WDPS31 PGTW 191500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 31P (THIRTYONE) WARNING 
NR 006//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 12.7S 141.0E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 375 NM NORTHWEST OF CAIRNS, AUSTRALIA
   MOVEMENT PAST 6 HOURS: SOUTHWESTWARD AT 01 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
CYCLONE 31P WITH A PARTIALLY EXPOSED LOW-LEVEL CIRCULATION CENTER
(LLCC) AND DEEP CONVECTION NOW FORMING CLOSER TO THE CENTER WITHIN
THE SOUTHWESTERN QUADRANT. THE SYSTEM HAS BEEN NEARLY STATIONARY
OVER THE PAST 6 HOURS AS THE STEERING RIDGE CONTINUES TO BUILD TO
THE SOUTHEAST. ENVIRONMENTAL ANALYSIS INDICATES THAT 31P IS IN A
MARGINAL ENVIRONMENT CHARACTERIZED BY WEAK POLEWARD OUTFLOW ALOFT,
MODERATE (15-20 KTS) EASTERLY VERTICAL WIND SHEAR, 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.
ADDITIONALLY, ANIMATED RADAR IMAGERY FROM WEIPA DEPICTS SHALLOW
RAIN BANDS WRAPPING INTO THE CENTER. THE INITIAL INTENSITY OF 30
KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE AGENCY DVORAK
FIXES AND OBJECTIVE INTENSITY ESTIMATES LISTED BELOW.

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

CURRENT STEERING MECHANISM: NORTHWESTERN PERIPHERY OF A SUBTROPICAL
RIDGE (STR) POSITIONED TO THE SOUTHEAST.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   KNES: T1.5 - 25 KTS
   ADRM: T2.0 - 30 KTS
   CIMSS SATCON: 35 KTS AT 191200Z
   CIMSS ADT: 34 KTS AT 191200Z
   CIMSS AIDT: 35 KTS AT 191200Z
   CIMSS DPRINT: 25 KTS AT 191200Z
   CIMSS DMINT: 27 KTS AT 190846Z

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

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: MEDIUM
   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: TC 31P IS FORECAST TO TRACK WESTWARD OVER THE
NEXT 24 HOURS AS THE STR BUILDS IN TO THE SOUTH OF THE VORTEX AND
PROPAGATES WESTWARD. NEAR TAU 24, THE SYSTEM IS FORECAST TO START
ON A WEST-NORTHWESTWARD TRACK AS THE SYSTEM RIDES ALONG THE
NORTHERN PERIPHERY OF THE STR THROUGH THE REMAINDER OF THE FORECAST
PERIOD. REGARDING INTENSITY, 31P IS FORECAST TO BEGIN SLIGHTLY
INTENSIFYING WITHIN THE GULF OF CARPENTARIA AS THE VORTEX BECOMES
BETTER ALIGNED THROUGH THE MID-LEVELS AND THE ENVIRONMENT MOISTENS.
GALES ARE EXPECTED TO BE CONFINED TO PRIMARILY THE SOUTHERN
SEMICIRCLE OF THE CIRCULATION. A PEAK OF AROUND 40 KTS IS FORECAST
TO OCCUR BETWEEN TAU 36 AND 48, BEFORE THE SYSTEM BEGINS TO SKIRT
ALONG THE NORTHERN TOP END. AFTER TAU 48, 31P IS FORECAST TO REMAIN
RIGHT AROUND 35 KTS THROUGH TAU 72 DUE TO LAND INTERACTION AND AN
OVERALL MARGINAL ENVIRONMENT. AFTER TAU 72, UPPER-LEVEL SOUTHERLY
SHEAR IS EXPECTED TO INCREASE AND POLEWARD OUTFLOW WILL BECOME
RESTRICTED, CAUSING THE VORTEX TO WEAKEN AND EVENTUALLY DISSIPATE
NEAR TAU 96. 

MODEL DISCUSSION: NUMERICAL MODEL TRACK GUIDANCE IS IN MODERATE
AGREEMENT THROUGH TAU 72 WITH A 90 NM CROSS-TRACK SPREAD AND 180 NM
ALONG-TRACK SPREAD AT TAU 72. AFTER TAU 72, ALONG-TRACK SPREAD
INCREASES SIGNIFICANTLY TO AROUND 300 NM WITH ECMWF AND GFS
ACCELERATING THE VORTEX NORTHWESTWARD WHILE GALWEM, UKMET, AND
NAVGEM DEPICT MUCH SLOWER TRACK SPEEDS. THE JTWC TRACK FORECAST IS
PLACED SLIGHTLY CLOSER TO THE GFS AND ECMWF MODELS WITH MEDIUM
CONFIDENCE THROUGH TAU 72 AND LOW CONFIDENCE THEREAFTER. INTENSITY
GUIDANCE AGREES ON MARGINAL INTENSIFICATION THROUGH TAU 36 AND THEN
A STEADY INTENSITY THROUGH TAU 72 AND THEN WEAKENING AFTERWARD.
NOTABLY, GFS KEEPS THE SYSTEM WEAKER THAN MESOSCALE MODELS DO, BUT
ALSO HAS THE VORTEX TRACK CLOSER TO THE TOP END THAN HAFS-A, HWRF,
AND COAMPS-TC DOES. OVERALL, THE JTWC INTENSITY FORECAST IS PLACED
WITH MEDIUM CONFIDENCE. THE AMOUNT OF LAND INTERACTION WILL PLAY A
BIG ROLE IN HOW MUCH THE SYSTEM IS ABLE TO DEVELOP.


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