WDPN32 PGTW 292100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (ETAU) WARNING NR 
012//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 30.7N 164.9E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 690 NM NORTH OF WAKE ISLAND
   MOVEMENT PAST 6 HOURS: NORTH-NORTHWESTWARD AT 08 KTS
   SIGNIFICANT WAVE HEIGHT: 25 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
STORM (TS) 21W (ETAU) AS A HIGHLY SHEARED AND MOSTLY EXPOSED
LOW-LEVEL CIRCULATION CENTER NEARLY COMPLETELY INUNDATED WITH DRY
AIR WRAPPING INTO THE CORE. ADDITIONALLY, 21W HAS NEARLY STALLED IN
ITS POLEWARD MOTION AS THE GRIP OF TYPHOON (TY) 20W STRENGTHENS
WITH CLOSER PROXIMITY. ENVIRONMENTAL ANALYSIS REVEALS AN
UNFAVORABLE ENVIRONMENT, CHARACTERIZED BY MODERATE TO HIGH (20-25
KTS) VERTICAL WIND SHEAR, DRY AIR ENTRAINMENT, MARGINAL (0-20 KJ
PER CM2) OCEAN HEAT CONTENT, AND TEMPERATE (26-27 C) SEA SURFACE
TEMPERATURES (SST). THE INITIAL POSITION IS PLACED WITH HIGH
CONFIDENCE BASED ON THE PARTIALLY EXPOSED LLCC IN ANIMATED EIR. THE
INITIAL INTENSITY OF 35 KTS IS ASSESSED WITH MEDIUM CONFIDENCE
BASED ON THE AUTOMATED CIMSS INTENSITY ESTIMATES AND AGENCY DVORAK
FIXES LISTED BELOW.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: THE WESTERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) CENTERED NEAR THE DATELINE AND BINARY
INTERACTION WITH TYPHOON 20W

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
   CIMSS ADT: 32 KTS AT 291940Z
   CIMSS AIDT: 36 KTS AT 291940Z
   CIMSS D-MINT: 33 KTS AT 291708Z
   CIMSS D-PRINT: 36 KTS AT 291910Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 20-25 KTS
   SST: 26-27 CELSIUS
   OUTFLOW: MODERATE POLEWARD
   OTHER FACTORS: DRY AIR ENTRAINMENT

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 IS FORECAST TO CONTINUE SLOWLY TRACKING
NORTHWESTWARD AS IT REMAINS TUGGED BETWEEN THE STEERING STR TO ITS
EAST AND 20W TO ITS SOUTHEAST. AN EMERGING POSSIBILITY IN MODEL
GUIDANCE SUGGESTS THAT 21W MAY BE FORCED EQUATORWARD BY 20W BETWEEN
TAU 0-24 BEFORE RESUMING ITS FORECASTED POLEWARD TRACK. AS 21W
TRACKS POLEWARD, IT WILL ENCOUNTER INCREASINGLY HOSTILE CONDITIONS,
WITH SSTS DROPPING BELOW 26C BY TAU 12 AND VWS INCREASING UP TO 30
KTS BY TAU 48, ULTIMATELY INDUCING SUBTROPICAL TRANSITION BY TAU 36.
21W IS FORECAST TO COMPLETE SUBTROPICAL TRANSITION BY TAU 48.
HOWEVER, SHOULD 21W MAKE A MOMENTARY DETOUR EQUATORWARD, CONDITIONS
WOULD IMPROVE TEMPORARILY BETWEEN TAU 0-24, OPENING THE POSSIBILITY
FOR SLIGHT STRENGTHENING. INTENSIFICATION WOULD NOT BE SIGNIFICANT
DURING THIS TIME, LIKELY 5-10 KTS, BUT REMAINS POSSIBLE SHOULD 21W
ENTER THE SLIGHTLY MORE FAVORABLE ENVIRONMENT. ADDITIONALLY, IN THE
EQUATORWARD LOOP SCENARIO, 21W WOULD LIKELY RETAIN TROPICAL
CHARACTERISTICS FOR LONGER THAN CURRENTLY FORECAST, WHICH WOULD
REQUIRE A SIGNIFICANT ADJUSTMENT TO THE CURRENT FORECAST
PHILOSOPHY.

MODEL DISCUSSION: DETERMINISTIC MODEL TRACK GUIDANCE REMAINS IN
POOR AGREEMENT, WITH MODELS GENERALLY SPLIT INTO TWO GROUPS: THOSE
THAT FAVOR AN EQUATORWARD LOOP IN RESPONSE TO INTERACTION WITH 20W
AND THOSE THAT FAVOR A CONTINUOUS POLEWARD TRACK. MODELS THAT
DEPICT THE LOOP INCLUDE GFS, GEFS ENSEMBLE MEAN, HAFS-A,
EXPERIMENTAL AIGFS. MODELS DEPICTING A DIRECT POLEWARD TRACK
INCLUDE ECMWF, UKMET, GALWEM, NAVGEM, GSM, AND EXPERIMENTAL AI
MODELS GOOGLE DEEPMIND AND AIGEFS. THE SPLIT IN MODEL GUIDANCE
INJECTS SIGNIFICANT UNCERTAINTY INTO THE JTWC TRACK FORECAST, WHICH
IS PLACED NEAR THE EXPERIMENTAL GOOGLE DEEPMIND AND MULTI-MODEL
CONSENSUS WITH VERY LOW CONFIDENCE. THE INTENSITY AIDS ALSO EXHIBIT
HIGH SPREAD, LARGELY ATTRIBUTED TO THE WIDELY VARYING SCENARIOS.
THE JTWC INTENSITY FORECAST IS HEDGED BETWEEN HAFS-A AND THE
MULTI-MODEL CONSENSUS WITH LOW CONFIDENCE.  

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