WDXS31 PGTW 111500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 17S (SEVENTEEN) 
WARNING NR 002//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.9S 119.1E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 147 NM NORTH OF PORT HEDLAND, AUSTRALIA
   MOVEMENT PAST 6 HOURS: SOUTHWARD AT 08 KTS
   SIGNIFICANT WAVE HEIGHT: 16 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
CYCLONE 17S WITH CONVECTION BEGINNING TO BE MORE CENTRALIZED OVER
THE LOW-LEVEL CIRCULATION CENTER (LLCC). WINDS FROM ROWLEY SHOALS
HAS SHIFTED TO WEST-SOUTHWESTERLY, INDICATING THAT THE SYSTEM HAS
PASSED TO ITS SOUTH. REPORTED WINDS ON STATION ARE STEADY AT AROUND
40 KTS (AFTER 1-MIN AVERAGE CONVERSION). ENVIRONMENTAL ANALYSIS
INDICATES THAT 17S IS IN A HIGHLY FAVORABLE ENVIRONMENT
CHARACTERIZED BY STRONG POLEWARD OUTFLOW ALOFT, LOW (10-15 KTS)
VERTICAL WIND SHEAR, AND VERY WARM (29-30 C) SEA SURFACE
TEMPERATURES. THE INITIAL POSITION IS PLACED WITH MEDIUM CONFIDENCE
BASED ON THE ANIMATED EIR IMAGERY. THE INITIAL INTENSITY OF 40 KTS
IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE MENTIONED NEARBY
SURFACE OBS, AGENCY DVORAK ESTIMATES, AND OBJECTIVE INTENSITY
ESTIMATES LISTED BELOW.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: WESTERN PERIPHERY OF A SUBTROPICAL
RIDGE (STR) POSITIONED OVER CENTRAL AUSTRALIA. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   KNES: T2.0 - 30 KTS
   APRF: T2.0 - 30 KTS
   CIMSS ADT: 41 KTS AT 111200Z
   CIMSS AIDT: 36 KTS AT 111200Z
   CIMSS DPRINT: 34 KTS AT 111200Z
   CIMSS DMINT: 31 KTS AT 111004Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: HIGHLY FAVORABLE
   VWS: 10-15 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG POLEWARD

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THE INTENSIFICATION RATE THROUGH TAU
48 IS SIGNIFICANTLY INCREASED COMPARED TO THE PREVIOUS FORECAST. 

FORECAST DISCUSSION: TC 17S IS FORECAST TO TRACK SOUTHWARD OVER THE
NEXT TWELVE HOURS AS THE RIDGING OVER AUSTRALIA REMAINS THE PRIMARY
STEERING MECHANISM. AFTER TAU 12, THE RIDGE BEGINS TO PROPOGATE
EASTWARD AND RIDGING FROM THE WEST EXTENDS TO THE SOUTH OF THE
SYSTEM, CAUSING A SOUTHWESTWARD TRACK. DURING THIS TIME, OVERALL
STEERING INFLUENCE IS WEAK, SO THERE IS A LARGE AMOUNT OF
UNCERTAINTY WITH THE TRACK THROUGH TAU 48. NEAR TAU 60, RIDGING
OVER AUSTRALIA BUILDS BACK IN AND THE RIDGING TO THE WEST MOVES
WESTWARD, ALLOWING 17S TO BEGIN ACCELERATING SOUTH-SOUTHEASTWARD AS
IT MAKES LANDFALL. THE LANDFALL LOCATION HAS SHIFTED WESTWARD
COMPARED TO THE PREVIOUS FORECAST AND IS CURRENTLY FORECAST TO
OCCUR NEAR TAU 60. REGARDING INTENSITY, 17S IS FORECAST TO BEGIN
INTENSIFYING AS ENVIRONMENTAL PARAMETERS ARE HIGHLY CONDUCIVE.
RAPID INTENSIFICATION FROM TAU 24-48 IS ANTICIPATED IN RESPONSE TO
THE INCREASINGLY WARM SEA SURFACE TEMPERATURES NEAR THE COAST. THE
CURRENT PEAK IS SET AT 95 KTS AT TAU 48, BUT THERE WILL LIKELY BE
AN EVEN HIGHER PEAK, NEAR TAU 60, THAT IS UNABLE TO BE REPRESENTED
IN THE FORECAST DUE TO TEMPORAL RESTRICTIONS. AFTER THE SYSTEM
MAKES LANDFALL, TERRAIN INTERACTION WILL QUICKLY ERODE THE VORTEX,
WITH DISSIPATION AROUND TAU 96.  

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN POOR AGREEMENT,
ESPECIALLY CONCERNING THE GEFS AND ECENS ENSEMBLE MEMBER SPREAD.
ECENS SIGNIFICANTLY SPREADS OUT STARTING FROM THE INITIAL POSITION,
EXTENDING WESTWARD, NEARLY TO LEARMONTH. CONSENSUS MODEL SPREAD
DOES NOT DEPICT QUITE AS MUCH UNCERTAINTY WITH A 120 NM CROSS-TRACK
SPREAD AT TAU 60, NEAR THE TIME OF LANDFALL. THE JTWC TRACK
FORECAST IS PLACED WITH LOW CONFIDENCE THROUGHOUT THE FORECAST
PERIOD, NEAR THE MULTI-MODEL CONSENSUS. INTENSITY GUIDANCE IS ALSO
IN POOR AGREEMENT WITH HAFS-A AND HWRF SUGGESTING EXTREMELY RAPID
INTENSIFICATION TO A PEAK OF AROUND 125 KTS AT TAU 60, WHICH IS
POSSIBLE WHEN TAKING INTO ACCOUNT THE ENVIRONMENT. RI AIDS ARE ALSO
NOW TRIGGERING (RI25 AND RI45). THE JTWC INTENSITY FORECAST IS
PLACED NEAR CONSENSUS AND WOULD LIKELY FORECAST A HIGHER PEAK, AT
TAU 60, IF ABLE.

FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: LOW
   TRACK 72-120 HR: LOW
   INTENSITY 0 - 72 HR: LOW
   INTENSITY 72-120 HR: LOW//
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