WDXS31 PGTW 230300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 27P (NARELLE) WARNING 
NR 021//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 14.5S 129.2E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 156 NM SOUTHWEST OF DARWIN, AUSTRALIA
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 09 KTS
   SIGNIFICANT WAVE HEIGHT: 14 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A
WELL-DEFINED CIRCULATION BRIEFLY TRANSITING OVER THE JOSEPH
BONAPARTE GULF, WITH SUSTAINED DEEP CONVECTION OVER THE NORTHERN
FLANK AND ROBUST UPPER-LEVEL EQUATORWARD EXHAUST. AS THE CIRCULATION 
TRAVERSES OVER THE VERY WARM WATERS OF THE GULF, THE SYSTEM IS 
BEGINNING TO EXHIBIT SIGNS OF SLIGHT DEVELOPMENT, EVIDENCED BY 
DEEPENING CLOUDS. A FORTUITOUS 222357Z ASCAT PASS MEASURED LOW-END 
GALES OVER THE GULF WATERS, NEAR THE CONVECTIVE BAND, AND OUTLINED A 
RATHER LARGE CIRCULATION CENTER. THE ASCAT PASS, A 222052Z RCM-2 SAR 
PASS, AND PGTW DVORAK FIX SUPPORT BOTH THE INITIAL POSITION AND 
INTENSITY OF 35 KTS WITH HIGH CONFIDENCE.

INITIAL WIND RADII BASIS: SAR DATA 

CURRENT STEERING MECHANISM: DEEP-LAYER SUBTROPICAL RIDGE OVER
CENTRAL AUSTRALIA

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   CIMSS ADT: 35 KTS AT 0020Z
   CIMSS AIDT: 34 KTS AT 0020Z
   CIMSS D-PRINT: 30 KTS AT 0020Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY
UNFAVORABLE
   VWS: 15-20 KTS
   SST: 30-31 CELSIUS
   OUTFLOW: MODERATE EQUATORWARD

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: TC 27P IS BEING STEERED TOWARDS THE
WEST-SOUTHWEST BY A STRONG, DEEP-LAYER RIDGE OVER CENTRAL
AUSTRALIA. THE RIDGE EXTENDS TOWARDS THE NW, WHICH SHOULD HOLD THE 
STORM MOTION THROUGH TAU 72. AS THE SYSTEM APPROACHES THE 
NORTHWESTERN TIP OF AUSTRALIA, THE EXTENSION OF THE RIDGE WILL WEAKEN 
AND ALLOW FOR A GRADUAL TURN POLEWARD. THE TIMING AND EXTENT OF THE 
WEAKENING OF THE RIDGE EXTENSION AND THE PLACEMENT OF THE CENTER OF 
THE ANTICYCLONE WILL GREATLY AFFECT THE TIMING AND SHARPNESS OF 
POLEWARD TURN. AFTER PASSING NEAR THE EXMOUTH COAST, TC 27P WILL 
ACCELERATE TOWARDS GERALDTON. THE INTENSITY FORECAST IS 
STRAIGHTFORWARD FOR THE FIRST 48 HOURS. THE SYSTEM MAY BRIEFLY 
INTENSIFY TO 40 KTS THROUGH TAU 12 AS IT TRACKS OVER THE WARM WATERS 
OF THE JOSEPH BONAPARTE GULF, THOUGH THIS IS NOT CAPTURED EXPLICITLY 
IN THE FORECAST. IT SHOULD EMERGE OVER THE EXCEPTIONALLY WARM WATERS 
OF 30-31 DEGREES CELSIUS OF THE SOUTH INDIAN OCEAN BETWEEN TAU 24 AND 
TAU 36, AT WHICH TIME THE SYSTEM WILL BEGIN A VERY SIGNIFICANT PHASE 
OF INTENSIFICATION. THE BROAD CIRCULATION WILL INITIALLY INTENSIFY AT 
A STEADY RATE AS IT BECOMES BETTER ORGANIZED AND FORMS A DEEP INNER 
CORE. RAPID INTENSIFICATION (RI) IS EXPLICITLY FORECAST AFTER TAU 48 
DUE TO THE HIGHLY FAVORABLE ENVIRONMENTAL CONDITIONS CONSISTING OF 
VERY WARM SEA SURFACE TEMPERATURES, LOW VERTICAL WIND SHEAR, 
PLENTIFUL MID-LEVEL MOISTURE, AND ENHANCED POLEWARD OUTFLOW, PEAKING 
NEAR 105 KTS. THE SYSTEM WILL BE QUITE EXTENSIVE IN SIZE WHEN IT 
MAKES A VERY CLOSE APPROACH TO LEARMONTH. WHILE THE CPA WILL 
FLUCTUATE BETWEEN FORECASTS, CPA MAY END UP NOT BEING THE MOST 
IMPORTANT FACTOR BECAUSE LEARMONTH IS FORECAST TO BE WELL WITHIN THE 
GALE- AND STORM-FORCE WIND RADII, LENDING TO INCREASING CONFIDENCE OF 
DESTRUCTIVE WINDS. AFTER IT TURNS POLEWARD, THE SYSTEM WILL STEADILY 
WEAKEN AS THE SEA SURFACE TEMPERATURES QUICKLY DROPS.

MODEL DISCUSSION: MODEL GUIDANCE IS IN TIGHT AGREEMENT THROUGH TAU
48, WITH A CROSS-TRACK SPREAD OF LESS THAN 100 NM. THE SPREAD
INCREASES TO 400 NM AT TAU 96, WITH THE DETERMINISTIC AND
PHYSICS-BASED MODELS PREFERRING A MORE OFFSHORE TRACK. THE AI
MODELS CONTINUE TO TURN THE SYSTEM INLAND SOONER AND AS FAR EASTWARD 
AS DAMPIER. BOTH GROUPINGS HAVE CONVERGED SLIGHTLY, SO THE FORECAST
TRACK IS HEDGED TOWARDS THE CONSENSUS (TES1), WHICH ACCOUNTS
FOR THE AI MODELS, AND IS PLACED CLOSER TO THE EXMOUTH COAST THAN THE 
PHYICS-BASED MODELS. ALL MODELS HOLD THE INTENSITY AT LOW-END
TROPICAL STORM INTENSITY THROUGH TAU 24. ALL MODELS DEPICT
SIGNIFICANT INTENSIFICATION AFTER TAU 24. IN FACT, THE GOOGLE
DEEPMIND AND COAMPS-TC MODELS SHOWCASE AN 80-90 PERCENT PROBABILITY 
OF RI AND A PEAK AT 105-110 KTS. THE INTENSITY FORECAST IS SET CLOSE 
TO THE CONSENSUS THROUGH TAU 24 AND THEN FOLLOWS THE MORE RAPID 
INCREASE OF THESE TWO MODELS. SOME UNCERTAINTY IN THE INTENSIFICATION 
RATE AND PEAK STILL EXIST GIVEN THAT THE SYSTEM WILL BE BROADER IN 
SIZE AND THE MODEL TRACK SPREAD FROM TAU 48 TO TAU 96 IS QUITE WIDE. 
ALL MODELS DEPICT STEADY TO RAPID WEAKENING BY TAU 120 DUE TO MUCH 
COOLER WATERS.

FORECAST CONFIDENCE:
   TRACK 00-72 HR: HIGH
   TRACK 72-120 HR: LOW
   INTENSITY 00-72 HR: HIGH
   INTENSITY 72-120 HR: MEDIUM//

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