WDXS32 PGTW 152100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 18S (EIGHTEEN) WARNING 
NR 019//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.8S 114.8E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 273 NM NORTH OF LEARMONTH, AUSTRALIA
   MOVEMENT PAST 6 HOURS: SOUTH-SOUTHWESTWARD AT 02 KTS
   SIGNIFICANT WAVE HEIGHT: 16 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A
PARTIALLY OBSCURED LOW LEVEL CIRCULATION CENTER (LLCC) WITH FLARING
DEEP CONVECTION DISPLACED WEST OF THE CENTER DUE TO ELEVATED
EASTERLY VERTICAL WIND SHEAR (VWS) BETWEEN 25-30KTS. MODERATE
POLEWARD OUTFLOW IS EVIDENCED BY EXPANDING CIRRUS FILAMENTS
EXTENDING WEST SOUTHWEST OF THE CENTER. ON THE LOWER LEVELS,
BANDING CLOUDS WRAP INTO THE LLCC INDICATING A MOIST ENVIRONMENT
SUPPORTED BY WARM SEA SURFACE TEMPERATURES OF 28-29C. CURRENTLY,
TROPICAL CYCLONE (TC) 18S IS IN A COMPETING STEERING ENVIRONMENT ON
THE LOWER LEVELS WITH A LOW- TO MID-LEVEL STR OVER SOUTHWESTERN
AUSTRALIA AND THE SOUTHERN PERIPHERY OF A NEAR EQUATORIAL RIDGE
(NER) TO THE NORTH. THE INITIAL POSITION WAS ASSESSED USING
ANIMATED EIR IMAGERY IDENTIFYING THE EXPOSED LLCC. HOWEVER, THE
POTENTIAL FOR A POSSIBLE SECONDARY CIRCULATION CENTER CANNOT BE
RULED OUT UNDERNEATH THE DEEP LAYER CONVECTION THAT MAY NOT BE
IDENTIFIED ON THE LOWER LEVELS DUE TO THE USE OF EIR. THE INITIAL
INTENSITY WAS ASSESSED USING PERSISTENCE AND THE AGENCY AND
OBJECTIVE FIXES LISTED BELOW.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: IN A COMPETING STEERING ENVIRONMENT ON
THE NORTHWESTERN PERIPHERY RIDGE TO THE SOUTHEAST AND THE SOUTHERN
PERIPHERY OF AN NER TO THE NORTH. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   KNES: T2.5 - 35 KTS
   APRF: T1.5 - 25 KTS
   CIMSS ADT: 33 KTS AT 151730Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 25-30 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE POLEWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: HIGH
   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 18S IS FORECAST TO REMAIN QUASI-STATIONARY
OVER THE NEXT 36 HOURS AS IT REMAINS TRAPPED IN A WEAK STEERING
ENVIRONMENT. AS A RIDGE OVER SOUTHWESTERN AUSTRALIA TRAVELS WEST, A
REGION OF WEAKNESS WILL REMAIN OVER THE WESTERN EXPANSE OF
AUSTRALIA THROUGH THE 36 HOUR POINT, AND TC 18S IS FORECAST TO
DRIFT GENERALLY SOUTHWARD BETWEEN TAU 24 AND 48. BY TAU 48, A
STRONG LOW- TO MID-LEVEL STR IS EXPECTED TO DEVELOP AND BUILD OVER
SOUTHWESTERN AUSTRALIA, DRIVING TC 18S ONTO A WESTWARD TRACK FROM
TAU 72 THROUGH THE END OF THE FORECAST PERIOD. THERE IS SIGNIFICANT
VARIATION IN HOW THE SYSTEM MAY MOVE WITHIN THE WEAK STEERING
ENVIRONMENT IN THE FIRST 48 HOURS, MEANING THAT A SLIGHT EASTWARD,
SOUTHWARD, OR WESTWARD TRACK ARE ALL POSSIBLITIES AT A SLOW TRACK
SPEED BEFORE TAKING OFF TO THE WEST AT TAU 72. THE INTENSITY FOR
THE TC18S IS FORECAST TO REMAIN STEADY FOR THE 48 HOURS DUE TO
ELEVATED VWS OF 25-30KTS AND MINIMAL UPPER-LEVEL DIVERGENCE TO
SUPPORT OUTFLOW. ONCE THE SYSTEM BEGINS TO TAP INTO THE JET MAXIMUM
TO THE SOUTH, TC 18S MAY BEGIN TO DEVELOP A OUTFLOW CHANNEL BETWEEN
TAU 48-60, AT WHICH POINT THE CYCLONE IS ANTICIPATED TO GRADUALLY
INTENSIFY TO A MAXIMUM OF 45KTS.

MODEL DISCUSSION: DETERMINISTIC MODEL GUIDANCE IS IN SIGNIFICANT
DISAGREEMENT AS TO THE INITIAL TRACK THAT TC 18S WILL INITIALLY
TAKE DUE TO THE HIGHLY WEAK STEERING ENVIRONMENT THAT PERSISTS. GFS
INITIALLY TAKES THE SYSTEM ON A NORTHEAST AND THEN SOUTHEAST TRACK
FOR THE FIRST 72 HOURS BEFORE SHARPLY TURNING NORTHEAST AGAIN AND
SUDDENLY SOUTHWEST BETWEEN TAUS 72-96 AND CONTINUING THROUGH THE
END OF THE FORECAST PERIOD. ECMWF, ON THE OTHER HAND, TAKES THE
SYSTEM TO THE WEST-NORTHWEST FOR THE FIRST 24 HOURS AND THEN
SHARPLY SOUTH-SOUTHWEST THROUGH TAU 72 BEFORE CONTINUING AGAIN TO
THE WEST-NORTHWEST THROUGH THE END OF THE FORECAST PERIOD. THE
SHARP VARIATIONS IN TRACK INDICATED A POSSIBLE ELONGATED
QUASI-STATIONARY PERIOD BEFORE PICKING UP AND TRACKING WESTWARD.
THE JTWC INTENSITY CONSENSUS GUIDANCE IS SPLIT BETWEEN BOTH GFS AND
NAVGEM BASED SHIPS GUIDANCE, HWRF, AND HAFS-A ANTICIPATING
SIGNIFICANT INTENSIFICATION THROUGHOUT THE FORECAST PERIOD AND
COAMPS-TC AND GFS INDICATING STAGNATION, EVEN POSSIBLE WEAKENING BE
THE END OF THE FORECAST. THE INTENSITY FORECAST REFLECTS THE MOST
LIKELY STAGNANT RATE OF INTENSIFICATION OVER THE NEXT 36-60 HOURS,
BUT ALSO THE POTENTIAL FOR INTENSIFICATION AS OUTFLOW IMPROVES AT
THE 48-60 HOUR MARK. 

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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