Advisories

WDPN32 PGTW 271500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (ETAU) WARNING NR 
003//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 22.0N 168.0E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 180 NM NORTH-NORTHEAST OF WAKE ISLAND
   MOVEMENT PAST 6 HOURS: NORTHWARD AT 08 KTS
   SIGNIFICANT WAVE HEIGHT: 16 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: 
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL 
STORM 21W (ETAU) WITH CURVED CONVECTIVE BANDING ALONG THE EASTERN 
PORTION OF THE NOW OBSCURED LOW-LEVEL CIRCULATION CENTER (LLCC). THE 
WIND FIELD CONTINUES TO APPEAR ASYMMETRIC WITH THE STRONGEST WINDS 
LOCATED WITHIN THE EASTERN SEMICIRCLE, THOUGH ASCAT MISSED THE SYSTEM 
TO CONFIRM THE WINDS NEAR THE CORE. THE INITIAL POSITION IS PLACED 
WITH MEDIUM CONFIDENCE BASED ON THE ANIMATED EIR. THE INITIAL 
INTENSITY OF 40 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE 
AGENCY DVORAK AND CIMSS INTENSITY ESTIMATES LISTED BELOW. 
ENVIRONMENTAL ANALYSIS INDICATES THAT 21W IS IN A FAVORABLE 
ENVIRONMENT CHARACTERIZED BY STRONG POLEWARD OUTFLOW ALOFT, LOW (10-15 
KTS) WESTERLY VERTICAL WIND SHEAR, AND WARM (29-30C) SEA SURFACE 
TEMPERATURES.

INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK 

CURRENT STEERING MECHANISM: THE WESTERN PERIPHERY OF A SUBTROPICAL
RIDGE (STR) POSITIONED TO THE EAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   RJTD: T2.5 - 35 KTS
   RCTP: T2.5 - 35 KTS
   KNES: T2.5 - 35 KTS
   PHFO: T2.5 - 35 KTS
   CIMSS ADT: 37 KTS AT 271140Z
   CIMSS AIDT: 38 KTS AT 271140Z
   CIMSS D-PRINT: 42 KTS AT 271140Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 10-15 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG POLEWARD

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: TS 21W (ETAU) IS FORECAST TO TRACK GENERALLY
NORTHWARD, ALONG THE WESTERN PERIPHERY OF THE STR TO THE EAST THROUGH 
TAU 72. AFTER TAU 72, AN UPPER-LEVEL TROUGH IS EXPECTED TO DIG IN FROM 
THE NORTH, BREAKING DOWN THE STEERING RIDGE. THIS WILL CAUSE TRACK 
SPEEDS TO SLOW THROUGH TAU 96 AS THE SYSTEM MOVES NORTHEASTWARD. THE 
INTERACTION WITH TS 20W NEAR THE END OF THE FORECAST PERIOD COULD ALSO 
CAUSE THE TRACK OF 21W TO BECOME ERRATIC. SUBTROPICAL TRANSITION IS 
FORECAST TO COMPLETE NEAR TAU 96 AS THE SYSTEM CROSSES THE 26 C SST 
ISOTHERM, BECOMES ENGULFED BY DRY AIR, AND INTERACTS WITH THE STRONG 
MID-LATITUDE TROUGH. REGARDING INTENSITY, 21W IS FORECAST TO GRADUALLY 
INTENSIFY THROUGH TAU 36, PRIMARILY DUE TO THE ROBUST POLEWARD OUTFLOW 
CHANNEL. SOUTHWESTERLY SHEAR AND DRY AIR ENTRAINMENT IS EXPECTED TO 
INCREASE AFTER TAU 36, CAUSING THE SYSTEM TO WEAKEN THROUGH TAU 96 AS 
THE SYSTEM CONTINUES TO TRACK POLEWARD. ADDITIONALLY, OCEAN HEAT
CONTENT (OHC) WILL SIGNIFICANTLY DROP AFTER TAU 48, FURTHER SUPPORTING 
THE WEAKENING TREND.

MODEL DISCUSSION: MODEL TRACK GUIDANCE IS IN MODERATE AGREEMENT WITH 
THE EXCEPTION OF NAVGEM, WHICH TRACKS THE VORTEX SIGNIFICANTLY FURTHER 
WEST THAN THE REMAINDER OF THE GUIDANCE. DISCOUNTING NAVGEM, THE  
CROSS-TRACK SPREAD AT TAU 72 IS MEASURED TO BE ABOUT 230 NM. THE JTWC 
TRACK FORECAST IS PLACED CLOSE TO THE MODIFIED CONSENSUS WITH MEDIUM 
CONFIDENCE. INTENSITY GUIDANCE GENERALLY AGREES ON SLIGHT 
INTENSIFICATION THROUGH TAU 36 FOLLOWED BY WEAKENING THEREAFTER. AS A 
RESULT, THE JTWC INTENSITY FORECAST IS ALSO PLACED CLOSE TO THE 
CONSENSUS WITH OVERALL MEDIUM CONFIDENCE.

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