Advisories

WDPN32 PGTW 291500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (ETAU) WARNING NR 
011//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 30.0N 165.4E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 645 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 ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A
LOW-LEVEL CIRCULATION CENTER (LLCC) PARTLY OBSCURED BY A MASSIVE
THUNDERSTORM. THIS CONVECTION HAS PERSISTED NEAR THE CENTER FOR
OVER 06 HOURS IN THE FACE OF HIGH SOUTHWESTERLY VERTICAL WIND SHEAR
(VWS) OF 25-30 KTS. THIS IS IN LARGE PART ENABLED BY STRONG
POLEWARD OUTFLOW ALOFT INTO A CHANNEL OF 50-60 KT GEOSTROPHIC WINDS
INDUCED BY A TROUGH NORTHWEST OF THE SYSTEM. THE SURFACE
CIRCULATION CONTINUES TO SPIN VIGOROUSLY IN THE SHORTWAVE-INFRARED
IMAGERY (SWIR), ALTHOUGH THE LACK OF CONVECTION ON THE UPSTREAM
SIDE OF THE VWS HAS RESULTED IN AN INCREASINGLY ELONGATED LLCC. THE
INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE
PARTIALLY EXPOSED LLCC OBSERVED IN THE SWIR IMAGERY. BECAUSE THE
SYSTEM STRUCTURE HAS CHANGED LITTLE IN THE PAST 06 HOURS, THE
INITIAL INTENSITY HAS BEEN HELD AT 40 KTS, WHICH IS SUPPORTED BY A
BLEND OF THE T2.5-3.0 SUBJECTIVE DVORAK FIXES AND THE RECENT
OBJECTIVE ESTIMATES FROM CIMSS RANGING FROM 33 KTS TO 38 KTS.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: WESTERN FLANK OF AN EXTENSIVE LOW- TO
MID-LEVEL RIDGE OVER THE CENTRAL NORTH PACIFIC AND POSSIBLE
INTERACTION WITH TYPHOON (TY) 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 SATCON: 38 KTS AT 290510Z
   CIMSS ADT: 33 KTS AT 291210Z
   CIMSS AIDT: 34 KTS AT 291210Z
   CIMSS D-MINT: 38 KTS AT 291113Z
   CIMSS D-PRINT: 33 KTS AT 291210Z

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

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: THE FORECAST CALLS FOR A CONTINUED POLEWARD
MOTION FOR THE NEXT 12 HOURS, FOLLOWED BY A TURN TOWARD THE
NORTH-NORTHEAST. THE NORTHWARD TRAJECTORY IS A RESULT OF AN
EXTENSIVE LOW- TO MID-LEVEL RIDGE CENTERED EAST OF THE DATELINE.
AFTER TAU 12, AN APPROACHING TROUGH WILL INDUCE AN EASTWARD TURN.
AS TS 21W GAINS LATITUDE, IT WILL TRAVERSE OVER PROGRESSIVELY
COOLER WATERS, CROSSING THE 26 DEGREE ISOTHERM INSIDE 12 HOURS AND
ENTER A MORE BAROCLINIC ENVIRONMENT. WHEN THIS HAPPENS, THE SYSTEM
WILL GRADUALLY LOSE TROPICAL CHARACTERISTICS AND TRANSITION INTO A
SUBTROPICAL STORM. THE INTENSITY CHANGE THROUGH THE FORECAST IS
MINOR AS A RESULT OF INCREASING BAROCLINICITY ASSOCIATED WITH LIGHT
THERMAL ADVECTION AND THE APPROACHING TROUGH. THERE REMAINS
SIGNIFICANT UNCERTAINTY IN THE FORECAST. SEVERAL MODELS, INCLUDING
GFS AND HAFS-A, EXPLICITLY DEPICT A LOOP TOWARD THE SOUTH IN
RESPONSE TO THE FUJIWHARA EFFECT WITH TY 20W. IF THIS OCCURS, THEN
TS 21W WILL ENCOUNTER MORE FAVORABLE CONDITIONS THAT MAY RESULT IN
SLIGHT INTENSIFICATION BETWEEN TAU 24 AND TAU 60 AND PROLONG ITS
DEMISE. THIS ALTERNATE SCENARIO WOULD NECESSITATE A DRASTIC CHANGE
TO THE TRACK AND INTENSITY FORECAST PHILOSOPHY. THE NEXT 12-24
HOURS WILL TELL THE STORY.

MODEL DISCUSSION: MODELS AGREE ON A GENERAL POLEWARD MOTION FOR THE
FIRST 12 HOURS. THE ECMWF AND THE EXPERIMENTAL GOOGLE DEEPMIND
SOLUTIONS TRACK THE SYSTEM NORTHWARD FOR THE FIRST 12 HOURS,
FOLLOWED BY A TURN TOWARD THE NORTHEAST. AS PREVIOUSLY MENTIONED,
THE GFS AND HAFS-A MODELS LOOP THE SYSTEM SOUTHWARD. SIGNIFICANT
ALONG-TRACK AND CROSS-TRACK SPREAD EXISTS, OWING TO DIFFERING
LEVELS OF BINARY INTERACTION WITH TY 20W. THE JTWC FORECAST TRACK
IS PLACED WITH LOW CONFIDENCE NEAR THE CENTER OF THE GROUP OF
MODELS THAT EXCLUDE THE SOLUTIONS LIKE GFS AND HAFS-A AS THEY
DEPICT A COMPLETELY DIFFERENT SCENARIO. THE INTENSITY AIDS ALSO
EXHIBIT HIGH SPREAD, LARGELY ATTRIBUTED TO THE WIDELY VARYING
SCENARIOS. THE JTWC INTENSITY FORECAST IS ALIGNED WITH THE
MULTI-MODEL CONSENSUS FOR THE FIRST 12 HOURS AND THEN LEANS INTO
DEEPMIND AFTER TAU 12. 

FORECAST CONFIDENCE:
   TRACK 00-72 HR: LOW
   TRACK 72-120 HR: ---
   INTENSITY 00-72 HR: LOW
   INTENSITY 72-120 HR: ---//
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