WDPS31 PGTW 200900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 31P (THIRTYONE) 
WARNING NR 009//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 12.6S 140.9E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 384 NM NORTHWEST OF CAIRNS, AUSTRALIA
   MOVEMENT PAST 6 HOURS: SOUTHWESTWARD AT 04 KTS
   SIGNIFICANT WAVE HEIGHT: 9 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A PARTIALLY
OBSCURED LOW LEVEL CIRCULATION CENTER (LLCC) AND ISOLATED POCKETS
OF DEEP CONVECTION ARRAYED IN AN IRREGULAR PATTERN TO THE EAST AND
SOUTH OF THE LLCC. ANIMATED RADAR DATA OUT OF WEIPA SHOWS DEFINED
LOW-LEVEL BANDING FEATURES, PARTICULARLY TO THE SOUTH AND WEST OF
THE CENTER, AND WEAKER BANDS TO THE NORTH AND EAST. THE CENTER HAS
BECOME INCREASINGLY DEFINED OVER THE PAST FEW HOURS IN THE RADAR
IMAGERY, SUGGESTING THE VORTEX, WHICH IS CURRENTLY TILTED WEST WITH
HEIGHT, MAY BE STARTING TO VERTICALLY ALIGN. A 200645Z SSMIS 37GHZ
MICROWAVE IMAGE SHOWED A DEFINED LOW EMISSIVITY REGION (LER)
DEFINING THE LLCC, WHICH MATCHED UP WELL WITH THE RADAR INDICATED
CENTER. COMPARISON WITH THE 91GHZ CHANNEL REVEALS A DECREASED
AMOUNT OF VORTEX TILT, THOUGH ALL OF THE CONVECTIVE ACTIVITY IS
CONFINED TO THE SOUTH AND EAST OF THE CENTER, INDICATIVE OF
RELATIVELY DRY AIR STILL ENSCONCED TO THE NORTH OF THE SYSTEM. THE
INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE
RADAR AND MSI. THE INITIAL INTENSITY REMAINS SET AT 30 KNOTS, BASED
ON AN AVERAGE OF THE SUBJECTIVE AND OBJECTIVE ESTIMATES NOTED
ABOVE, AS WELL AS A RECENT SCATTEROMETER PASS FROM 200130Z WHICH
INDICATED WINDS HOVERING AT 25-30 KNOTS, WITH THE HIGHEST WINDS TO
THE SOUTH, UNDER THE DEEP CONVECTION. ANALYSIS REVEALS A FAVORABLE
ENVIRONMENT, WITH LOW (AND LOWERING) NORTHEASTERLY SHEAR, VERY WARM
SSTS, HIGH OHC AND MODERATE POLEWARD OUTFLOW ALOFT.

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

CURRENT STEERING MECHANISM: WEAK STEERING PATTERN. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   KNES: T2.0 - 30 KTS
   ADRM: T2.5 - 35 KTS
   CIMSS SATCON: 38 KTS AT 200400Z
   CIMSS ADT: 45 KTS AT 200630Z
   CIMSS AIDT: 38 KTS AT 200530Z
   CIMSS D-PRINT: 27 KTS AT 200600Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 10-15 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: WEAK POLEWARD
   OTHER FACTORS: DRY MID-LEVEL AIR TO THE NORTH AND VORTEX
VERTICAL ASYMMETRY. 

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: TC 31P HAS TRACED A RATHER MEANDERING TRACK
OVER THE PAST 12 HOURS OR SO, TRAPPED IN A WEAK STEERING PATTERN
WITH NO CLEAR-CUT STEERING INFLUENCES. THIS HAS STARTED TO CHANGE
OVER THE LAST SIX HOURS, WITH THE SYSTEM NOW MOVING AT ABOUT FOUR
KNOTS TOWARDS THE SOUTHWEST AS THE A DEEP-LAYER SUBTROPICAL RIDGE
(STR) CENTER HAS BEGUN TO BUILD IN THE CORAL SEA. OVER THE NEXT 12
HOURS, A LOW- TO MID-LEVEL RIDGE IS EXPECTED TO BUILD OVER NORTHERN
AUSTRALIA WHICH WILL PUSH TC 31P ONTO A MORE WESTWARD TRACK AT
FIRST, SLOWLY ARCING ONTO A MORE NORTHWESTWARD TRACK THROUGH THE
REMAINDER OF THE FORECAST PERIOD. IN TERMS OF INTENSITY, AS THE
SYSTEM MOVES FURTHER AWAY FROM THE COAST AND FURTHER INTO THE GULF
OF CARPENTARIA, SHEAR IS FORECAST TO STEADILY LOWER, WHICH WILL
ALLOW FOR VORTEX SYMMETRIZATION AND VERTICAL ALIGNMENT. ONCE THE
VORTEX ALIGNS, TC 31P WILL QUICKLY TAKE ADVANTAGE OF THE WARM, HIGH
OHC WATERS OF THE GULF AND STEADILY INTENSIFY. THE CLOSE APPROACH
TO THE NORTHERN SHORES OF ARNHEM LAND AFTER TAU 24 WILL CAUSE SOME
DISRUPTION TO THE SYSTEM WHICH WILL COMPETE WITH OTHERWISE
FAVORABLE CONDITIONS, TO ARREST FURTHER DEVELOPMENT THROUGH TAU 48.
IF THE SYSTEM REMAINS FURTHER OFFSHORE HOWEVER, IT IS POSSIBLE IT
COULD INTENSIFY FURTHER. BY TAU 72, SHEAR WILL BEGIN TO INCREASE
AGAIN AS THE SYSTEM TRACKS EQUATORWARD, WHICH ALONG WITH DRY AIR
INTRUSIONS, WILL STEADILY WEAKEN THE SYSTEM, WHICH IS FORECAST BY
TAU 96 AS IT ENTERS THE BANDA SEA. 

MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE IS IN GOOD AGREEMENT
THROUGH TAU 72, WITH VERY LITTLE IN THE WAY OF CROSS-TRACK
DISPERSION, PROVIDING HIGH CONFIDENCE TO THE JTWC FORECAST THROUGH
TAU 72. AFTER THIS POINT, MODELS BEGIN TO DIVERGE SOMEWHAT AS THE
SYSTEM WEAKENS AND SOME MODELS BEGIN TO LOSE TRACK OF THE VORTEX.
THE GFS IS THE WESTERN-SOUTHERN MOST MODEL THROUGH THE ENTIRE
FORECAST PERIOD, TAKING THE CENTER OVER GOVE, AUSTRALIA AND THEN
VERY CLOSE TO THE ARNHEM LAND COASTLINE THROUGH TAU 48 BEFORE
TURNING TOWARDS BABA ISLAND. THE ECMWF DETERMINISTIC TRACK MARKS
THE EASTERN SIDE OF THE ENVELOPE, TAKING THE CENTER OVER RARAGALA
ISLAND, THEN NORTHWEST TOWARDS THE CHANNEL EAST OF BABA ISLAND. THE
JTWC FORECAST IS PLACED CLOSE TO THE ECMWF SOLUTION WITH HIGH
CONFIDENCE. INTENSITY GUIDANCE IS IN FAIRLY GOOD AGREEMENT THAT THE
SYSTEM WILL INTENSIFY TO A PEAK AROUND 40 KNOTS WITHIN THE NEXT 24
HOURS BEFORE LEVELING OFF THEN WEAKENING AFTER TAU 48. 

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