WDXS32 PGTW 172100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 18S (EIGHTEEN) WARNING 
NR 027//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.4S 112.7E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 304 NM NORTH-NORTHWEST OF LEARMONTH,
AUSTRALIA
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 02 KTS
   SIGNIFICANT WAVE HEIGHT: 24 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
CYCLONE (TC) 18S WITH FLARING CONVECTION PARTIALLY OBSCURING THE
BROAD LOW-LEVEL CIRCULATION CENTER (LLCC) RIGHT AS THE SYSTEM
REACHES DIURNAL MAXIMUM. MODERATE EASTERLY VERTICAL WIND SHEAR
(VWS) OF 20-25KTS HAS DECOUPLED THE VORTEX AND HAS PREVENTED THE
CYCLONE FROM INTENSIFYING AND HAS CAUSED A WESTWARD VERTICAL TILT
WITH HEIGHT. THE SYSTEM REMAINS QUASI-STATIONARY (QS) AS THE A
SUBTROPICAL RIDGE BUILDS TO THE SOUTHEAST, CAUSING TC 18S TO DRIFT
IN A WESTWARD DIRECTION OVER THE LAST SIX HOURS. THE SYSTEM REMAINS
IN RELATIVELY COOL BUT FAVORABLE SEA SURFACE TEMPERATURES (SST)
RESULTING FROM OCEANIC UPWELLING OVER THE LAST FEW DAYS. THE
INITIAL POSITION WAS ASSESSED WITH MEDIUM CONFIDENCE BASED ON
ANIMATED EIR DEPICTING A PARTIALLY EXPOSED LLCC THAT BECOMES
OBSCURED AS CONVECTION FLARES BUT IS UNLIKELY TO BE REPRESENTATIVE
OF AN IMPROVED ENVIRONMENT DUE TO DIURNAL HEATING. THE INITIAL
INTENSITY WAS ASSESSED WITH LOW CONFIDENCE BASED ON AGENCY AND
OBJECTIVE FIXES BELOW. HOWEVER, A 171416Z METOP-B ASCAT UHR WIND
SPEED PRODUCT DEPICTS AN ELONGATED CIRCULATION WITH A MAXIMUM OF
25KT WINDS 50NM TO THE NORTHEAST OF THE SYSTEM.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: COMPETING STEERING ENVIRONMENT.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   KNES: T2.0 - 30 KTS
   DEMS: T2.5 - 35 KTS
   APRF: T2.0 - 30 KTS
   CIMSS SATCON: 39 KTS AT 1900Z
   CIMSS ADT: 41 KTS AT 171900Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY
UNFAVORABLE
   VWS: 5-10 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: WEAK RADIAL

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   INITIAL INTENSITY: LOW
   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 CONTINUE DRIFTING IN A
WEST-SOUTHWESTWARD DIRECTION FOR THE NEXT 24 HOURS BEFORE COMING
UNDER THE INFLUENCE OF A STR TO THE SOUTHEAST WHICH WILL DRIVE THE
SYSTEM WEST THROUGH THE END OF THE FORECAST PERIOD. THE WEAK NATURE
OF THE STEERING ENVIRONMENT MAY RESULT IN THE SYSTEM TRANSITING
FARTHER WEST OR SOUTH THAN REPRESENTED ON THE FORECAST IN THE
NEAR-TERM. THE INTENSITY IS FORECAST TO REMAIN STAGNANT OVER THE
NEXT 48 HOURS AS OCEAN HEAT CONTENT (OHC) REMAINS LOW AND THE
UPPER-LEVEL FLOW IS NEARLY CONVERGENT. AS THE SYSTEM MOVES FARTHER
OUT OF THE AREA, THERE IS A LIKELIHOOD THAT CONDITIONS WILL IMPROVE
FOR INTENSIFICATION TO 50KTS BY TAU 120 AS TC 18S TRACKS OVER
REGIONS OF HIGHER OHC AND VWS MAY DECREASE. 

MODEL DISCUSSION: DETERMINISTIC MODEL GUIDANCE IS IN GOOD AGREEMENT
THAT TC 18S WILL REMAIN QS FOR THE NEXT 18-24 HOURS BEFORE TRACKING
WEST ALONG THE NORTHERN SIDE OF A STR TO THE SOUTHEAST. HOWEVER,
THE JTWC INTENSITY CONSENSUS IS NOT IN GOOD AGREEMENT WITH A
MAXIMUM DIFFERENCE OF 85KTS. HAFS-A AND HWRF NOW PREDICT A PEAK OF
110KTS BETWEEN TAU 60-108, HOWEVER COAMPS-TC PREDICTS A BRIEF
INCREASE TO 50KTS BY TAU 24 BEFORE CONSISTENTLY DECREASING TO 25KTS
BY THE END OF THE FORECAST PERIOD. IT IS WORTH NOTING THAT HWRF,
HAFS-A, SHIPS, AND GFS HAS BEEN FLAKY IN CONSISTENTLY FORECASTING
SIGNIFICANT INTENSIFICATION THAT HAS NOT OCCURRED AND THEN
FLIP-FLOPPING, WITH DRASTIC CHANGES OVER SHORT PERIODS OF TIME.
GIVEN THE EXTREME RUN-TO-RUN INCONGRUENCE IN THE MODEL DATA, THE
INTENSITY FORECAST IS MADE WITH LOW CONFIDENCE.

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