WDXS31 PGTW 230900
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: 16.7S 107.3E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 511 NM NORTHWEST OF LEARMONTH, AUSTRALIA
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 13 KTS
   SIGNIFICANT WAVE HEIGHT: 19 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS DEEP
CONVECTION PARTIALLY OBSCURING THE LOW-LEVEL CIRCULATION CENTER
(LLCC) OF TROPICAL CYCLONE (TC) 20S. A SLIGHT VERTICAL TILT OF THE
SYSTEM REVEALS LOW-LEVEL CLOUD BANDING CIRCULATING IN THE EASTERN
PERIPHERY OF THE SYSTEM. CIRRUS FILAMENTS EXHAUST OUTWARDS FROM THE
CENTER DUE TO STRONG UPPER-LEVEL DIVERGENCE AND POLEWARD OUTFLOW
INTO THE JET TO THE SOUTH. THE INITIAL POSITION IS PLACED WITH
MEDIUM CONFIDENCE BASED ON THE AFOREMENTIONED ANIMATED MSI. THE
INITIAL INTENSITY IS PLACED WITH MEDIUM CONFIDENCE BASED ON TEH
AGENCY AND OBJECTIVE FIXES LISTED BELOW.

INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK 

CURRENT STEERING MECHANISM: A NEAR EQUATORIAL RIDGE TO THE
NORTHEAST.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   PHFO: T3.0 - 45 KTS
   APRF: T2.5 - 35 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 0-5 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: STRONG POLEWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: LOW

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TROPICAL CYCLONE (TC) 20S WILL ROUND THE
PERIPHERY OF A SUBTROPICAL RIDGE LOCATED OVER WESTERN AUSTRALIA,
TRACKING SOUTHWESTWARD UNTIL TAU 48, AT WHICH TIME THE SYSTEM WILL
CROSS OVER THE RIDGE AXIS AND CURVE SOUTH-SOUTHEASTWARD. THE
ENVIRONMENT WILL BE FAVORABLE FOR INTENSIFICATION TO A PEAK OF
70KTS DURING THE FIRST 48 HOURS OF THE FORECAST WHILE VERTICAL WIND
SHEAR IS LOW AND A POLEWARD OUTFLOW CHANNEL FEEDS INTO THE
UPPER-LEVEL JET TO THE SOUTH. BETWEEN TAU 48 AND TAU 72, THE SYSTEM
WILL PASS INTO COOL SEA SURFACE TEMPERATURES AND EMBED WITHIN THE
JET, RESULTING IN HIGH VERTICAL WIND SHEAR AND WEAKENING TO 60KTS
BY THE END OF THE FORECAST. THE SYSTEM WILL UNDERGO SUBTROPICAL
TRANSITION BETWEEN TAU 48-72 AND BECOME FULLY SUBTROPICAL AT TAU
72.

MODEL DISCUSSION: DETERMINISTIC MODEL GUIDANCE IS IN GOOD AGREEMENT
THAT TC 20S WILL ROUND THE WESTERN PERIPHERY OF THE SUBTROPICAL
RIDGE AND CURVE SOUTH-SOUTHEASTWARD BY THE END OF THE FORECAST. HOW
TIGHTLY THE SYSTEM CURVES VARIES 135NM AT TAU 72 WTIH GFS LYING TO
THE WEST AND GALWEM LYING TO THE EAST. THE INTENSITY GUIDANCE IS IN
GOOD AGREEMENT THAT A PEAK INTENSITY WILL OCCUR AT TAU 48, WITH A
SPREAD OF 60KTS(GFS) TO 80KTS (HWRF).

FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: MEDIUM
   INTENSITY 0 - 72 HR: MEDIUM//
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