WDXS31 PGTW 012100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 01S (ONE) WARNING NR 
001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 9.9S 76.1E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 272 NM SOUTHEAST OF DIEGO GARCIA
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 04 KTS
   SIGNIFICANT WAVE HEIGHT: 14 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL CYCLONE (TC) 01S HAS DEVELOPED RELATIVELY QUICKLY OVER THE
PAST DAY OR SO, TO THE SOUTHEAST OF DIEGO GARCIA. ANIMATED
MULTISPECTRAL SATELLITE IMAGERY (MSI) SHOWS THE LOW LEVEL
CIRCULATION CENTER (LLCC) BECOMING MUCH BETTER DEFINED SINCE
YESTERDAY, WITH A SMALL BUT DISCRETE CENTRAL DENSE OVERCAST FORMING
OVER THE LLCC IN THE LAST SIX HOURS OR SO. MICROWAVE IMAGERY HAS
BEEN RELATIVELY SPARSE DUE TO CONTINUED UPSTREAM OUTAGES, BUT A
011659Z METOP-C AMSU-B 89GHZ IMAGE REVEALED A TIGHTLY DEFINED
CIRCULATION WITH WELL-DEFINED SPIRAL BANDING FEATURES WRAPPING INTO
AN OBSCURED LLCC. ANALYSIS OF THE ACCOMPANYING ASCAT PASS
AMBIGUITIES PROVIDED HIGH CONFIDENCE TO THE INITIAL POSITION. THE
INITIAL INTENSITY WAS ASSESSED WITH HIGH CONFIDENCE, MUCH HIGHER
THAN THE MAJORITY OF THE AGENCY DVORAK CURRENT INTENSITY ESTIMATES
BASED ON THE ASCAT PASS NOTED ABOVE, WHICH SHOWED 35-40 KNOT WIND
BARBS DRAPED ACROSS THE SOUTHERN HEMISPHERE OF THE SYSTEM. THE
OVERALL ENVIRONMENT IS GENERALLY FAVORABLE; THE SYSTEM SITS UNDER
AN UPPER-LEVEL RIDGE PROVIDING GOOD POLEWARD OUTFLOW, SHEAR IS
CURRENTLY LOW AND SSTS REMAIN WARM AT 27-28C.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: THE SYSTEM IS MOVING WEST-SOUTHWESTWARD
ALONG THE WESTERN PERIPHERY OF A DEEP-LAYER SUBTROPICAL RIDGE (STR)
SITUATED TO THE SOUTHEAST. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   KNES: T1.5 - 25 KTS
   FMEE: T2.5 - 35 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 10-15 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: MODERATE POLEWARD

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING
MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY.

FORECAST DISCUSSION: TC 01S IS FORECAST TO TRACK GENERALLY TOWARDS
THE WEST-SOUTHWEST THROUGH THE ENTIRETY OF THE FORECAST PERIOD,
ALONG THE WESTERN SIDE OF THE DEEP STR TO THE EAST. THE PASSAGE OF
A DEEP UPPER-LEVEL TROUGH TO THE SOUTH AROUND TAU 48 IS EVIDENT IN
THE GLOBAL MODEL FIELDS, INTRODUCING SOME UNCERTAINTY INTO THE
TRACK FORECAST, BUT AT THIS TIME, THE TROUGH DOES NOT APPEAR DEEP
ENOUGH TO CAPTURE TC 01S AND PULL IT POLEWARD. IN TERMS OF INTENSITY, 
WHILE THE SYSTEM IS CURRENTLY EXPERIENCING FAVORABLE CONDITIONS, DRY 
AIR CURRENTLY POSITIONED TO THE WEST WILL BEGIN TO BE ENTRAINED INTO 
THE SYSTEM WITH THE NEXT 6 TO 12 HOURS. THE INFLUX OF DRY AIR WILL 
TEND TO OFFSET THE OTHERWISE FAVORABLE UPPER-LEVEL CONDITIONS, WITH A 
RESULTANT STEADY INTENSITY FORECAST FOR THE FIRST 24 HOURS OF THE 
FORECAST. AFTER TAU 24, THE DRY AIR BEGINS TO EXERT AN INCREASING 
INFLUENCE, LEADING TO A SLOW WEAKENING TREND. AFTER TAU 36, SHEAR IS 
FORECAST TO INCREASE, WHICH WILL SEAL THE FATE OF TC 01S, THOUGH THE 
DISSIPATION WILL STILL TAKE TIME AS THE WINDS, PARTICULARLY ACROSS THE 
SOUTHERN PORTION OF THE CIRCULATION, REMAINING ELEVATED FOR A TIME. 
DISSIPATION AS A TROPICAL CYCLONE EXPECTED NO LATER THAN TAU 72, AND 
LIKELY AS EARLY AS TAU 60.   

MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE IS IN GOOD AGREEMENT
ON THE TRACK SCENARIO, WITH A MODEST AMOUNT OF CROSS-TRACK SPREAD OF 
ABOUT 200NM BY TAU 120, BETWEEN THE GEFS ENSEMBLE MEAN ON THE NORTH 
SIDE AND THE ECEPS MEAN ON THE SOUTH SIDE OF THE GUIDANCE ENVELOPE. 
THE JTWC TRACK IS LAID JUST SOUTH OF THE CONSENSUS MEAN WITH HIGH 
CONFIDENCE. INTENSITY GUIDANCE IS ALSO IN GOOD AGREEMENT, WITH ALL OF 
THE AVAILABLE MEMBERS INDICATING A STEADY INTENSITY THROUGH ABOUT TAU 
24, FOLLOWED BY A GENERAL WEAKENING TREND TO 30 KNOTS BY TAU 72. 

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