Advisories

WDPN33 PGTW 271500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 20W (TWENTY) WARNING NR 
006//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.1N 163.3E
   INITIAL INTENSITY: 45 KTS
   GEOGRAPHIC REFERENCE: 230 NM SOUTHWEST OF WAKE ISLAND
   MOVEMENT PAST 6 HOURS: EAST-NORTHEASTWARD AT 09 KTS
   SIGNIFICANT WAVE HEIGHT: 14 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: 
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
STORM 20W WITH AN EXTREMELY COMPACT AND FULLY OBSCURED LOW-LEVEL 
CIRCULATION CENTER (LLCC). THE CENTER IS NOW EMBEDDED UNDERNEATH AN 
INCREASINGLY SYMMETRIC CENTRAL DENSE OVERCAST (CDO) WITH ASSOCIATED 
CLOUD TOPS MEASURING COLDER THAN -85 C IN SOME AREAS. A 270727Z RCM-2 
SAR PASS SHOWED A STRIKINGLY SMALL CIRCULATION WITH A RADIUS OF 
MAXIMUM WINDS AROUND 10 NM AND A RING OF 45-50 KTS COMPLETELY 
SURROUNDING THE CENTER. A 271040Z METOP-C ASCAT PASS ALLOWED FOR HIGH 
CONFIDENCE IN THE INITIAL WIND RADII, BUT WAS UNABLE TO MEASURE THE 
STRONGEST WINDS DUE TO THE RESOLUTION LIMITATIONS OF THE INSTRUMENT. 
THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE 
ANIMATED EIR AND EXTRAPOLATION FROM THE ASCAT. THE INITIAL INTENSITY 
OF 45 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED PRIMARILY ON 
PERSISTENCE FROM THE AFOREMENTIONED SAR PASS. THE AGENCY DVORAK FIXES 
ARE ANALYZED TO BE UNDERESTIMATING THE CURRENT INTENSITY DUE TO THE 
EXTREMELY SMALL SIZE OF THE SYSTEM.

INITIAL WIND RADII BASIS: 271040Z METOP-C ASCAT SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: SOUTHWESTWARD EXTENSION OF A SUBTROPICAL 
RIDGE (STR) POSITIONED TO THE NORTHEAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   RJTD: T1.5 - 25 KTS
   KNES: T2.0 - 30 KTS
   PHFO: T2.5 - 35 KTS
   CIMSS ADT: 41 KTS AT 271140Z
   CIMSS AIDT: 38 KTS AT 271140Z
   CIMSS D-MINT: 38 KTS AT 271157Z
   CIMSS D-PRINT: 48 KTS AT 271140Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 10-15 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE POLEWARD
   OTHER FACTORS: BINARY INTERACTION WITH TS 21W (ETAU) TO THE 
NORTHEAST

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: A FASTER RATE OF INTENSIFICATION IS
FORECAST, REACHING A PEAK INTENSITY OF 85 KTS.

FORECAST DISCUSSION: TS 20W IS FORECAST TO TRACK NORTHEASTWARD, ALONG 
THE SOUTHWESTERN EXTENSION OF THE STR THROUGH TAU 24. AROUND TAU 36, 
THE TRAJECTORY WILL BECOME MORE NORTHWARD AS THE SYSTEM TRACKS ALONG 
THE WESTERN PERIPHERY OF THE STR THROUGH TAU 72. AFTER TAU 72, AN 
UPPER-LEVEL TROUGH IS EXPECTED TO DIG IN FROM THE NORTH AND WEAKEN THE 
RIDGE. THE TROUGH WILL THEN INTERACT WITH 20W AND PULL IT FURTHER 
POLEWARD. SUBTROPICAL TRANSITION IS FORECAST TO BEGIN AROUND TAU 72 AS 
THE SYSTEM CROSSES THE 26 C ISOTHERM AND INTERACTS WITH THE STRONG 
MID-LATITUDE TROUGH. SUBTROPICAL TRANSITION IS THEN FORECAST TO 
COMPLETE AROUND TAU 96, ONCE THE SYSTEM BECOMES COMPLETELY EMBEDDED 
UNDERNEATH THE TROUGH. NEAR TAU 96, BINARY INTERACTION WITH 21W (ETAU) 
WILL INCREASE AS THE DISTANCE BETWEEN THE TWO CIRCULATIONS DECREASES. 
AS A RESULT, THE EXACT TRACK NEAR THE END OF THE FORECAST PERIOD COULD 
DIFFER DEPENDING ON THE RELATIVE STRENGTH AND DISTANCE BETWEEN THE TWO 
SYSTEMS. THE LIKELIHOOD OF THE ABSORPTION OF 20W WITHIN THE WIND FIELD 
OF 21W IS LOOKING LESS AND LESS LIKELY; HOWEVER, IT IS STILL A 
POSSIBILITY IF 20W DOES NOT INTENSIFY AS MUCH AS ANTICIPATED. IN TERMS 
OF INTENSITY, 20W IS FORECAST TO QUICKLY INTENSIFY OVER THE NEXT 36 
HOURS AS THE ENVIRONMENT REMAINS CONDUCIVE. THE COMPACT NATURE OF THE 
VORTEX IS HIGHLY SUSCEPTIBLE TO SMALL CHANGES IN THE ENVIRONMENT, 
CAUSING THE INTENSITY TO POTENTIALLY BE EXTREMELY VOLATILE AS A 
RESULT. AS IT STANDS, A PEAK INTENSITY OF 85 KTS IS FORECAST TO OCCUR 
AT AROUND TAU 48. AFTER TAU 48, WESTERLY SHEAR AND DRY AIR IS EXPECTED 
TO INCREASE, CAUSING THE SYSTEM TO WEAKEN THROUGH THE REMAINDER OF THE 
FORECAST PERIOD. ADDITIONALLY, OCEAN HEAT CONTENT (OHC) WILL BEGIN TO 
SIGNIFICANTLY LOWER AFTER TAU 48, FURTHER SUPPORTING THE WEAKENING 
TREND.

MODEL DISCUSSION: MODEL TRACK GUIDANCE IS NOW IN MODERATE AGREEMENT
THROUGH TAU 72 WITH THE EXCEPTION OF GALWEM AND NAVGEM, WHICH CONTINUE 
TO BE THE WESTERN OUTLIERS. THE GFS DETERMINISTIC REMAINS THE 
EASTERNMOST MODEL. THE JTWC TRACK FORECAST IS PLACED NEAR THE 
EXPERIMENTAL GOOGLE DEEPMIND ENSEMBLE MEAN WITH MEDIUM CONFIDENCE 
THROUGH TAU 72 AND LOW CONFIDENCE THEREAFTER. INTENSITY GUIDANCE 
CONTINUES TO BE IN VERY POOR AGREEMENT. NEARLY EVERY AVAILABLE RAPID 
INTENSIFICATION (RI) AID HAS TRIGGERED THIS MODEL RUN. HAFS-A IS NOW 
MORE IN LINE WITH THE RI AIDS AS WELL. PEAK INTENSITIES RANGE FROM 70 
KTS (GFS) TO 115 KTS (RIPA) AT TAU 48 WITH HAFS-A SOMEWHERE IN THE 
MIDDLE WITH A PEAK OF 85 KTS. THE JTWC INTENSITY FORECAST IS PLACED 
VERY CLOSE TO THE HAFS-A WITH OVERALL LOW CONFIDENCE.

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