WDXS32 PGTW 180300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 18S (EIGHTEEN) WARNING
NR 028//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.4S 112.3E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 311 NM NORTH-NORTHWEST OF LEARMONTH,
AUSTRALIA
   MOVEMENT PAST 6 HOURS: WESTWARD AT 04 KTS
   SIGNIFICANT WAVE HEIGHT: 18 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS COLLAPSING
DIURNAL CONVECTION PARTIALLY OBSCURING A LOW-LEVEL CIRCULATION
CENTER (LLCC) AND WARMING CLOUD TOPS THROUGH WHICH GLIMPSES OF THE
LLCC ARE REVEALED. THE SYSTEM IS CURRENTLY BEING WEAKLY STEERED
WESTWARD ON THE NORTHERN PERIPHERY OF A SUBTROPICAL RIDGE (STR)
BUILDING TO THE SOUTHEAST. SEA SURFACE TEMPERATURES ARE FAVORABLE;
HOWEVER, OCEAN HEAT CONTENT HAS DRAINED OVER THE PAST FEW DAYS AND
WEAKLY SUPPORTS THE SYSTEM. MODERATE VERTICAL WIND SHEAR (VWS) OF
20-25KTS SHEARS THE VORTEX, PREVENTING VERTICAL ALIGNMENT OF THE
VORTEX. WINDS ALOFT ARE CONVERGENT ABOVE THE CYCLONE, COMPLETELY
ELIMINATING OUTFLOW SUPPORTING INTENSIFICATION. THE INITIAL
POSITION WAS ASSESSED WITH MEDIUM CONFIDENCE BASED ON ANIMATED MSI
SHOWING A PARTIALLY EXPOSED LLCC. THE INITIAL INTENSITY WAS
ASSESSED BASED ON PERSISTENCE AND THE AGENCY AND OBJECTIVE FIXES
LISTED BELOW.

INITIAL WIND RADII BASIS: PERSISTENCE

CURRENT STEERING MECHANISM: THE NORTHERN PERIPHERY OF A STR TO THE
EAST.

AGENCY DVORAK AND AUTOMATED FIXES:
   PGTW: T2.5 - 35 KTS
   KNES: T2.5 - 35 KTS
   DEMS: T2.5 - 35 KTS
   APRF: T2.0 - 30 KTS
   CIMSS ADT: 45 KTS AT 172330Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY
UNFAVORABLE
   VWS: 20-25 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 TO WEAKLY DRIFT
TO THE WEST-SOUTHWEST OVER THE NEXT 12 HOURS AS THE STR TO THE
SOUTHEAST BUILDS, CURVING THE CYCLONE TO A WEST-NORTHWEST TRACK
THROUGH TAU 36. BY TAU 48, THE SYSTEM IS EXPECTED TO BE DRIVEN DUE
WEST THROUGH TAU 96, PICKING UP TRACK SPEED TO NINE KTS BY TAU 96.
BETWEEN TAU 96 AND 120, TC 18S IS EXPECTED TO SLIGHTLY CURVE TO THE
WEST-SOUTHWEST ONCE MORE WHILE ACCELERATING TO A SPEED OF 12KTS.
THE INTENSITY IS FORECAST TO REMAIN UNCHANGED THROUGH THE FIRST 48
HOURS. SPECIFICALLY IN THE UPPER LEVELS, CONVERGENCE ALOFT WILL CAP
THE VORTEX AND PREVENT AN EXHAUST MECHANISM THAT COULD LEAD TO
INTENSIFICATION, DESPITE RELATIVELY LOW VWS AND FAVORABLE SEA
SURFACE TEMPERATURES. THE UPPER-LEVEL FLOW IS EXPECTED TO IMPROVE
BY TAU 60, ALLOWING FOR SLOW INTENSIFICATION TO 50KTS BY THE END OF
THE FORECAST PERIOD.

MODEL DISCUSSION: DETERMINISTIC MODEL GUIDANCE IS IN GOOD AGREEMENT
THAT TC 18S WILL CRAWL, WALK, AND THEN RUN IN A WESTWARD DIRECTION.
THE MAXIMUM CROSS-TRACK ERROR IS 150NM BY TAU 120, WITH ECM2 TO THE
NORTH AND GFS TO THE SOUTH. THE INTENSITY GUIDANCE HAS DRASTICALLY
INCREASED ONCE AGAIN, SEEMINGLY IN RESPONSE TO THE DIURNAL
CONVECTION THAT WAS NOTED EARLIER. THE INCONSISTENT GUIDANCE THAT
HAS DRASTICALLY CHANGED FROM ONE MODEL RUN TO THE NEXT OVER THE
LAST 36 HOURS IS SUSPECTED TO BY HIGHLY SENSITIVE TO THE DIURNAL
MAXIMUMS THAT WERE REACHED BETWEEN THE 12Z AND 18Z MODEL RUNS,
WHICH CAN GIVE THE FALSE IMPRESSION THAT THE SYSTEM IS BECOMING
WELL-FORMED. LEADING THE CHARGE ON INTENSIFICATION IS HAFS-A, WHICH
PREDICTS A PEAK OF 115KTS AT TAU 108, AND CLOSE BEHIND IS HWRF. GFS
AND SHIPS GUIDANCE HAVE BOTH INCREASED TO A PEAK OF 80KTS AT
APPROXIMATELY TAU 84. THE JTWC FORECAST DISCOUNTS THE UNTRUSTWORTHY
MODEL GUIDANCE AND REMAINS CONSISTENT WITH PREVIOUS FORECASTS.

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