WDPN32 PGTW 272100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (ETAU) WARNING NR 
004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 23.1N 167.8E
   INITIAL INTENSITY: 45 KTS
   GEOGRAPHIC REFERENCE: 238 NM NORTH-NORTHEAST OF WAKE ISLAND
   MOVEMENT PAST 6 HOURS: NORTHWARD AT 11 KTS
   SIGNIFICANT WAVE HEIGHT: 20 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS AN 
ASYMMETRIC SYSTEM, WITH A COMMA-SHAPED AREA OF DEEP CONVECTION NEAR 
THE ASSESSED LOW LEVEL CIRCULATION CENTER, AT THE HEAD OF THE COMMA, 
AND A BAND OF DEEP CONVECTION EXTENDING SOUTHEASTWARD. A SERIES OF 
37GHZ MICROWAVE IMAGES INCLUDING A 271415Z AMSR3, 271427Z AMSR2 AND 
271739Z SSMIS DEPICTED A PARTIALLY EXPOSED LOW LEVEL CIRCULATION 
CENTER (LLCC) DISPLACED TO THE NORTHWEST OF THE DEEP CONVECTION 
FORMING THE COMMA HEAD FEATURE. ANIMATED WATER VAPOR IMAGERY AND 
ATMOSPHERIC MOTION VECTORS SHOW AN UPPER-LEVEL ANTICYCLONE POSITIONED 
TO THE SOUTHWEST NEAR TS 20W, WHICH IS INCLUDING WEST-SOUTHWESTERLY 
FLOW AND MODERATE SHEAR OVER TOP OF TS 20W. HOWEVER THE WATER VAPOR 
IMAGERY ALSO SHOWS A WELL-FORMED ARC OF UPPER-LEVEL CLOUDS PUSHING 
WESTWARD, INDICATING THAT THE SHEAR AT THE OUTFLOW LAYER IS LIKELY 
LOWER THAN ENVIRONMENTAL SHEAR ESTIMATE. THE INITIAL POSITION IS 
ASSESSED WITH HIGH CONFIDENCE BASED ON THE EXPOSED LLCC IN THE MOST 
RECENT MICROWAVE IMAGE NOTED ABOVE. THE INITIAL INTENSITY IS ASSESSED 
WITH MEDIUM CONFIDENCE, BASED ON A BLEND OF THE OBJECTIVE AND 
SUBJECTIVE INTENSITY ESTIMATES OUTLINED BELOW. ENVIRONMENTAL 
CONDITIONS ARE MARGINALLY FAVORABLE, WITH THE WARM SSTS AND ROBUST 
OUTFLOW ALOFT, OFFSET BY THE MODERATE WESTERLY SHEAR AND SOME DRY AIR 
PRESENT ALONG THE WESTERN FLANK OF THE SYSTEM.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: WESTERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) CENTERED TO THE EAST. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   RJTD: T2.5 - 35 KTS
   RCTP: T3.0 - 45 KTS
   KNES: T2.5 - 35 KTS
   PHFO: T3.0 - 45 KTS
   CIMSS SATCON: 42 KTS AT 271434Z
   CIMSS ADT: 41 KTS AT 271740Z
   CIMSS AIDT: 38 KTS AT 271740Z
   CIMSS D-MINT: 33 KTS AT 271736Z
   CIMSS D-PRINT: 40 KTS AT 271840Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 20-25 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG POLEWARD

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THE FORECAST HAS BEEN EXTENDED OUT TO
120 HOURS TO BETTER CAPTURE THE BINARY INTERACTION WITH TS 20W. 

FORECAST DISCUSSION: TROPICAL STORM (TS) 21W (ETAU) WILL CONTINUE TO 
TRACK QUICKLY NORTHWARD ALONG THE WESTERN FLANK OF THE STR OVER THE 
NEXT 48 HOURS. AFTER TAU 48, THE SYSTEM MOVES INTO A WEAK STEERING 
PATTERN, AS THE REMNANTS OF TS LALA MOVE INTO A BLOCKING POSITION 
DOWNSTREAM AND WEAKEN THE RIDGE SOUTH OF THE ALEUTIANS, WHILE AT THE 
SAME TIME TS 20W WILL RAPIDLY APPROACH FROM THE SOUTHEAST AND BEGIN 
BINARY INTERACTION WITH 21W. BY TAU 60, TS 21W WILL HAVE BECOME 
NEARLY STATIONARY, OR BE DRIFTING WESTWARD, IN THIS WEAK STEERING 
ENVIRONMENT. BY TAU 72, 20W WILL HAVE MOVED TO A POSITION NORTHEAST 
OF 21W, AND 21W WILL BEGIN TO MOVE BACK TO THE EAST-NORTHEAST AS THE 
TWO SYSTEMS UNDERGO A FUJIWHARA WITH ONE ANOTHER. BY TAU 96 THE 
SYSTEM WILL ACCELERATE NORTHEASTWARD AS IT IS PICKED UP BY THE 
SUBTROPICAL WESTERLIES AHEAD OF AN APPROACHING TROUGH. THE SYSTEM 
WILL BEGIN SUBTROPICAL TRANSITION (STT) BY TAU 96 AND WILL COMPLETE 
STT NO LATER THAN TAU 120. REGARDING INTENSITY, THE RECENT EVOLUTION 
OF THE WATER VAPOR IMAGERY SUGGESTS THE SYSTEM MAY BE SETTING UP SOME 
LOWER MESOSCALE SHEAR OVER THE LLCC. ASSUMING THIS IS CORRECT, THE 
FORECAST CALLS FOR A SLIGHT AMOUNT OF INTENSIFICATION IN THE NEXT 12 
TO 24 HOURS, BEFORE THE SYSTEM STEADILY IF SLOWLY WEAKENS THROUGH THE 
REMAINDER OF THE FORECAST PERIOD UNDER PERSISTENT MODERATE TO HIGH 
SHEAR AND DRY AIR ENTRAINMENT. 

MODEL DISCUSSION: TRACK GUIDANCE IS IN FAIR AGREEMENT THROUGH TAU 48, 
WITH A CROSS-TRACK SPREAD INCREASING TO 160NM BY THIS POINT IN THE 
FORECAST. THE GALWEM AND NAVGEM ARE CONSISTENT WESTERN OUTLIERS, 
WHILE THE AI MODELS ARE CLUSTERED WITH THE GFS ON THE EASTERN SIDE OF 
THE ENVELOPE. BY TAU 60, THE MODELS BEGIN A RAPID DISPERSAL, WITH 
ROUGHLY HALF (GFS, ECMWF, GALWEM, JGSM AND EC-AIFS) DEPICTING A SHORT 
WESTWARD JOG, A BRIEF QUASISTATIONARY PERIOD FOLLOWED BY AN EJECTION 
EASTWARD BY TAU 96. THE REMAINDER (MOGREPS, GEFS AND THE AI-BASED 
MODELS) ARE TRACK THE SYSTEM STRAIGHT THROUGH WITH LITTLE SLOWDOWN 
AND LITTLE TO NO INDICATION OF BINARY INTERACTION WITH TS 20W, WHICH 
IS NOT SURPRISING GIVEN THE CLIMATOLOGICALLY RARE BINARY INTERACTION. 
THE JTWC FORECAST IS PLACED NEAR THE ECMWF TRACKER, THOUGH CONFIDENCE 
IS VERY LOW DUE TO THE UNCERTAINTY SURROUNDING THE FUJIWHARA 
INTERACTION AT THIS LONG-RANGE FORECAST POINT. INTENSITY GUIDANCE IS 
IN MUCH BETTER AGREEMENT, WITH ALL MEMBERS OF THE CONSENSUS CONFINED 
TO AN ENVELOPE BETWEEN 40-55 KNOTS, GRADUALLY WEAKENING AFTER TAU 36. 

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