WDPS31 PGTW 180900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 31P (THIRTYONE) WARNING 
NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 11.2S 139.7E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 523 NM EAST OF DARWIN, AUSTRALIA
   MOVEMENT PAST 6 HOURS: EAST-SOUTHEASTWARD AT 08 KTS
   SIGNIFICANT WAVE HEIGHT: 14 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
CYCLONE 31P WITH A WELL-DEFINED, PARTIALLY EXPOSED LOW-LEVEL
CIRCULATION CENTER (LLCC) AND FLARING CONVECTION ALONG THE
SOUTHWESTERN QUADRANT OF THE SYSTEM. ENVIRONMENTAL ANALYSIS
INDICATES THAT 31P IS IN A MARGINALLY FAVORABLE ENVIRONMENT
CHARACTERIZED BY MODERATE (15-20 KTS) EASTERLY VERTICAL WIND SHEAR,
WEAK OUTFLOW ALOFT, AND WARM (29-30 C) SEA SURFACE TEMPERATURES.
THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE
PARTIALLY EXPOSED LLCC IN ANIMATED MSI. THE INITIAL INTENSITY OF 35
KTS IS ASSESSED WITH HIGH CONFIDENCE BASED ON A PARTIAL 180045Z
METOP-C ASCAT IMAGE SHOWING 30-35 KTS WITHIN THE WESTERN
SEMICIRCLE. ADDITIONALLY, AGENCY DVORAK ESTIMATES RANGE FROM
2.0-2.5, FURTHER SUPPORTING THE INITIAL INTENSITY ASSESSMENT.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA AND OBJECTIVE BEST
TRACK

CURRENT STEERING MECHANISM: SOUTHERN PERIPHERY OF A NEAR-EQUATORIAL
RIDGE (NER) EXTENDING TO THE NORTH OF THE SYSTEM.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   KNES: T2.0 - 30 KTS
   ADRM: T2.5 - 35 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 15-20 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: WEAK POLEWARD

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING
MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY.

FORECAST DISCUSSION: TC 31P IS FORECAST TO TRACK SOUTHEASTWARD
THROUGH TAU 24, ALONG THE SOUTHERN PERIPHERY OF THE NER. NEAR TAU
24, RIDGING WILL BEGIN TO BUILD TO THE SOUTH OF THE SYSTEM, CAUSING
31P TO TURN SHARPLY WESTWARD. AS THE RIDGE BUILDS TO THE SOUTH AND
BEGINS TO PROPAGATE WESTWARD, THE STEERING PATTERN WILL BECOME MORE
DEFINED, ALLOWING FOR AN INCREASE IN TRACK SPEEDS AS THE VORTEX
RIDES ALONG THE NORTHEASTERN PERIPHERY OF THE RIDGE THROUGH THE END
OF THE FORECAST PERIOD. REGARDING INTENSITY, 31P IS EXPECTED TO
REMAIN IN A MARGINAL ENVIRONMENT DUE TO THE ABSENCE OF A GOOD
OUTFLOW CHANNEL AND SOME SLIGHT DRY AIR ENTRAINMENT ALONG THE
WESTERN PERIPHERY. HOWEVER, SHEAR IS EXPECTED TO BEGIN SLIGHTLY
DECREASING AFTER TAU 12 TO AROUND 10-15 KTS. OVERALL, NO
SIGNIFICANT INTENSIFICATION IS CURRENTLY EXPECTED THROUGHOUT THE
FORECAST PERIOD AT THIS TIME. 

MODEL DISCUSSION: NUMERICAL MODEL TRACK GUIDANCE IS IN GOOD
AGREEMENT WITH ALL MODELS DEPICTING THE SHARP TURN AFTER TAU 24.
THE ECMWF AND GFS ENSEMBLES ALSO HAVE NO MAJOR SPREAD BETWEEN
MEMBERS, SUPPORTING THE JTWC TRACK FORECAST. INTENSITY GUIDANCE IS
IN AGREEMENT THROUGH TAU 84, WITH ALL MODELS SUGGESTING A NEARLY
STEADY INTENSITY OF AROUND 35 KTS. AFTER TAU 84, COAMPS-TC AND HWRF
SUGGEST INTENSIFICATION. THOSE MODEL TRACKERS DO KEEP THE VORTEX
FURTHER AWAY FROM LAND INTERACTION THAN THE REMAINDER OF GUIDANCE
THOUGH. THE JTWC INTENSITY FORECAST IS PLACED WITH MEDIUM
CONFIDENCE THROUGH TAU 72 AND LOW CONFIDENCE THEREAFTER.

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