WDXS31 PGTW 121500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 17S (ZELIA) WARNING NR 
006//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 19.1S 118.6E
   INITIAL INTENSITY: 70 KTS
   GEOGRAPHIC REFERENCE: 72 NM NORTH OF PORT HEDLAND, AUSTRALIA
   MOVEMENT PAST 6 HOURS: SOUTHWESTWARD AT 01 KTS
   SIGNIFICANT WAVE HEIGHT: 24 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
CYCLONE 17S (ZELIA) WITH A COMPACT, SYMMETRICAL CENTRAL DENSE
OVERCAST (CDO) AND CIRRUS FILAMENTS EXPANDING OUTWARD. 17S IS
ANALYZED TO HAVE RAPIDLY INTENSIFIED FROM 40 KTS 24 HOURS AGO, TO
70 KTS. ENVIRONMENTAL ANALYSIS INDICATES THAT 17S IS IN A HIGHLY
FAVORABLE ENVIRONMENT CHARACTERIZED BY STRONG POLEWARD AND
EQUATORWARD OUTFLOW ALOFT, LOW (10-15 KTS) VERTICAL WIND SHEAR, AND
VERY WARM (30-31 C) SEA SURFACE TEMPERATURES. THE INITIAL POSITION
IS PLACED WITH HIGH CONFIDENCE BASED ON ANIMATED RADAR IMAGERY FROM
PORT HEDLAND REVEALING THE WELL-DEFINED CIRCULATION. ADDITIONALLY,
A 121048Z SSMIS 91 GHZ MICROWAVE IMAGE DEPICTS A COMPACT CORE WITH
A DEVELOPED MICROWAVE EYE FEATURE. THE INITIAL INTENSITY OF 70 KTS
IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE AGENCY DVORAK AND
OBJECTIVE INTENSITY ESTIMATES LISTED BELOW.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: A COMPETING STEERING ENVIRONMENT
BETWEEN TWO SUBTROPICAL RIDGES (STR) TO THE EAST AND WEST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T4.0 - 65 KTS
   KNES: T4.0 - 65 KTS
   APRF: T3.5 - 55 KTS
   CIMSS ADT: 72 KTS AT 121130Z
   CIMSS AIDT: 64 KTS AT 121130Z
   CIMSS DPRINT: 65 KTS AT 121130Z
   CIMSS DMINT: 69 KTS AT 121050Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: HIGHLY FAVORABLE
   VWS: 10-15 KTS
   SST: 30-31 CELSIUS
   OUTFLOW: STRONG POLEWARD AND EQUATORWARD

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 17S IS FORECAST TO TRACK SLOWLY
WEST-SOUTHWESTWARD OVER THROUGH TAU 36 AS RIDGING FROM THE WEST
EXTENDS OVER TO THE SOUTH OF THE VORTEX. HOWEVER, THE STEERING
PATTERN WILL REMAIN WEAK, SO THERE IS UNCERTAINTY REGARDING THE
TRACK THROUGH THIS TIME. NEAR TAU 36, THE RIDGE TO THE WEST BEGINS
TO MOVE WESTWARD AND RIDGING OVER WESTERN AUSTRALIA BUILDS BACK IN.
THIS WILL CAUSE THE SYSTEM TO TRACK SOUTHWARD THROUGH THE REMAINDER
OF THE FORECAST PERIOD. LANDFALL IS CURRENTLY FORECAST TO OCCUR
JUST AFTER TAU 48, BETWEEN DAMPIER AND PORT HEDLAND. REGARDING
INTENSITY, 17S IS FORECAST TO CONTINUE RAPIDLY INTENSIFYING TO 105
KTS AT TAU 24 IN RESPONSE TO THE CONTINUED HIGHLY CONDUCIVE
ENVIRONMENT. A PEAK INTENSITY OF 120 KTS IS FORECAST AT TAU 48,
JUST BEFORE THE SYSTEM MAKES LANDFALL. AFTER LANDFALL, TERRAIN
INTERACTION WILL QUICKLY ERODE THE VORTEX WITH DISSIPATION FORECAST
TO OCCUR NO LATER THAN TAU 96.

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE CONTINUES TO BE IN POOR
AGREEMENT. THE ECMWF ENSEMBLE CONTINUES TO HAVE A WIDE SPREAD OF
POSSIBLE TRACKS, STARTING FROM THE INITIAL POSITION. IT NOW HAS
MORE MEMBERS EXTENDING OUT WESTWARD, TOWARD LEARMONTH AS WELL.
HOWEVER, THE ECMWF ENSEMBLE MEAN CONTINUES TO STAY FAR EAST (200
NM) OF LEARMONTH THOUGH. THE GFS ENSEMBLE, ON THE OTHER HAND, HAS
TIGHTENED UP A BIT, DEPICTING LESS MEMBERS EXTENDING FAR WESTWARD.
NEAR THE TIME OF LANDFALL, AT TAU 48, THERE IS A 160 NM CROSS-TRACK
SPREAD BETWEEN CONSENSUS MEMBERS, EXTENDING FROM PORT HEDLAND
(ECMWF DETERMINISTIC) TO DAMPIER (GFS DETERMINISTIC). THE JTWC
TRACK FORECAST IS PLACED SLIGHTLY TO THE WEST OF CONSENSUS WITH LOW
CONFIDENCE. INTENSITY GUIDANCE IS IN SLIGHTLY BETTER AGREEMENT WITH
PEAK INTENSITIES RANGING FROM 100 KTS (GFS) TO 125 KTS HAFS-A.
NEARLY ALL RI AIDS ARE STILL TRIGGERING AS WELL. THE JTWC INTENSITY
FORECAST IS PLACED SLIGHTLY HIGHER THAN CONSENSUS, CALLING FOR A
PEAK OF 120 KTS. THE INTENSITY FORECAST IS ALSO PLACED WITH LOW
CONFIDENCE DUE TO THE POSSIBILITY FOR HIGH TRACK VARIANCE. THE
AMOUNT OF TIME OVER WATER WILL HAVE A LARGE INFLUENCE ON THE
INTENSITY OF THE SYSTEM.

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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