Prognostic Meteorological Discussion
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FXAK02 KWNH 241855
PMDAK

ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
254 PM EDT SAT JUN 24 2017

VALID 12Z WED JUN 28 2017 - 12Z SUN JUL 02 2017

LATEST GUIDANCE GENERALLY SHOWS A PERSISTENT MEAN RIDGE ALOFT OVER
NWRN CANADA.  CONSENSUS SUGGESTS THAT A PORTION OF THIS RIDGE WILL
LIKELY BUILD ACROSS CNTRL-NRN MAINLAND ALASKA WHILE ENERGY ASSOC
WITH A BERING SEA UPR LOW AND/OR SURROUNDING FLOW GRADUALLY
MIGRATES TOWARD THE NERN PACIFIC/GULF OF ALASKA.  FARTHER UPSTREAM
MOST MODELS/MEANS INDICATE HIGH LATITUDE FLOW DROPPING SWD AND
HELPING TO CARVE OUT A SHARPENING TROUGH WITH AN AXIS NEAR
160-170E LONGITUDE BY NEXT WEEKEND.  THE MOST PROBLEMATIC PART OF
THE FCST WITHIN THIS RELATIVELY AGREEABLE LARGE SCALE EVOLUTION
CONTINUES TO BE RESOLVING INDIVIDUAL WAVES ASSOC WITH MID-LATITUDE
PACIFIC ENERGY STREAMING TOWARD THE GULF OF ALASKA AND EVENTUAL
MOVEMENT OF THE BERING SEA UPR LOW.

PREDICTABILITY AND CONFIDENCE STILL LOOK BELOW AVG FOR ONE OR MORE
WAVES THAT WILL LIKELY TRACK FROM THE MID-LATITUDE PAC TOWARD
KODIAK ISLAND/GULF OF ALASKA WITH UPR SUPPORT CARRIED ALONG BY
FAST LOW AMPLITUDE MEAN FLOW.  WITHIN THIS STREAM THE MOST
TRACKABLE UPR SUPPORT ORIGINATES FROM A SMALL UPR LOW CURRENTLY TO
THE N OF JAPAN AND IS FCST TO REACH EITHER SIDE OF 170E LONGITUDE
AS AN OPEN WAVE BY THE START OF THE PERIOD 12Z WED.  FOR A TIME
THE 00Z-06Z GFS SFC REFLECTION SEEMS TO LAG THE UPR SUPPORT,
LENDING SOME DEGREE OF SKEPTICISM TO GFS DETAILS.  THROUGH THE
ARRIVAL OF THE 12Z GFS, RECENT GFS RUNS HAVE BEEN AT THE LEADING
EDGE OF THE FULL ENSEMBLE SPREAD FOR A PSBL LEADING WAVE REACHING
JUST OFF KODIAK ISLAND AS EARLY AS DAY 4 WED.  THE 00Z ECMWF (AND
00Z CMC ALBEIT WITH DIFFERING DETAILS ELSEWHERE) AND ECMWF MEAN
OFFERED AN EVOLUTION THAT SEEMED A LITTLE MORE LIKELY THAN
ALTERNATIVES WHILE MAINTAINING A FAIR DEGREE OF CONTINUITY FOR THE
SFC GRADIENT OVER THE GULF OF ALASKA.  WITH THE ARRIVAL OF
REMAINING 12Z GUIDANCE THE POTENTIAL REMAINS FOR A LEADING WAVE
THOUGH PERHAPS SLOWER THAN THE GFS/UKMET, WHILE DEFINITION OF
UPSTREAM WRN PAC ENERGY IS UP FOR DEBATE.  WHILE 12Z ECMWF/CMC
DETAILS HAVE CHANGED SOMEWHAT THE 12Z ECMWF STILL TRENDS CLOSER TO
CONTINUITY ONCE LOW PRESSURE NEARS KODIAK ISLAND.

MODELS/MEANS AS A WHOLE HAVE TRENDED TOWARD SOMEWHAT FASTER
MIGRATION OF THE BERING UPR LOW (AND/OR SURROUNDING SHRTWV ENERGY)
INTO THE GULF BY THE LATTER HALF OF THE PERIOD, WITH THE ECMWF
MEAN TENDING TO BE THE MOST STABLE PIECE OF GUIDANCE OVER THE PAST
1-2 DAYS.  THUS BERING SEA SFC LOW PRESSURE IS QUICKER TO WEAKEN
VS 24 HRS AGO.

BASED ON FCST PREFS THROUGH 12Z GFS ARRIVAL ALONG WITH CONTINUITY
CONSIDERATIONS, TODAY`S FCST BLEND STARTED WITH THE 00Z
ECMWF/ECMWF MEAN FOR DAY 4 WED SINCE THE 06Z GEFS MEAN SHOWED SOME
HINT OF OPERATIONAL GFS DETAILS AT THAT TIME.  THEN DAYS 5-8
THU-SUN INCORPORATED THE 06Z GEFS/00Z ECMWF MEANS WHILE
OPERATIONAL 00Z ECMWF WEIGHT GRADUALLY DECREASED FROM 50 PCT ON
THU TO 20 PCT BY SUN.

RAUSCH

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