WDXS31 PGTW 110900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 17S (SEVENTEEN) 
WARNING NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 16.9S 119.8E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 215 NM NORTH-NORTHEAST OF PORT HEDLAND,
AUSTRALIA
   MOVEMENT PAST 6 HOURS: SOUTHWESTWARD AT 05 KTS
   SIGNIFICANT WAVE HEIGHT: 16 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
CYCLONE 17S WITH TIGHTLY-WRAPPING LOW-LEVEL CLOUD LINES FEEDING
INTO AN OBSCURED LOW-LEVEL CIRCULATION CENTER (LLCC). CONVECTIVE
STRUCTURE HAS MARKEDLY ORGANIZED OVER THE PAST 12 HOURS AND A
110154 ASCAT-C IMAGE REVEALS THAT THE WIND FIELD STRENGTHENED.
35-40 KTS IS PRESENT PRIMARILY WITHIN THE WESTERN PORTION OF THE
CIRCULATION. SURFACE OBSERVATIONS FROM ROWLEY SHOALS ARE
CONSISTENTLY REPORTING AROUND 35 KTS FROM THE SOUTH-SOUTHWEST. THE
INITIAL POSITION IS PLACED WITH MEDIUM CONFIDENCE BASED ON THE
ANIMATED MSI. THE INITIAL INTENSITY OF 40 KTS IS ASSESSED WITH HIGH
CONFIDENCE BASED ON THE MENTIONED OBSERVATIONS (38 KTS WHEN
CONVERTED TO 1-MIN AVERAGES) AND ASCAT-C IMAGE. ENVIRONMENTAL
ANALYSIS INDICATES THAT 17S IS IN A FAVORABLE ENVIRONMENT
CHARACTERIZED BY MODERATE POLEWARD OUTFLOW ALOFT, LOW (10-15 KTS)
VERTICAL WIND SHEAR, AND WARM (29-30 C) SEA SURFACE TEMPERATURES.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   KNES: T2.0 - 30 KTS
   APRF: T2.0 - 30 KTS

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

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: 17S IS FORECAST TO TRACK GENERALLY SOUTHWARD
ALONG THE WESTERN PERIPHERY OF THE RIDGE OVER AUSTRALIA THROUGH TAU
36. AFTER TAU 36, THE SYSTEM WILL ENTER A WEAK STEERING ENVIRONMENT
AS THE RIDGE WEAKENS. THERE IS VERY LOW CONFIDENCE IN THE TRACK
THAT 17S WILL TAKE DURING THIS TIME. TRACK SPEEDS WILL CERTAINLY
SLOW AND COULD POSSIBLY BE ERRATIC. NEAR TAU 72, RIDGING BUILDS
BACK UP OVER AUSTRALIA AND BEGINS TO DRIVE THE SYSTEM
SOUTHWESTWARD, THROUGH THE REMAINDER OF THE FORECAST PERIOD.
CURRENTLY, A LANDFALL EAST OF PORT HEDLAND AROUND TAU 72 IS
FORECAST, BUT THERE IS A POSSIBILITY OF A LARGE CHANGE IN LANDFALL
LOCATION. REGARDING INTENSITY, 17S IS FORECAST TO STRENGTHEN AS IT
APPROACHES THE COAST. SHEAR IS EXPECTED TO DROP AND SEA SURFACE
TEMPERATURES WILL BE VERY WARM. IF THE SYSTEM STALLS OFF THE COAST
FOR AN EXTENDED PERIOD, FURTHER INTENSIFICATION THAN CURRENTLY
FORECASTED COULD OCCUR. POLEWARD OUTFLOW IS ALSO EXPECTED TO
IMPROVE NEAR TAU 72, JUST BEFORE LANDFALL, FURTHER ASSISTING IN
INTENSIFICATION. AS IT STANDS, THE JTWC INTENSITY FORECAST CALLS
FOR STEADY INTENSIFICATION TO A PEAK OF 80 KTS AS THE SYSTEM MAKES
LANDFALL. AFTER LANDFALL, TERRAIN INTERACTION WILL ERODE THE VORTEX
WITH DISSIPATION BY TAU 120. 

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN POOR AGREEMENT,
ESPECIALLY CONCERNING THE GFS AND ECENS ENSEMBLE MEMBER SPREAD.
ECENS SPREADS OUT STARTING FROM THE INITIAL POSITION SIGNIFICANTLY,
FROM LEARMONTH TO BROOME. CONSENSUS MODEL SPREAD DOES NOT DEPICT AS
MUCH UNCERTAINTY WITH A 140 NM CROSS-TRACK SPREAD AT TAU 72, NEAR
THE TIME OF LANDFALL. AFTER TAU 72, MODELS DIVERGE DUE TO VARYING
WEAKENING TIMELINES OVER LAND. THE HTWC TRACK FORECAST IS PLACED
WITH LOW CONFIDENCE THROUGHOUT THE FORECAST PERIOD. INTENSITY
GUIDANCE IS ALSO IN POOR AGREEMENT WITH HAFS-A BEING QUITE A BIT
MORE AGGRESSIVE THAN THE REMAINDER OF GUIDANCE, SUGGESTING A PEAK OF
95 KTS AT TAU 72. GFS KEEPS THE SYSTEM WEAKER, PARTICULARLY AT THE
BEGINNING OF THE FORECAST PERIOD. THE JTWC INTENSITY FORECAST IS
PLACED MORE IN LINE WITH HAFS-A WITH OVERALL LOW CONFIDENCE DUE TO
THE UNCERTAINTY ASSOCIATED WITH THE TRACK.

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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