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 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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