATES LISTED BELOW.

INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND
RADII).

CURRENT STEERING MECHANISM: COMPETING STEERING BETWEEN RIDGING OVER
THE EASTERN INDIAN OCEAN AND RIDGING CENTERED OVER CENTRAL
AUSTRALIA

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   CIMSS D-PRINT: 31 KTS AT 060200Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 5-10 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE POLEWARD AND EQUATORWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: NOT APPLICABLE

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING
MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY.

FORECAST DISCUSSION: TROPICAL CYCLONE (TC) 20S WILL PROGRESS
STEADILY SOUTHWESTWARD AS SUBTROPICAL RIDGING OVER THE EASTERN
INDIAN OCEAN RETREATS WEST AND RIDGING OVER THE CENTRAL AUSTRALIAN
LANDMASS EXERTS MORE INFLUENCE. AROUND TAU 48 THE STATE OF STEERING
WILL CAUSE A SHARPER SOUTHWESTWARD TURN, AS THE RIDGING OVER
AUSTRALIA BECOMES THE PRIMARY DRIVER, AND WHILE THE SYSTEM ROUNDS
THE WESTERN PERIPHERY OF THE RIDGE. LANDFALL IS EXPECTED BETWEEN
TAU 48 AND TAU 60 ALONG THE AUSTRALIAN COAST SOUTHWEST OF BARROW
ISLAND. STEADY WEAKENING IS FORECASTED PAST TAU 60 AS LOW ELEVATION
TOPOGRAPHY AND DRY AIR ENTRAINMENT WILL CAUSE THE DISSIPATION OF TC
20S BY TAU 96. REGARDING INTENSITY THE ENVIRONMENT IS FORECAST TO
REMAIN FAVORABLE FOR DEVELOPMENT UNTIL LANDFALL, WITH WARM (29 C TO
30 C) SEA SURFACE TEMPERATURES, LOW TO MODERATE VERTICAL WIND SHEAR
IN THE AREA, AND MODERATE POLEWARD AND EQUATORWARD OUTFLOW. DRY AIR
ENTRAINMENT AND PROXIMITY TO LAND WILL BE THE MAIN INHIBITING
FACTORS MANAGED BY THE STRENGTH OF STEERING FROM THE RIDGE OVER THE
AUSTRALIAN LANDMASS. PEAK INTENSITY IS CURRENTLY FORECAST AT AROUND
75 KTS BETWEEN TAUS 36 AND 48, IF NO MAJOR CHANGES TO THE TRACK
OCCUR. HOWEVER, ELEVATED INTENSITY CAN BE EXPECTED IF SYSTEM SLOWS
DOWN OR SPENDS MORE TIME OVER WARM WATERS JUST PRIOR TO LANDFALL.

MODEL DISCUSSION: MODELS ARE IN GOOD AGREEMENT ON THE WEST
SOUTHWESTWARD PROGRESSION OF TC 20S WITH CROSS TRACK SPREAD OF
30-40 KTS PRIOR TO LANDFALL, EXCLUDING NAVGEM, WHICH IS CURRENTLY
THE ONLY OUTLIER, SKIMMING NORTH OF BARROW ISLAND. MAJORITY OF THE
REMAINING CONSENSUS MEMBERS DEPICT A SHARP SOUTHWESTARD TURN EAST
OF BARROW ISLAND BRINGING THE CENTER INLAND SHORTLY AFTER TAU 48.
THE JTWC FORECASTED TRACK LIES JUST SOUTH OF THE MULTI MODEL
CONSENSUS WITH MEDIUM CONFIDENCE ACCOUNTING FOR THE OUTLYING NAVGEM
TRACK. INTENSITY GUIDANCE IS ALSO ASSESSED WITH MEDIUM CONFIDENCE,
DUE TO PREDICTED PEAK WIND SPEEDS SPREAD OF UP TO 30 KTS ACROSS THE
GUIDANCE ENVELOPE. THE ASSOCIATED UNCERTAINTY IS AMPLIFIED BY
SIGNIFICANT DEPENDENDENCY ON THE POSITION OF THE TRACK. GFS AND
HAFS-A ARE DEPICTING HIGHER INTENSITIES IN LINE WITH THE PREDICTED
ENVIRONMENTAL CONDITIONS, WHILE COAMPS-TC IS CURRENTLY OFFERS THE
MOST CONSERVATIVE PREDICTION. 

FORECAST CONFIDENCE:
   TRACK 00-72 HR: HIGH
   TRACK 72-120 HR: MEDIUM
   INTENSITY 00-72 HR: MEDIUM
   INTENSITY 72-120 HR: MEDIUM//
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