Advisories

WDPN32 PGTW 281500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (ETAU) WARNING NR 
007//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 25.2N 166.5E
   INITIAL INTENSITY: 45 KTS
   GEOGRAPHIC REFERENCE: 354 NM NORTH OF WAKE ISLAND
   MOVEMENT PAST 6 HOURS: NORTH-NORTHWESTWARD AT 07 KTS
   SIGNIFICANT WAVE HEIGHT: 22 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: 
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
STORM 21W (ETAU) WITH A FULLY EXPOSED LOW-LEVEL CIRCULATION CENTER 
(LLCC) WITH DEEP CONVECTION ATTEMPTING TO BUILD CLOSER TO THE CENTER. 
THE STRONG WESTERLY SHEAR ACTING ON THE SYSTEM HAS CONTINUED TO CAUSE 
THE CONVECTION TO BE DISPLACED TO THE EAST. A 280959Z METOP-B ASCAT 
PASS REVEALED A HIGHLY ASYMMETRIC WIND FIELD WITH THE 34 KNOT WINDS 
ENTIRELY CONFINED TO THE EASTERN SEMICIRCLE. THE INITIAL POSITION IS 
PLACED WITH HIGH CONFIDENCE BASED ON THE EXPOSED LLCC IN ANIMATED EIR 
IMAGERY. THE INITIAL INTENSITY OF 45 KTS AND THE INITIAL WIND RADII 
ARE ASSESSED WITH HIGH CONFIDENCE BASED ON THE ASCAT PASS SHOWING A 
SWATH OF 40 KTS TO THE EAST AND THE KNOWN SLIGHT LOW BIAS OF THE 
SENSOR AT THESE INTENSITIES. ENVIRONMENTAL ANALYSIS INDICATES THAT 21W 
IS PLACED WITHIN 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: 280959Z METOP-B ASCAT SCATTEROMETER DATA 

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: T3.0 - 45 KTS

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: HIGH
   INITIAL WIND RADII: HIGH

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 AND CAUSING TRACK SPEEDS TO SLOW. BINARY 
INTERACTION WITH TY 20W (BANG-LANG) WILL CAUSE 21W TO BE TUGGED 
NORTHEASTWARD THROUGH AFTER TAU 72. A DEVELOPING FRONTAL BOUNDARY IS 
EXPECTED TO FORM NEAR THE VERY END OF THE FORECAST PERIOD, WHICH COULD 
CAUSE TRACK SPEEDS TO BE FASTER IF THE FRONTAL BOUNDARY PICKS UP THE 
SYSTEM SOONER THAN ANTICIPATED. SUBTROPICAL TRANSITION IS FORECAST TO 
BEGIN AT TAU 60 WITH COMPLETION BY TAU 72 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 24, PRIMARILY DRIVEN BY WARM SST AND 
STRONG POLEWARD OUTFLOW. AFTER TAU 24, DRY AIR, HIGH 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 
THROUGH TAU 72 WITH THE PRIMARY OUTLIER BEING THE JGSM, WHICH DEPICTS 
A STRAIGHT NORTHWARD TRACK CROSSING THE 40N LATITUDE AT TAU 72. 
DISCOUNTING JGSM, THE REMAINDER OF GUIDANCE GENERALLY SHOWS A SLOW 
TURN TO THE EAST OR NORTHEAST AFTER TAU 48, ONCE THE SYSTEM BEGINS TO 
INTERACT WITH 20W. THE JTWC TRACK FORECAST IS PLACED CLOSE TO THE 
MODIFIED CONSENSUS WITH MEDIUM CONFIDENCE. INTENSITY GUIDANCE AGREES 
ON A STEADY INTENSITY THROUGH TAU 24 FOLLOWED BY GRADUAL WEAKENING 
THROUGH TAU 72 WITH NO MAJOR OUTLIERS PRESENT. AS A RESULT, THE JTWC 
INTENSITY FORECAST IS PLACED NEAR THE CONSENSUS WITH MEDIUM 
CONFIDENCE.

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