WDPS31 PGTW 210900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 31P (THIRTYONE) 
WARNING NR 013//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 12.1S 137.6E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 393 NM EAST OF DARWIN, AUSTRALIA
   MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 09 KTS
   SIGNIFICANT WAVE HEIGHT: 14 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A RATHER
DISORGANIZED SYSTEM, WITH POCKETS OF DEEP CONVECTION PERSISTING
AROUND THE LOW LEVEL CIRCULATION CENTER BUT ARRAYED IN A MORE
LINEAR FASHION THAN WRAPPING INTO A CLEAR-CUT CENTER. A 210420Z
AMSR2 COLOR ENHANCED 37GHZ MICROWAVE IMAGE SHOWED A WEAK LLCC
LOCATED WEST OF A BAND OF ENHANCED CONVECTION WITH A WELL-DEFINED
BANDING FEATURE EXTENDING TO THE NORTH AND NORTHWEST TO NEAR CAPE
WESSEL. RADAR DATA OUT OF GOVE, AUSTRALIA, IS NOT PICKING UP THE
LLCC WELL AND PROVIDED ONLY LIMITED ASSISTANCE TO PLACING THE
INITIAL POSITION. SURFACE OBSERVATIONS FROM GOVE SHOW STEADILY
INCREASING SOUTHEASTERLY WINDS AND FALLING PRESSURES BUT THE TRENDS
HAVE BEGUN TO REVERSE AS OF THE 0700Z OBSERVATION WHICH SHOWS WINDS
BACKING SLIGHTLY AND PRESSURE BEGINNING TO RISE, SUGGESTING THE
LLCC MAY ACTUALLY BE AT ITS CLOSEST POINT TO THE NORTH-NORTHEAST OF
GOVE AT THIS POINT. MEANWHILE OBS FROM CAPE WESSEL HAVE VEERED
SHARPLY FROM SOUTHEAST TO WEST-NORTHWEST OVER THE PAST HOUR
SUGGESTING THE SURFACE CENTER IS RAPIDLY APPROACHING FROM THE
SOUTHEAST. THE INITIAL POSITION IS ASSESSED WITH LOW CONFIDENCE
BASED ON AN EXTRAPOLATION OF THE EARLIER AMSR2 DATA AND THE LIMITED
RADAR DATA. THE INITIAL INTENSITY IS ASSESSED WITH MEDIUM
CONFIDENCE BASED ON EXTRAPOLATION OF A 210241Z OSCAT-3 PASS WHICH
REVEALED A SMALL AREA OF 35 KNOT WINDS LOCATED IN THE SOUTHERN HALF
OF THE CIRCULATION. THE ENVIRONMENT IS MARGINALLY FAVORABLE, WITH
WARM SSTS, LOW DEEP-LAYER SHEAR AND MODEST POLEWARD ALOFT, OFFSET
BY LOW- TO MID-LEVEL DRY AIR INTRUSION AND PROXIMITY TO LAND.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: NORTHERN PERIPHERY OF A LOW- TO
MID-LEVEL SUBTROPICAL RIDGE (STR) OVER NORTHERN AUSTRALIA. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   KNES: T1.5 - 25 KTS
   ADRM: T2.0 - 30 KTS
   CIMSS ADT: 43 KTS AT 210530Z
   CIMSS AIDT: 37 KTS AT 210530Z
   CIMSS D-MINT: 24 KTS AT 210420Z
   CIMSS D-PRINT: 27 KTS AT 210610Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 5-10 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE POLEWARD
   OTHER FACTORS: LOW- TO MID-LEVEL DRY AIR INTRUSION. 

ANALYSIS CONFIDENCE:
   INITIAL POSITION: LOW
   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 31P IS MOVING SWIFTLY WEST-NORTHWESTWARD
ALONG THE NORTHERN SIDE OF A LOW- TO MID-LEVEL RIDGE SITUATED OVER
NORTHERN AUSTRALIA. NO SIGNIFICANT CHANGES ARE EXPECTED IN THE
STEERING PATTERN THROUGH THE SHORT FORECAST PERIOD, THOUGH TRACK
SPEEDS WILL BEGIN TO SLOW DOWN BY THE END OF THE FORECAST AS THE
SYSTEM MOVES INTO A GENERALLY WEAKER STEERING PATTERN. THE CENTER
IS EXPECTED TO PASS NORTH OF GOVE WITHIN THE NEXT COUPLE OF HOURS,
THEN PASS SOUTH OF CAPE WESSEL BEFORE MOVING FURTHER INTO THE
ARAFURA SEA OVER THE NEXT 24 HOURS, WITH THE CENTER REMAINING WELL
OFFSHORE OF THE NORTHERN COAST OF ARNHEM LAND. HAVING REACHED 35
KNOTS PRIOR TO ITS CLOSEST APPROACH TO LAND, THE SYSTEM WILL LIKELY
SEE SOME DISRUPTION IN LOW LEVEL STRUCTURE AS THE HIGHEST WINDS
MOVE OVER THE GOVE PENINSULA. INTENSITIES COULD DIP DOWN BELOW 35
KNOTS DURING THIS TIMEFRAME, BUT AS THE SYSTEM MOVES BACK OVER MORE
OPEN WATERS AND THE ENTIRETY OF THE WINDFIELD REMAINS OFFSHORE,
REINTENSIFICATION IS LIKELY. HOWEVER, COMPETING ENVIRONMENTAL
FACTORS (LOW SHEAR, WARM SSTS AND MODERATE OUTFLOW ALOFT) WILL BE
OFFSET BY REPEATED INTRUSIONS OF DRY AIR PULLED INTO THE SYSTEM
FROM THE SOUTH. THUS THE SYSTEM IS FORECAST TO MAINTAIN AS STEADY
INTENSITY THROUGH TAU 36. BY TAU 36 HOWEVER AS THE SYSTEM CONTINUES
EQUATORWARD, IT WILL MOVE INTO INCREASINGLY UNFAVORABLE SHEAR
CONDITIONS, AND QUICKLY DISSIPATE NO LATER THAN TAU 48 OVER THE
ARAFURA SEA. 

MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE IS IN GOOD
CROSS-TRACK AGREEMENT, WITH ONLY MINIMAL SPREAD THROUGH THE
FORECAST PERIOD. ALONG-TRACK DISPERSION INCREASES STEADILY FROM TAU
00 HOWEVER, WITH THE ECMWF AND THE GEFS MEAN OPENING UP A
COMMANDING LEAD ON THE GFS OF A LITTLE OVER 100NM BY TAU 48. THE
JTWC FORECAST IS PLACED NEAR THE GFS AND MULTI-MODEL CONSENSUS MEAN
WITH HIGH CONFIDENCE. INTENSITY GUIDANCE IS IN GOOD OVERALL
AGREEMENT. DISCARDING THE UNREALISTIC SHIPS OUTPUT, THE REMAINDER OF
THE GUIDANCE IS CONSISTENT WITH A STEADY-STATE INTENSITY THROUGH
TAU 36 BEFORE WEAKENING TO 30 KNOTS BY TAU 48. THE JTWC FORECAST IS
PLACED ON TOP OF THE COAMPS-TC, HAFS-A AND HWRF WITH HIGH
CONFIDENCE. 

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