WDXS32 PGTW 160300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 18S (EIGHTEEN) WARNING 
NR 020//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.5S 114.5E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 289 NM NORTH OF LEARMONTH, AUSTRALIA
   MOVEMENT PAST 6 HOURS: NORTHWESTWARD AT 04 KTS
   SIGNIFICANT WAVE HEIGHT: 16 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A PARTIALLY
EXPOSED LOW LEVEL CIRCULATION CENTER OBSCURED BY FLARING CONVECTION
OFFSET TO THE WEST. LOW-LEVEL CLOUD BANDING IS ALSO VISIBLE
WRAPPING INTO THE CIRCULATION CENTER. THE WESTWARD VERTICAL TILT
WITH HEIGHT CAN BE ATTRIBUTED TO A TIGHT GRADIENT OF ELEVATED
VERTICAL WIND SHEAR (VWS) OF 20-25KTS, COUPLED WITH DRY AIR
ENTRAINMENT INTO THE SYSTEM FROM THE NORTHWEST CREATING AN
UNFAVORABLE ENVIRONMENT FOR INTENSIFICATION. MODERATE
WEST-SOUTHWESTWARD OUTFLOW AND FAVORABLE SEA SURFACE TEMPERATURES
SUPPORT THE STAGNANT NATURE OF THE SYSTEM. CURRENTLY TROPICAL
CYCLONE (TC) 18S IS DRIFTING NORTHWESTWARD IN A COMPETING STEERING
ENVIRONMENT IN THE LOWER-LEVELS WHILE RIDING THE SOUTHERN PERIPHERY
OF A NEAR EQUATORIAL RIDGE (NER) TO THE NORTH AND THE NORTHWESTERN
PERIPHERY OF A SUBTROPICAL RIDGE (STR) TO THE SOUTHEAST. THE
INITIAL POSITION WAS ASSESSED WITH HIGH CONFIDENCE BASED ON
ANIMATED MSI AND A 162205Z 37GHZ SSMIS MICROWAVE IMAGE REVEALING A
LLCC. THE INITIAL INTENSITY WAS ASSESSED WITH LOW CONFIDENCE BASED
ON PERSISTENCE AND AGENCY AND OBJECTIVE FIXES LISTED BELOW.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: COMPETING STEERING ENVIRONMENT.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   KNES: T2.5 - 35 KTS
   APRF: T1.0 - 25 KTS
   CIMSS ADT: 35 KTS AT 152330Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 20-25 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: MODERATE POLEWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   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 TO DRIFT
WESTWARD FOR THE NEXT 36 HOURS BEFORE SLOWLY CURVING TO THE
WEST-SOUTHWEST FROM TAU 36-48 AS THE RIDGE TO THE SOUTHEAST OVER
AUSTRALIA WEAKENS. BY HOUR 72, THE SYSTEM IS DRIVEN WESTWARD BY A
SUBTROPICAL RIDGE THAT BUILDS BETWEEN TAU 60-72 THAT WILL CARRY TC
18S THROUGH TO THE END OF THE FORECAST PERIOD. GIVEN THE WEAK
NATURE OF THE STEERING ENVIRONMENT UNTIL TAU 48, THE POTENTIAL
CANNOT BE RULED OUT THAT THE SYSTEM MAY DRIFT IN ANY DIRECTION
BEFORE BEING DRIVEN WESTWARD. THE INTENSITY FORECAST HAS CHANGED
SLIGHTLY OVER THE LAST SIX HOURS, REMAINING STAGNANT AT 35KTS FOR
THE FIRST 24 HOURS BEFORE SLIGHTLY INTENSIFYING FROM TAU 24-48 AS
THE VWS MODERATES AND THE UPPER-LEVEL ENVIRONMENT FAILS TO SUPPORT
SIGNIFICANT OUTFLOW. UNTIL TAU 96, THE SYSTEM IS EXPECTED TO
STRUGGLE TO INTENSIFY AGAINST THE MODERATE VWS UNTIL POTENTIALLY
TAPPING INTO A JET MAXIMUM TO THE SOUTH THAT MAY CREATE A STRONG
POLEWARD CHANNEL SUPPORTING MODEST INTENSIFICATION. 

MODEL DISCUSSION: DETERMINISTIC MODEL GUIDANCE IS IN POOR AGREEMENT
AS TO THE INITIAL DIRECTION AS HOW TC 18S WILL DRIFT THROUGH THE
FIRST 48-72 HOURS. GFS AND AEMI INITIALLY DRIFT EAST FOR THE FIRST
24 HOURS AND THEN SOUTHEAST THROUGH TAU 60 BEFORE BEING PICKED UP
BY THE BUILDING STR TO THE SOUTHEAST AND TRAVELING WEST. THE ECMWF
AND ENSEMBLE GUIDANCE INDICATE A SLOW DRIFT WESTWARD BEFORE DIVING
SOUTH THROUGH TAU 48 AND WEST AT TAU 72 WHEN PICKED UP BY A
SOUTHERN STR. JTWC INTENSITY GUIDANCE ALSO IS IN POOR AGREEMENT
WITH A OVERALL 150KT SPREAD BETWEEN HAFS-A WHICH HAS A PEAK OF
175KTS AND COAMPS-TC WHICH ANTICIPATES A WEAKENING TO 25KTS BY TAU
120. THE JTWC INTENSITY FORECAST IS ASSESSED WITH LOW CONFIDENCE
ACKNOWLEDGING THE POTENTIAL FOR NEAR STAGNATION THROUGH THE FIRST
48 HOURS AT 35KTS AND A SLOW INCREASE TO 45KTS BY TAU 96. BOTH
ECMWF AND GFS AGREE THAT THE ENVIRONMENT FOR UPPER-LEVEL DIVERGENCE
AND OUTFLOW WILL BECOME MORE FAVORABLE BEYOND TAU 96. 

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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