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 EMBBEDDED 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 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
VOLITLE 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,
FUTHER 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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