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 20W 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 20W 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//
NNNN