Advisories

WDPN32 PGTW 290900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (ETAU) WARNING NR 
010//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 28.2N 165.7E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 536 NM NORTH OF WAKE ISLAND
   MOVEMENT PAST 6 HOURS: NORTHWARD AT 18 KTS
   SIGNIFICANT WAVE HEIGHT: 20 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A PARTIALLY
EXPOSED LOW-LEVEL CIRCULATION CENTER (LLCC) THAT HAS RECENTLY
BECOME MORE OBSCURED BY A FLARE-UP OF DEEP CONVECTION OVER ITS
NORTHEAST PERIPHERY. THE LLCC IS QUITE ASYMMETRIC, BECOMING MORE
ELONGATED ALONG A SOUTHWEST-NORTHEAST AXIS. WHILE THE SYSTEM HAS
GOOD POLEWARD OUTFLOW INTO 80 KT GEOSTROPHIC FLOW INDUCED BY AN
UPPER-LEVEL TROUGH TO THE NORTHWEST, STRONG SOUTHWESTERLY VERTICAL
WIND SHEAR (VWS) OF 25-30 KTS MITIGATES ANY DEVELOPMENT POTENTIAL
FROM THE WARM SEA SURFACE TEMPERATURES (SST) OF 26-27 DEGREES
CELSIUS. OCEAN WIND SPEED VALUES DERIVED FROM A 290207Z AMSR2 PASS
WERE 40-45 KTS IN THE NORTHEAST QUADRANT. THE INITIAL POSITION IS
ASSESSED WITH HIGH CONFIDENCE BASED ON THE MSI. THE INITIAL
INTENSITY SET AT 40 KTS WITH MEDIUM CONFIDENCE, HEDGED TOWARD THE
AMSR2 WIND SPEED DATA AND THE HIGHER SIDE OF THE SUBJECTIVE DVORAK
AND OBJECTIVE INTENSITY ESTIMATES LISTED BELOW.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: WESTERN FLANK OF A LARGE RIDGE OVER THE
CENTRAL NORTH PACIFIC

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: 38 KTS AT 290510Z
   CIMSS ADT: 29 KTS AT 290600Z
   CIMSS AIDT: 30 KTS AT 290600Z
   CIMSS D-MINT: 33 KTS AT 290438Z
   CIMSS D-PRINT: 32 KTS AT 290600Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 25-30 KTS
   SST: 26-27 CELSIUS
   OUTFLOW: MODERATE POLEWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: MEDIUM

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: TROPICAL STORM (TS) 21W WILL CONTINUE POLEWARD
ALONG THE WESTERN PERIPHERY OF A LARGE RIDGE OVER THE CENTRAL NORTH
PACIFIC. AFTER TAU 24, A LOW- TO MID-LEVEL TROUGH APPROACHING FROM
THE NORTHWEST WILL PICK UP THE SYSTEM, TURNING IT TOWARD THE
NORTHEAST. THE HOSTILE ENVIRONMENTAL CONDITIONS WILL NOT RELENT.
STRONG VWS, DRY AIR, AND COOL SST WILL LEAD TO CONTINUED WEAKENING.
THE FORECAST PERMITS THE RECENT BURST OF CONVECTION TO REMAIN
INTACT OVER THE CENTER FOR THE FIRST 12 HOURS AND THEREFORE HOLDS
THE INTENSITY. HOWEVER, AS IT GAINS LATITUDE, IT WILL ENCOUNTER
EVEN COOLER WATERS AND LOSE MOST OF ITS CONVECTION. THE FORECAST
DEPICTS A GRADUAL SPIN-DOWN OF THE VORTEX AS THE SYSTEM UNDERGOES
SUBTROPICAL TRANSITION. SIGNIFICANT UNCERTAINTY EXISTS IN HOW
TYPHOON (TY) 20W WILL INTERACT WITH THIS STORM. THERE IS A DISTINCT
POSSIBILITY THAT, AS TY 20W CATCHES UP OVER THE NEXT 24 HOURS, TS
21W WILL BE PUSHED WESTWARD AT FIRST AND THEN SOUTHWARD UNDER THE
WELL-DOCUMENTED FUJIWHARA EFFECT. THIS SCENARIO WOULD NECESSITATE A
DRASTIC CHANGE IN THE FORECAST.

MODEL DISCUSSION: WHILE THE MAJORITY OF MODEL TRACKERS DEPICT THE
MOTION IN THE JTWC TRACK FORECAST, IT IS DIFFICULT TO TEASE OUT
WHICH MODELS LOSE THE CIRCULATION OF TS 21W AND LATCHES ONTO TY 20W
AND WHICH MODELS GENUINELY PREDICT A NORTHEASTWARD TRAJECTORY. THE
JGSM, GFS, AND ECMWF SOLUTIONS CLEARLY LOSE THE CIRCULATION AFTER
TAU 12-24, WHILE HAFS-A AND VARIOUS OTHER AI MODELS CONTINUE WITH A
POLEWARD MOTION. DUE TO THE UNCERTAINTY ASSOCIATED WITH THE BINARY
INTERACTION OF TS 21W AND TY 20W, THE JTWC TRACK FORECAST IS
POSITIONED WITH LOW CONFIDENCE NEAR THE AI CONSENSUS. LIKEWISE, THE
JTWC INTENSITY FORECAST IS SET WITH LOW CONFIDENCE, LEANING INTO
THE HAFS-A, GFS, AND EXPERIMENTAL GOOGLE DEEPMIND SOLUTIONS, WHICH
DEPICT GRADUAL WEAKENING. GFS REDEVELOPS THE SYSTEM UNDER THE
FUJIWHARA SCENARIO IN WHICH AN EVENTUAL TRACK TOWARD THE SOUTH
WOULD RESULT IN MORE FAVORABLE ENVIRONMENTAL CONDITIONS.

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