WDXS31 PGTW 130300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 17S (ZELIA) WARNING NR 
008//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 19.1S 118.2E
   INITIAL INTENSITY: 120 KTS
   GEOGRAPHIC REFERENCE: 75 NM NORTH-NORTHWEST OF PORT HEDLAND,
AUSTRALIA
   MOVEMENT PAST 6 HOURS: NORTHWESTWARD AT 01 KTS
   SIGNIFICANT WAVE HEIGHT: 31 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
CYCLONE (TC) 17S (ZELIA), AS IT IS CURRENTLY UNDERGOING RAPID
INTENSIFICATION. ADDITIONALLY, THE SYSTEM CAN BE CHARACTERIZED BY
STRONG POLEWARD AND EQUATORWARD OUTFLOW, A 300NM DIAMETER WIDE
CENTRAL DENSE OVERCAST (CDO), AS WELL AS A WELL-DEFINED AND
SYMMETRICAL 10NM EYE. ENVIRONMENTAL CONDITIONS REMAIN HIGHLY FAVORABLE 
FOR CONTINUOUS DEVELOPMENT, WITH STRONG OUTFLOW, LOW (5-10KTS) 
VERTICAL WIND SHEAR (VWS) AND VERY WARM (30-31C) SEA SURFACE 
TEMPERATURES (SST). THE INITIAL POSITION IS PLACED WITH HIGH 
CONFIDENCE BASED ON THE EASILY IDENTIFYING EYE FEATURE. THE INITIAL 
INTENSITY OF 120 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE 
AGENCY DVORAK AND OBJECTIVE INTENSITY ESTIMATES.

INITIAL WIND RADII BASIS: PERSISTENCE 

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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T6.0 - 115 KTS
   KNES: T7.0 - 140 KTS 
   APRF: T6.0 - 115 KTS
   CIMSS SATCON: 118 KTS AT 130000Z
   CIMSS ADT: 109 KTS AT 130000Z
   CIMSS AIDT: 114 KTS AT 130000Z
   CIMSS D-PRINT: 123 KTS AT 130000Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: HIGHLY FAVORABLE
   VWS: 5-10 KTS
   SST: 30-31 CELSIUS
   OUTFLOW: STRONG POLEWARD AND EQUATORWARD

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: STRONGER THAN PREVIOUSLY ASSESSED
RAPID INTENSIFICATION

FORECAST DISCUSSION: TC ZELIA IS CURRENTLY QUASI-STATIONARY, AS THE
STEERING MECHANISM IS STILL DRIVEN BY COMPETING SUBTROPICAL RIDGES
(STR) TO THE WEST AND EAST. AS THE STR TO THE WEST MOVES WESTWARD,
AWAY FROM TC 17S, THE OTHER STR WILL STEER THE SYSTEM ON A
SOUTHWARD TRACK, TOWARDS LANDFALL. CONSIDERING THE ONGOING RAPID
INTENSIFICATION, AS WELL AS THE POTENTIAL FOR EYEWALL REPLACEMENT
CYCLES, THE MAXIMUM INTENSITY TIMELINE IS HIGHLY UNCERTAIN, BUT THE
SYSTEM IS CURRENTLY FORECAST TO PEAK AT 155KTS AROUND TAU 24.
HOWEVER, IF TC ZELIA SPENDS MORE TIME THAN FORECASTED OVER WATER, 
THERE IS POTENTIAL FOR FURTHER INTENSIFICATION. ON THE CONTRARY, IF 
THE SYSTEM BECOMES COMPLETELY STATIONARY FOR LONGER PERIOD OF TIME, 
THE DYNAMICALLY INDUCED COOLING OF THE WATER MAY LEAD TO SLIGHT 
WEAKENING. TC ZELIA IS FORECAST TO MAKE LANDFALL AROUND TAU 36 AND 
CONTINUE TRACKING SOUTHWARD AFTERWARDS. AS THE SYSTEM PASSES TO THE 
EAST OF THE HAMERSLEY MOUNTAIN RANGE, IT WILL RAPIDLY WEAKEN AND IS 
FORECAST TO DISSIPATE BY TAU 96.

MODEL DISCUSSION: DETERMINISTIC AND ENSEMBLE NUMERICAL MODEL
GUIDANCE REMAINS IN POOR AGREEMENT IN REGARDS TO BOTH TRACK AND
INTENSITY. SHORT-TERM FORECAST IS PARTICULARLY UNCERTAIN, DUE TO A
NEARLY 180 DEGREE SPREAD BETWEEN THE MODELS FOR TAU 12. GFS
ENSEMBLE AND DETERMINISTIC TRACKERS ARE THE MOST WEST-NORTHWESTWARD, 
WHILE NAVGEM AND GALWEM FORECAST THE SYSTEM TO TRACK SOUTHEASTWARD. 
MULTI-MODEL CONSENSUS GUIDANCE HOWEVER IS PREDICTING A CONTINUATION OF 
THE PREVIOUS FORECAST, WITH WEST-SOUTHWESTWARD TRACK BEGINNING AS SOON 
AS THE SYSTEM STARTS MOVING, FOLLOWED BY A MORE SOUTHWARD TURN, AND 
LANDFALL WEST OF PORT HEDLAND. PAST THE LANDFALL, ALL TRACKERS ARE 
PREDICTING A SOUTHWARD MOVEMENT, HOWEVER THE INITIAL LANDFALL LOCATION
UNCERTAINTY LEADS TO A 250NM CROSS-TRACK SPREAD AT TAU 96. JTWC
FORECAST IS THEREFORE PLACED WITH LOW CONFIDENCE THROUGHOUT THE
FORECAST, CLOSE TO THE MULTI-MODEL CONSENSUS. IT IS ASSESSED SLIGHTLY 
TO THE WEST, AS A RESULT OF TRACKING INDIVIDUAL ENSEMBLE MEMBERS, 
WHICH MOST CLOSELY REFLECT THE CURRENT POSITION OF THE SYSTEM. IN 
TERMS OF INTENSITY, JTWC INTENSITY FORECAST IS ALSO ASSESSED WITH LOW
CONFIDENCE THROUGHOUT THE FORECAST PERIOD. RAPID INTENSIFICATION RATE, 
AS WELL AS LENGTH OF IT ARE STRONGLY INFLUENCED BY THE TRACK GUIDANCE 
UNCERTAINTY. FOR THE INITIAL 24 HOURS, JTWC FORECAST IS ASSESSED ABOVE 
MULTI-MODEL CONSENSUS, AND CLOSE TO THE DTOP AND HAFS GUIDANCE, WHICH 
ARE PEAKING AT 155-160KTS. MEANWHILE THE DETERMINISTIC GFS GUIDANCE IS
THE LOW-END OUTLIER, ASSESSING THE SYSTEM TO ALREADY BE AT ITS PEAK
INTENSITY. AFTER TAU 24, THE GUIDANCE AGREEMENT IMPROVES, AS ALL
TRACKERS REFLECT THE IMPACTS OF LAND INTERACTION. HOWEVER THE
TIMELINE OF THE DISSIPATION REMAINS IN QUESTION, WITH SOME MODELS
DISSIPATING THE SYSTEM AS FAST AS TAU 60 (COAMPS-TC) AND HAFS
KEEPING THE 50-55 KTS MAXIMUM WIND SPEEDS THROUGHOUT THE FORECAST
PERIOD.

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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