WDPS31 PGTW 041500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 18P (ALFRED)     
WARNING NR 023//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 27.8S 158.5E
   INITIAL INTENSITY: 55 KTS
   GEOGRAPHIC REFERENCE: 292 NM EAST OF BRISBANE, AUSTRALIA
   MOVEMENT PAST 6 HOURS: WESTWARD AT 06 KTS
   SIGNIFICANT WAVE HEIGHT: 30 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL CYCLONE (TC) 18P IS LOCATED UNDER A SUBTROPICAL JET AND
WITHIN A MARGINAL ENVIRONMENT CHARACTERIZED BY STRONG (45-55 KNOTS)
VERTICAL WIND SHEAR (VWS), MARGINAL SST VALUES OF 25-26C, AND
EXTENSIVE DRY AIR THROUGH THE MID- AND UPPER-LEVELS OF THE SYSTEM.
THE SYSTEM HAS HYBRID CHARACTERISTICS, WITH MODERATE BAROCLINICITY,
AND A HIGHLY ASYMMETRIC WIND FIELD. A 041111Z ASCAT-B IMAGE REVEALS
EXTENSIVE GALE-FORCE WINDS OVER THE SOUTHERN SEMICIRCLE, WITH A
SMALL PATCH OF 50 KNOT WINDS. THE ASCAT-B IMAGE SUPPORTS THE
INITIAL POSITION WITH HIGH CONFIDENCE AND SUPPORTS THE 34-KNOT AND
50-KNOT WIND RADII ANALYSIS WITH MEDIUM CONFIDENCE. DUE TO THE
STRONG VWS AND PRESENCE OF SIGNIFICANT DRY AIR, ANIMATED ENHANCED
INFRARED (EIR) SATELLITE IMAGERY DEPICTS AN EXPOSED, WELL-DEFINED
LOW-LEVEL CIRCULATION CENTER (LLCC), WITH FLARING DISORGANIZED DEEP
CONVECTION OVER THE SOUTHERN QUADRANT. A FORTUITOUS ALTIMETER PASS
AT 040705Z INDICATES SIGNIFICANT WAVE HEIGHTS OF 26-27 FEET JUST TO
THE WEST OF THE CENTER AT 040600Z. THE INITIAL INTENSITY OF 55 KTS
IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE ASCAT-B DATA.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: SUBTROPICAL RIDGE (STR) CENTERED OVER
WESTERN AUSTRALIA WITH A NARROW RIDGE EXTENDING SOUTH OF THE
SYSTEM.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   ABRF: T2.0 - 30 KTS
   CIMSS SATCON: 43 KTS AT 040930Z
   CIMSS ADT: 25 KTS AT 041130Z
   CIMSS AIDT: 33 KTS AT 041130Z
   CIMSS D-MINT: 30 KTS AT 040835Z
   CIMSS D-PRINT: 33 KTS AT 041200Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 30+ KTS
   SST: 25-26 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: EXTENSIVE DRY AIR THROUGH THE MID- AND
       UPPER-LEVELS.

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: TC 18P HAS SLOWED AND TURNED SHARPLY WESTWARD
OVER THE PAST 12 HOURS UNDER THE STEERING INFLUENCE OF THE DOMINANT
STEERING RIDGE TO THE SOUTH. TRACK SPEEDS ARE EXPECTED TO REMAIN
CLIMATOLOGICALLY SLOW DUE TO THE COMPETING NEAR-EQUATORIAL RIDGE TO
THE NORTH. TC 18P WILL TRACK GENERALLY WESTWARD, WITH LANDFALL
ANTICIPATED NEAR BRISBANE AT AROUND TAU 48. THE INTENSITY SHOULD
MAINTAIN AT 55 KNOTS THROUGH TAU 48, WITH SOME POTENTIAL FOR SLIGHT
INTENSIFICATION IN THE TAU 36 TO TAU 48 PERIOD AS THE CORE MOISTENS
AND VWS RELAXES TO ABOUT 25-30 KNOTS. ADDITIONALLY, AS THE SYSTEM
APPROACHES THE GOLD COAST, SST VALUES WILL INCREASE SLIGHTLY TO
26-27C. DUE TO THE SLOW TRACK MOTION, THIS SYSTEM IS EXPECTED TO BRING 
HEAVY RAINFALL OVER THE COASTAL REGION OF AUSTRALIA. AFTER TAU 48, THE 
SYSTEM WILL TRACK INLAND AND GRADUALLY DISSIPATE BY TAU 72.     

MODEL DISCUSSION: DETERMINISTIC MODEL GUIDANCE REMAINS IN TIGHT
AGREEMENT SUPPORTING THE JTWC TRACK FORECAST WITH HIGH CONFIDENCE.
RELIABLE INTENSITY GUIDANCE IS IN GOOD AGREEMENT, WITH HAFS-A AND
HWRF INDICATING A PEAK INTENSITY OF 58 KNOTS AT TAU 36. THE GEFS AND 
EPS ENSEMBLES ARE ALSO IN GOOD AGREEMENT AND INDICATE A HIGH 
PROBABILITY OF A 50-64 KNOT SYSTEM MAKING LANDFALL OVER OR NEAR 
BRISBANE. 

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