Advisories

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 REVEALED 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 SHEAR, AND WARM (29-30C) 
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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