Advisories

WDPN32 PGTW 282100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (ETAU) WARNING NR 
008//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 26.2N 166.0E
   INITIAL INTENSITY: 45 KTS
   GEOGRAPHIC REFERENCE: 415 NM NORTH OF WAKE ISLAND
   MOVEMENT PAST 6 HOURS: NORTH-NORTHWESTWARD AT 11 KTS
   SIGNIFICANT WAVE HEIGHT: 23 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
STORM (TS) 21W (ETAU) AS A FULLY EXPOSED AND COMPACT LOW-LEVEL
CIRCULATION CENTER (LLCC). THE PERSISTENT CONVECTION THAT HAD
PREVIOUSLY PARTIALLY OBSCURED THE LLCC HAS NOW BEEN SHEARED TO THE
EAST BY THE VERY STRONG WESTERLY VERTICAL WIND SHEAR (VWS). THE
281721 SSMIS MICROWAVE IMAGERY CONFIRMS 21W IS A VERY SHALLOW
SYSTEM, AND ALSO REVEALS ETAU'S LOW LEVEL CLOUD BANDS ARE FEEDING
INTO TYPHOON (TY) 20W TO THE SOUTH. ENVIRONMENTAL ANALYSIS REVEALS
A MARGINAL ENVIRONMENT, CHARACTERIZED BY HIGH (30-35 KTS) VWS, WARM
(27-28 C) SEA SURFACE TEMPERATURES (SST), GOOD POLEWARD OUTFLOW.
THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE
FULLY EXPOSED NATURE OF THE LLCC. THE INITIAL INTENSITY OF 45 KTS
IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE AGENCY DVORAK FIXES
AND THE AUTOMATED CIMSS ESTIMATES LISTED BELOW.

INITIAL WIND RADII BASIS: PERSISTENCE 

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: T2.0 - 30 KTS
   PHFO: T3.0 - 45 KTS
   CIMSS SATCON: 40 KTS AT 281700Z
   CIMSS ADT: 34 KTS AT 281940Z
   CIMSS AIDT: 37 KTS AT 281940Z
   CIMSS D-MINT: 34 KTS AT 281721Z
   CIMSS D-PRINT: 35 KTS AT 281830Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 30-35 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: MODERATE POLEWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: LOW
   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 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 24, 21W IS EXPECTED TO CURVE NORTH-NORTHEASTWARD AS AN
UPPER-LEVEL TROUGH BREAKS DOWN THE STEERING RIDGE. ADDITIONALLY,
21W IS EXPECTED TO BEGIN INTERACTING WITH TY 20W AROUND TAU 36,
WHICH WILL FURTHER REINFORCE THE NORTHEASTWARD TRACK THROUGH TAU
72. INTERACTION WITH TS 21W 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 60 AND COMPLETE TRANSITION NO LATER THAN TAU 72.
BY TAU 72, 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 24, BUT QUICKLY SPREADS THEREAFTER AS PROXIMITY TO 20W
DECREASES AND THE STORMS INFLUENCE ONE ANOTHER. MODEL GUIDANCE IS
VERY POOR AFTER TAU 36. 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 LOW CONFIDENCE. RELIABLE INTENSITY AIDS
ARE IN GOOD AGREEMENT IN A GRADUAL WEAKENING THROUGH TAU 72. AN
EMERGING POSSIBILITY WITHIN MODEL GUIDANCE IS THAT 21W WILL BE
FORCIBLY DISSIPATED BY 20W MUCH EARLIER THAN TAU 72, FURTHER
DEGRADING THE JTWC INTENSITY FORECAST. THE JTWC INTENSITY FORECAST
IS ALIGNED WITH THE MULTI-MODEL CONSENSUS WITH LOW CONFIDENCE.

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