Advisories

WDPN32 PGTW 280900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (ETAU) WARNING NR 
006//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 24.6N 166.8E
   INITIAL INTENSITY: 45 KTS
   GEOGRAPHIC REFERENCE: 318 NM NORTH OF WAKE ISLAND
   MOVEMENT PAST 6 HOURS: NORTHWESTWARD AT 09 KTS
   SIGNIFICANT WAVE HEIGHT: 20 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
STORM 21W (ETAU) WITH A FULLY EXPOSED LOW-LEVEL CIRCULATION CENTER
(LLCC) AND CONVECTION OFFSET TO THE EAST. THE HIGH WESTERLY SHEAR
ACTING ON THE SYSTEM IS THE REASON FOR THE LACK OF CONVECTION OVER
THE CENTER. THE WIND FIELD REMAINS HIGHLY ASYMMETRIC WITH THE
STRONGEST WINDS CONFINED TO THE EASTERN SEMICIRCLE, CLOSER TO WHERE
THE CONVECTIVE ACTIVITY IS LOCATED. THE INITIAL POSITION IS PLACED
WITH HIGH CONFIDENCE BASED ON THE EXPOSED LLCC IN ANIMATED MSI. THE
INITIAL INTENSITY OF 45 KTS IS ASSESSED WITH MEDIUM CONFIDENCE
BASED ON THE AGENCY DVORAK AND CIMSS INTENSITY ESTIMATES LISTED
BELOW. ENVIRONMENTAL ANALYSIS INDICATES THAT 21W IS PLACED WITH A
MARGINAL ENVIRONMENT CHARACTERIZED BY STRONG OUTFLOW ALOFT AND WARM
(29-30 C) SEA SURFACE TEMPERATURES OFFSET BY HIGH (30-35 KTS)
WESTERLY SHEAR AND DRY AIR ENTRAINMENT.

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: T3.0 - 45 KTS
   KNES: T2.5 - 35 KTS
   PHFO: T3.0 - 45 KTS
   CIMSS ADT: 34 KTS AT 280610Z
   CIMSS AIDT: 37 KTS AT 280610Z
   CIMSS D-PRINT: 36 KTS AT 280610Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 30-35 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: DRY AIR ENTRAINMENT FROM THE WEST

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   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
NORTH-NORTHWESTWARD ALONG THE WESTERN PERIPHERY OF THE SUBTROPICAL
RIDGE (STR) POSITIONED TO THE EAST THROUGH TAU 48. NEAR TAU 48, AN
INCOMING UPPER-LEVEL TROUGH IS EXPECTED TO DIG IN FROM THE NORTH,
BREAKING DOWN THE STEERING RIDGE. THIS WILL CAUSE THE TRACK SPEEDS
OF 21W TO SLOW CONSIDERABLY THROUGH TAU 72. AFTER TAU 72, THE EXACT
TRACK OF 21W IS HIGHLY UNCERTAIN DUE TO POTENTIAL INTERACTION WITH
TS 20W (BANG-LANG) AND THE DEVELOPING FRONTAL BOUNDARY. AS IT
STANDS, 21W IS FORECAST TO TURN SLOWLY NORTHEASTWARD, THROUGH TAU
96, THOUGH TRACK SPEEDS COULD BE FASTER NEAR THE END OF THE
FORECAST PERIOD IF THE FRONTAL BOUNDARY PICKS UP THE SYSTEM SOONER
THAN ANTICIPATED. SUBTROPICAL TRANSITION IS FORECAST TO BEGIN AT
TAU 72 WITH COMPLETION BY TAU 96 AS THE SYSTEM CROSSES THE 26 C
ISOTHERM, BECOMES ENGULFED BY DRY AIR, AND INTERACTS WITH THE
MID-LATITUDE TROUGH. IN TERMS OF INTENSITY, 21W IS FORECAST TO
MAINTAIN ITS CURRENT INTENSITY THROUGH TAU 36, PRIMARILY DRIVEN BY
WARM SST AND STRONG POLEWARD OUTFLOW. AFTER TAU 36, DRY AIR, SHEAR,
AND QUICKLY LOWERING OCEAN HEAT CONTENT WILL CAUSE 21W TO WEAKEN
THROUGH THE REMAINDER OF THE FORECAST PERIOD. 

MODEL DISCUSSION: MODEL TRACK GUIDANCE IS IN MODERATE AGREEMENT
ONLY THROUGH TAU 48 WITH ALL MODELS DEPICTING A NORTH-NORTHWESTWARD
TRACK. AFTER TAU 48, THE ECMWF DETERMINISTIC BECOMES AN OUTLIER BY
SHOWING THE SYSTEM STAYING QUASISTATIONARY RATHER THAN TURNING
NORTHEASTWARD. NAVGEM IS ALSO AN OUTLIER, SHOW FAR FASTER TRACK
SPEEDS THAN THE REMAINDER OF GUIDANCE AND WAS DISCOUNTED FROM THE
GUIDANCE ENVELOPE. THE JTWC TRACK FORECAST IS PLACED NEAR THE
CONSENSUS THROUGH TAU 48 AND THEN CLOSER TO THE GFS THROUGH TAU 96
WITH LOW CONFIDENCE. INTENSITY GUIDANCE GENERALLY AGREES  ON A
STEADY INTENSITY THROUGH TAU 36 FOLLOWED BY STEADY WEAKENING
THEREAFTER. AS A RESULT, THE JTWC INTENSITY FORECAST IS PLACED
CLOSE TO THE CONSENSUS WITH MEDIUM CONFIDENCE.

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