WDPS31 PGTW 181500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 31P (THIRTYONE) WARNING 
NR 002//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 11.6S 140.5E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 566 NM EAST OF DARWIN, AUSTRALIA
   MOVEMENT PAST 6 HOURS: EAST-SOUTHEASTWARD AT 09 KTS
   SIGNIFICANT WAVE HEIGHT: 14 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
CYCLONE 31P WITH A WELL-DEFINED, YET COMPLETELY EXPOSED LOW-LEVEL
CIRCULATION CENTER (LLCC). THE CONVECTION THAT WAS PREVIOUSLY
FLARING OVER THE CENTER OF THE SYSTEM HAS DISSIPATED. MODERATE
NORTHEASTERLY SHEAR CONTINUES TO ACT ON THE SYSTEM AND IS LIKELY
THE CAUSE FOR THE LACK OF CENTRAL CONVECTION. AN 180901Z F17 SSMIS
91 GHZ MICROWAVE IMAGE REVEALED THE LOW-LEVEL STRUCTURE WITH A
SMALL PATCH OF REMNANT CONVECTION IN THE SOUTHWESTERN PERIPHERY.
ENVIRONMENTAL ANALYSIS INDICATES THAT 31P IS IN A MARGINAL
ENVIRONMENT CHARACTERIZED BY WEAK POLEWARD OUTFLOW ALOFT, MODERATE
(15-20 KTS) VWS, AND WARM (29-30 C) SEA SURFACE TEMPERATURES. THE
INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE
EXPOSED LLCC IN ANIMATED EIR IMAGERY. THE INITIAL INTENSITY OF 35
KTS IS ASSESSED WITH LOW CONFIDENCE BASED ON THE AGENCY DVORAK AND
OBJECTIVE INTENSITY ESTIMATES LISTED BELOW. LOW CONFIDENCE IS DUE
IN PART TO THE WEAK APPEARANCE OF THE SYSTEM CONFLICTING WITH THE
CURRENT INTENSITY ESTIMATES.

INITIAL WIND RADII BASIS: PERSISTENCE 

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
   CIMSS ADT: 35 KTS AT 181140Z
   CIMSS AIDT: 35 KTS AT 181140Z
   CIMSS DPRINT: 28 KTS AT 181210Z
   CIMSS DMINT: 30 KTS AT 180901Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 15-20 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: WEAK 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 31P IS FORECAST TO TRACK SOUTH-SOUTHEASTWARD
OVER THE NEXT 12 HOURS BEFORE RIDGING BEGINS TO BUILD IN TO THE
SOUTHEAST OF THE SYSTEM. AS THE RIDGING BUILDS IN, THE SYSTEM WILL
BEGIN TO MAKE A SHARP WESTWARD TURN. AS THE RIDGE CONTINUES TO
BUILD AND PROPAGATES WESTWARD, THE STEERING PATTERN WILL BECOME
MORE DEFINED, ALLOWING FOR AN INCREASE IN TRACK SPEEDS AS THE
VORTEX RIDES ALONG THE NORTHERN 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 24. OVERALL, NO SIGNIFICANT INTENSIFICATION IS
CURRENTLY EXPECTED THROUGHOUT THE FORECAST PERIOD AT THIS TIME. DUE
TO THE BORDERLINE 35-KNOT THRESHOLD OF THE SYSTEM, THERE IS A
POSSIBILITY FOR A DROP TO BELOW THE BASIN WARNING CRITERIA IF THE
SYSTEM IS UNABLE TO ATTAIN PERSISTENT CONVECTION.

MODEL DISCUSSION: NUMERICAL MODEL TRACK GUIDANCE IS IN GOOD
AGREEMENT WITH ALL MODELS DEPICTING THE SHARP WESTWARD TURN NEAR
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 96, WITH ALL MODELS SUGGESTING
A NEARLY STEADY INTENSITY OF AROUND 30-40 KTS. AFTER TAU 96,
COAMPS-TC BECOMES AN OUTLIER AND SUGGESTS A BOUT OF INTENSIFICATION
THROUGH THE REMAINDER OF THE FORECAST PERIOD. THE JTWC INTENSITY
FORECAST IS PLACED WITH MEDIUM CONFIDENCE THROUGH TAU 72 AND LOW
CONFIDENCE THEREAFTER.

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