WDPN32 PGTW 270900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (ETAU) WARNING NR 
002//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 21.2N 168.0E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 139 NM NORTHEAST OF WAKE ISLAND
   MOVEMENT PAST 6 HOURS: NORTH-NORTHEASTWARD AT 08 KTS
   SIGNIFICANT WAVE HEIGHT: 14 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
STORM 21W (ETAU) WITH CURVED CONVECTIVE BANDING ALONG THE EASTERN
PORTION OF A MOSTLY OBSCURED LOW-LEVEL CIRCULATION CENTER (LLCC). A
270218Z AMSR2 36 GHZ MICROWAVE IMAGE REVLEAED THAT THE LOW-LEVEL
STRUCTURE HAS SIGNIFICANTLY IMPROVED OVER THE PAST 6 HOURS, SHOWING
A PARTIAL MICROWAVE EYE FEATURE BEGINNING TO FORM. THE WIND FIELD
CONTINUES TO APPEAR ASYMMETRIC WITH THE STRONGEST WINDS CONFINED TO
THE EASTERN SEMICIRCLE. THE INITIAL POSITION IS PLACED WITH MEDIUM
CONFIDENCE BASED ON LOW-LEVEL CLOUD TRACING IN THE ANIMATED MSI.
THE INITIAL INTENSITY OF 40 KTS IS ASSESSED WITH LOW CONFIDENCE DUE
TO AGENCY DVORAK ESTIMATES SUGGESTING A T2.0 (30 KTS). THE INITIAL
INTENSITY IS SET A BIT HIGHER DUE TO THE STRONG STRUCTURE IN THE
AFOREMENTIONED AMSR2 MICROWAVE IMAGE. ENVIRONMENTAL ANALYSIS
INDICATES THAT 21W IS IN A FAVORABLE ENVIRONMENT CHARACTERIZED BY
STRONG POLEWARD OUTFLOW ALOFT, LOW (10-15 KTS) WESTERLY VERTICAL
WIND SEHAR, AND WARM (29-30 C) SEA SURFACE TEMPERATURES.

INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK 

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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   RJTD: T2.0 - 30 KTS
   KNES: T2.0 - 30 KTS
   PHFO: T2.0 - 30 KTS
   CIMSS ADT: 31 KTS AT 270540Z
   CIMSS AIDT: 35 KTS AT 270540Z
   CIMSS D-PRINT: 34 KTS AT 270540Z

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 RIDGE. THIS WILL CAUSE
TRACK SPEEDS TO SIGNIFICANTLY SLOW THROUGH THE REMAINDER OF THE
FORECAST PERIOD. 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 BEGIN TO 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,
CROSS-TRACK SPREAD AT TAU 72 IS ABOUT 190 NM WITH GFS MAKING UP THE
EASTERNMOST TRACKER. 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 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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