Advisories

WDPN32 PGTW 290300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (ETAU) WARNING NR 
009//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 26.4N 165.8E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 428 NM NORTH OF WAKE ISLAND
   MOVEMENT PAST 6 HOURS: NORTHWARD AT 05 KTS
   SIGNIFICANT WAVE HEIGHT: 20 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
STORM (TS) 21W (ETAU) AS A HIGHLY SHEARED AND VERY ELONGATED FULLY
EXPOSED LOW-LEVEL CIRCULATION CENTER (LLCC). THE SOUTHERN
HEMISPHERE OF 21W IS BEING PULLED INTO TYPHOON (TY) 20W TO ITS
SOUTH, WHILE THE NORTHERN HEMISPHERE IS BEING STRETCHED BY THE MASS
OF DEEP CONVECTION DISPLACED TO THE NORTHEAST. THE 282224Z PARTIAL
ASCAT PASS REVEALS A HIGHLY ASYMMETRIC WINDFIELD, IN TERMS OF BOTH
WIND SPEED AND SHAPE. THE PASS REVEALED WEAK (5-10 KTS) WINDS ALONG
THE SOUTHEASTERN QUADRANT AND COMPETING WIND DIRECTIONS SPLIT
BETWEEN 20W AND 21W. THE HIGHEST WIND SPEEDS ARE LOCALIZED TO THE
NORTHEASTERN QUADRANT AND EXCLUSIVE TO UNDER THE CONVECTION.
ENVIRONMENTAL ANALYSIS REVEALS A MARGINAL ENVIRONMENT,
CHARACTERIZED BY VERY HIGH (35-40 KTS) VERTICAL WIND SHEAR AND
MARGINAL OCEAN HEAT CONTENT, OFFSET SLIGHTLY BY WARM (27-28 C) SEA
SURFACE TEMPERATURES (SST) AND GOOD POLEWARD OUTFLOW. THE INITIAL
POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE FULLY EXPOSED
LLCC IN ANIMATED MSI. THE INITIAL INTENSITY OF 35 KTS IS ASSESSED
WITH HIGH CONFIDENCE BASED ON THE 282224Z PARTIAL ASCAT PASS.

INITIAL WIND RADII BASIS: ADJUSTED WIND RADII USING THE 282224Z
PARTIAL ASCAT PASS

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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   RJTD: T2.5 - 35 KTS
   RCTP: T2.5 - 35 KTS
   KNES: T2.0 - 30 KTS
   PHFO: T3.0 - 45 KTS
   CIMSS ADT: 34 KTS AT 290140Z
   CIMSS AIDT: 33 KTS AT 290140Z
   CIMSS D-MINT: 39 KTS AT 282211Z
   CIMSS D-PRINT: 32 KTS AT 290110Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 35-40 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: MODERATE POLEWARD

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 IS FORECAST TO CONTINUE TRACKING
NORTH-NORTHWESTWARD FOR THE NEXT 24 HOURS WHILE IT REMAINS UNDER
THE INFLUENCE OF THE STEERING STR CENTERED TO ITS EAST. AFTER ABOUT
TAU 12, 21W IS EXPECTED TO CURVE NORTH-NORTHEASTWARD AS AN
UPPER-LEVEL TROUGH BREAKS DOWN THE STEERING RIDGE. ADDITIONALLY,
21W HAS BEGUN INTERACTING WITH TY 20W, WHICH WILL FURTHER REINFORCE
THE NORTHEASTWARD TRACK -- AND POTENTIALLY EXPEDITE THE
NORTHEASTWARD CURVE -- THROUGH TAU 60. INTERACTION WITH TY 20W MAY
RESULT IN STORM MOTION NOT REFLECTED IN THE OFFICIAL FORECAST, AS
THE DEGREE OF INTERACTION IS STILL UNCERTAIN. TS 21W IS EXPECTED TO
BEGIN SUBTROPICAL TRANSITION (STT) AROUND TAU 48 AND COMPLETE
TRANSITION NO LATER THAN TAU 60. BY TAU 60, 21W IS FORECAST TO HAVE
CROSSED THE 26C ISOTHERM AND BE EMBEDDED UNDER THE UPPER-LEVEL
TROUGH. REGARDING INTENSITY, MAINTAIN ITS CURRENT INTENSITY THROUGH
TAU 12, PRIMARILY DRIVEN BY WARM SST AND GOOD POLEWARD OUTFLOW.
AFTER TAU 12, DRY AIR, HIGH SHEAR, AND QUICKLY LOWERING OCEAN HEAT
CONTENT WILL CAUSE 21W TO WEAKEN THROUGH THE REMAINDER OF THE
FORECAST PERIOD. 

MODEL DISCUSSION: DETERMINISTIC MODEL GUIDANCE IS IN GOOD AGREEMENT
THROUGH TAU 12, BUT QUICKLY SPREADS THEREAFTER AS PROXIMITY TO 20W
DECREASES AND THE STORMS HAVE GREATER INFLUENCE ON ONE ANOTHER.
MODEL GUIDANCE IS VERY POOR AFTER TAU 24. THE DEGREE OF UNCERTAINTY
IN BINARY INTERACTION IS MORE SIGNIFICANT FOR 21W SINCE IT IS
FORECAST TO BE THE WEAKER STORM AND THEREFORE MORE SUSCEPTIBLE TO
INFLUENCE BY 20W. AS A RESULT, THE JTWC TRACK FORECAST IS PLACED
CLOSE TO THE MULTI-MODEL CONSENSUS WITH VERY LOW CONFIDENCE.
RELIABLE INTENSITY AIDS ARE IN GOOD AGREEMENT IN A GRADUAL
WEAKENING THROUGH TAU 60. AN EMERGING POSSIBILITY WITHIN MODEL
GUIDANCE IS THAT 21W WILL BE FORCIBLY DISSIPATED BY 20W MUCH
EARLIER THAN TAU 60, FURTHER DEGRADING THE JTWC INTENSITY FORECAST.
THE JTWC INTENSITY FORECAST IS ALIGNED WITH THE MULTI-MODEL
CONSENSUS, HAFS-A, AND EXPERIMENTAL GOOGLE DEEPMIND WITH LOW
CONFIDENCE.

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