WDPS31 PGTW 182100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 31P (THIRTYONE) 
WARNING NR 003//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 11.9S 141.1E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 599 NM EAST OF DARWIN, AUSTRALIA
   MOVEMENT PAST 6 HOURS: EAST-SOUTHEASTWARD AT 07 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A 
PARTIALLY-EXPOSED, DEFINED LOW-LEVEL CIRCULATION, WITH FLARING DEEP
CONVECTION OVER THE SOUTHWEST QUADRANT. ANIMATED RADAR IMAGERY FROM
THE WEIPA RADAR REVEALS SHALLOW RAINBANDS WRAPPING INTO A BROAD LOW-
LEVEL CIRCULATION CENTER (LLCC), WHICH SUPPORTS THE INITIAL POSITION 
WITH HIGH CONFIDENCE. AN 181822Z SSMIS 37GHZ MICROWAVE IMAGE REVEALS 
SHALLOW BANDING WRAPPING INTO A WELL-DEFINED LLCC, WHICH ALSO 
SUPPORTS THE INITIAL POSITION. SURFACE OBSERVATIONS FROM WEIPA AERO, 
APPROXIMATELY 70NM TO THE SOUTHEAST, INDICATE UNIMPRESSIVE 10-15 KNOT 
EAST-SOUTHEASTERLY SUSTAINED WINDS, WITH MINIMUM SLP NEAR 1004.7MB. 
ENVIRONMENTAL CONDITIONS ARE MARGINAL DUE TO MODERATE TO HIGH 
VERTICAL WIND SHEAR (VWS) AND WEAK POLEWARD OUTFLOW. ADDITIONALLY, 
THE SYSTEM IS VERTICALLY STACKED BUT SHALLOW, WITH THE UPPER-LEVEL 
CIRCULATION CENTER LOCATED WEST OF THE LLCC. THE 172300Z WEIPA 
SOUNDING ALSO INDICATES A VERY DRY MID- TO UPPER-LEVEL ENVIRONMENT, 
WITH SEVERAL STABLE POCKETS CAPPED WITH SUBSIDENCE INVERSIONS. DUE TO 
THE MARGINAL ENVIRONMENT AND INTERACTION WITH CAPE YORK PENINSULA, 
THE SYSTEM HAS WEAKENED SLIGHTLY TO 30 KNOTS. THE INTENSITY IS BASED 
ON A 181224Z ASCAT-B UHR IMAGE SHOWING MAXIMUM WINDS OF 28 KNOTS OVER 
THE SOUTHERN QUADRANT. DVORAK ESTIMATES HAVE DECREASED AND NOW RANGE 
FROM 25 TO 35 KNOTS. CIMSS OBJECTIVE INTENSITY ESTIMATES RANGE FROM 
27 TO 37 KNOTS.

INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND
RADII).

CURRENT STEERING MECHANISM: COMPETING STEERING ENVIRONMENT, WITH A
NEAR-EQUATORIAL RIDGE (NER) TO THE NORTH AND LOW- TO MID-LEVEL
SUBTROPICAL RIDGE (STR) EXTENDING TO THE SOUTHEAST AND WEST.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   KNES: T1.5 - 25 KTS
   ADRM: T2.5 - 35 KTS
   CIMSS SATCON: 37 KTS AT 181623Z
   CIMSS ADT: 31 KTS AT 181800Z
   CIMSS AIDT: 33 KTS AT 181800Z
   CIMSS D-MINT: 24 KTS AT 181822Z
   CIMSS D-PRINT: 27 KTS AT 181800Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 20-25 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: WEAK POLEWARD
   OTHER FACTORS: DRY AIR ENTRAINMENT

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: HIGH
   INITIAL WIND RADII: NOT APPLICABLE

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TROPICAL CYCLONE (TC) 31P WILL TRACK SLOWLY AND 
PERHAPS ERRATICALLY THROUGH TAU 24 BUT SHOULD BEGIN TO TURN WESTWARD 
BY TAU 36 AS A STR BUILDS TO THE SOUTH. THIS STR WILL DRIVE THE 
SYSTEM WEST-NORTHWESTWARD THROUGH THE REMAINDER OF THE FORECAST 
PERIOD. TC 31P SHOULD BEGIN TO RE-INTENSIFY AFTER TAU 12 AS THE 
VORTEX BECOMES BETTER ALIGNED THROUGH THE MID-LEVELS AND THE 
ENVIRONMENT MOISTENS. PEAK INTENSITY COULD POSSIBLY PEAK SLIGHTLY 
HIGHER THAN 35 KNOTS AS THE SYSTEM TRAVERSES THE GULF OF CARPENTARIA. 
HOWEVER, GRADUAL WEAKENING IS ANTICIPATED AFTER TAU 72 AS THE SYSTEM 
SKIRTS THE NORTHERN COAST OF AUSTRALIA AND TRACKS EQUATORWARD.     

MODEL DISCUSSION: THE BULK OF THE DETERMINISTIC GUIDANCE SUPPORTS THE 
JTWC TRACK FORECAST WITH MEDIUM OVERALL CONFIDENCE, WITH NAVGEM AND 
UKMET THE PRIMARY OUTLIERS. THE 181200Z ECMWF AND GFS ENSEMBLES
(EPS, GEFS) SHOW A SIMILAR SPREAD OF SOLUTIONS, WITH THE LARGE 
MAJORITY OF SOLUTIONS IN THE 20-40 KNOT INTENSITY RANGE. 
ADDITIONALLY, THE BULK OF THE SOLUTIONS SUPPORT A TRACK NORTHEAST
OF DARWIN AND NORTH OF THE NORTHERN COAST OF AUSTRALIA AFTER TAU
72. RELIABLE INTENSITY GUIDANCE IS FAIRLY FLAT, WITH COAMPS-TC (GFS) 
PEAKING AT 40 KNOTS AND HAFS-A MUCH LOWER PEAKING AT 30 KNOTS.      

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