Advisories

WDPN33 PGTW 280300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 20W (TWENTY) WARNING NR 
008//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 18.2N 164.9E
   INITIAL INTENSITY: 60 KTS
   GEOGRAPHIC REFERENCE: 117 NM SOUTHWEST OF WAKE ISLAND
   MOVEMENT PAST 6 HOURS: NORTHEASTWARD AT 10 KTS
   SIGNIFICANT WAVE HEIGHT: 23 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: 
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A STEADILY 
CONSOLIDATING, EXTREMELY COMPACT SYSTEM WITH VORTICAL HOT TOWERS 
(VHT) DEVELOPING AROUND THE ASSESSED LOW LEVEL CIRCULATION CENTER 
(LLCC) AS WELL AS ALONG THE OUTER RAIN BANDS, PARTICULARLY ALONG THE 
EASTERN FLANK. A 272311Z ASCAT-C PASS SHOWED THE COMPACT WIND FIELD 
ASSOCIATED WITH TS 20W, WITH GALE FORCE WINDS FOCUSED IN THE SOUTHERN 
HEMISPHERE OF THE CIRCULATION, AND THE HIGHEST WINDS IN THE 
SCATTEROMETER DATA, OF APPROXIMATELY 40-45 KNOTS, LOCATED IN THE 
SOUTHEASTERN QUADRANT. OF NOTE, THE SYSTEM IS SO COMPACT THAT THE 
SCATTEROMETER DOES NOT HAVE HIGH ENOUGH RESOLUTION TO CAPTURE THE 
HIGHEST WINDSPEEDS, WHICH ARE ASSESSED TO BE 15-20 KNOTS HIGHER. THE 
INITIAL POSITION AND INTENSITY ARE ASSESSED WITH HIGH CONFIDENCE 
BASED ON THE ASCAT DATA. NOTABLY, THE BULK OF THE OBJECTIVE AND 
SUBJECTIVE INTENSITY ESTIMATES (EXCEPT PHFO AND ADT) ARE 
SIGNIFICANTLY LOW BIASED DUE TO THE COMPACT STRUCTURE OF THE SYSTEM. 
ANIMATED WATER VAPOR IMAGERY DEPICTS AN UPPER-LEVEL ANTICYCLONE 
LOCATED JUST SOUTHEAST OF THE LLCC, SUPPORTING STRONG POLEWARD 
OUTFLOW AND LOW DEEP-LAYER SHEAR OVER TS 20W. OVERALL ENVIRONMENTAL 
CONDITIONS ARE FAVORABLE TO HIGHLY FAVORABLE, CHARACTERIZED BY LOW 
DEEP LAYER SHEAR, STRONG POLEWARD OUTFLOW, AND WARM SSTS, WITH AN 
ADDITIONAL BOOST PROVIDED BY THE FACT THAT THE SYSTEM IS CROSSING 
OVER A PATCH OF HIGH OHC.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: NORTHERN FLANK OF AN EXPANSIVE REGION OF 
RIDGING THAT EXTENDS FROM THE SOUTH OF GUAM TO NEAR THE DATELINE AT 
25N. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS
   RJTD: T1.5 - 25 KTS
   KNES: T3.0 - 45 KTS
   PHFO: T3.5 - 55 KTS
   CIMSS ADT: 57 KTS AT 272330Z
   CIMSS AIDT: 41 KTS AT 280000Z
   CIMSS D-PRINT: 39 KTS AT 280030Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 5-10 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG POLEWARD

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: TROPICAL STORM (TS) 20W (TWENTY) CONTINUES TO 
TRACK NORTHEASTWARD ALONG THE NORTHERN FLANK OF AN EXTENSIVE RIDGING 
PATTERN THAT EXTENDS FROM SOUTH OF THE GUAM TO NEAR THE DATELINE. 
OVER THE NEXT 48 TO 60 HOURS, THE SYSTEM GRADUALLY TURNS NORTHWARD AS 
THE RIDGE BUILDS SOUTHWARD. BY TAU 60, TS 20W WILL REACH AN EQUAL 
LATITUDE WITH TS 21W AND BEGIN TO TURN NORTHWESTWARD AS IT IS 
CAPTURED AND BEGINS TO FUJIWHARA AROUND TS 21W. BY TAU 72, TS 20W 
WILL HAVE ROTATED TO A POSITION NORTH OF TS 21W AND AFTER THIS POINT, 
20W WILL BE EJECTED TO THE NORTHEAST. THE SYSTEM BEGINS SUBTROPICAL 
TRANSITION BY TAU 72 AS IT CROSSES NORTH OF A FINGER OF THE 
SUBTROPICAL JET AND REACHES THE 26C SST ISOTHERM. TRANSITION TO A 
SUBTROPICAL LOW IS EXPECTED AS EARLY AS TAU 96 AND NO LATER THAN TAU 
120. REGARDING INTENSITY, TS 20W WILL REMAIN IN A VERY FAVORABLE 
ENVIRONMENT FOR THE NEXT 36 HOURS. THE SYSTEM HAS CLEARLY ATTAINED 
VORTEX ALIGNMENT AND SYMMETRIZATION, AND IS FORECAST TO CONTINUE 
INTENSIFICATION AT A NEAR-RAPID PACE, TO A PEAK OF AT LEAST 85 KNOTS. 
AFTER TAU 36, STEADILY INCREASING SHEAR AND DRY AIR ENTRAINMENT WILL 
LEAD TO STEADY WEAKENING THROUGH THE REMAINDER OF THE FORECAST 
PERIOD. 

MODEL DISCUSSION: TRACK GUIDANCE REMAINS HIGHLY UNCERTAIN, 
PARTICULARLY AFTER THE SYSTEM BEGINS TO FUJIWHARA WITH TS 21W AROUND 
TAU 60. BUT EVEN PRIOR TO THAT POINT, CROSS-TRACK SPREAD RAPIDLY 
INCREASES TO 450NM BY TAU 60, WITH NAVGEM AND GALWEM MAKING THE MOST 
AGGRESSIVE NORTHWESTWARD TRACK AND THE AI MODELS ALONG WITH GEFS 
TAKING THE MOST NORTHWARD TRAJECTORY. OF THE TWO, THE NORTHWESTWARD 
TRACK IS THE LEAST LIKELY AND THESE MODELS HAVE BEEN DISCARDED FROM 
THE CONSENSUS. THROUGH TAU 60, THE JTWC FORECAST IS PLACED JUST EAST 
OF THE MODIFIED CONSENSUS MEAN AND NEAR THE AI CONSENSUS. BEYOND TAU 
60, BOTH ALONG- AND CROSS-TRACK UNCERTAINTY INCREASES SIGNIFICANTLY 
DUE TO THE UNCERTAINTY ASSOCIATED WITH THE BINARY INTERACTION AND 
FUJIWHARA WITH THE MEMBERS OF CONSENSUS EACH HANDLING IT DIFFERENTLY, 
LEADING TO SPREAD EXCEEDING OVER 1000NM. THE JTWC FORECAST IS 
POSITIONED CLOSE TO THE AI CONSENSUS MEAN AND THE HAFS-A, BOTH OF 
WHICH CAPTURE THE FUJIWHARA WELL. HOWEVER, AT THIS LONG-RANGE POINT 
IN THE FORECAST, THE EXACT TIMING AND NATURE OF THE INTERACTION OF 
20W AND 21W IS ITSELF HIGHLY UNCERTAIN LEADING TO EXTREMELY LOW 
CONFIDENCE IN THE TRACK FORECAST. INTENSITY GUIDANCE IS ALSO VERY 
UNCERTAIN DUE TO THE COMPACT NATURE OF THE SYSTEM. THE BULK OF THE RI 
GUIDANCE CONTINUES TO TRIGGER AND ALL GUIDANCE EXCEPT THE GFS IS 
ABOVE 80 KNOTS FOR THE PEAK. THE JTWC FORECAST IS PLACED JUST BELOW 
THE NON-RI-INTEGRATED CONSENSUS MEAN THROUGH THE FORECAST PERIOD. DUE 
TO THE SMALL NATURE OF THE SYSTEM, SIGNIFICANT INTENSIFICATION BEYOND 
85 KNOTS IS POSSIBLE.  

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