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Alaska Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1848Z Jun 16, 2018)
 
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Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
248 PM EDT Sat Jun 16 2018

Valid 12Z Wed Jun 20 2018 - 12Z Sun Jun 24 2018

Latest ensemble means paint a fairly agreeable picture for the
overall evolution aloft during the period.  Upper troughing with
one or more embedded lows over and west of the extreme western
mainland may weaken somewhat over the course of the period while
western Canada upper ridging should extend into portions of the
mainland.  A central Pacific shortwave and possibly some upstream
energy over the western Aleutians may track toward/into the Gulf
of Alaska while a western Pacific system comes into the picture
south of the western Aleutians next weekend.  Across the Arctic
the means show continued ejection of short range energy while
subsequent high latitude troughing will most likely set up farther
westward, well northwest of the mainland.

Looking more closely at individual models/ensemble members, there
is a fair degree of uncertainty regarding specifics within the
overall Bering Sea and extreme western mainland upper trough.  In
particular there is the question of relative strength and
interaction among an initial upper low center near the
southwestern mainland and another bundle of energy most likely to
drop southward from the Bering Strait/eastern Siberia.  Confidence
is low in any particular solution so would overall prefer a
centered/intermediate scenario while awaiting better convergence
for details.  Farther northward individual model runs are quite
varied/inconsistent with Arctic flow, favoring a blended/ensemble
mean approach over the higher latitudes.

GFS/ECMWF runs have shown decent continuity since the 12Z/15 cycle
for the central Pacific wave that may track northeastward
toward/into the Gulf of Alaska.  Individual ensemble members
display improved clustering compared to 24 hours ago but still
display a decent amount of spread.  It may still take some time to
resolve the details of this system as they will depend on the
combination of initial middle Pacific shortwave energy and
upstream impulse over the western Aleutians.  The relative scale
of these upper features is sufficiently small to suggest lower
predictability at extended time frames.  The 06Z GFS strayed south
of the best model/mean cluster late in the period while the 12Z
run has adjusted back somewhat closer to the majority cluster. 
Maintaining some operational model input through day 8 Sun helps
to maintain better definition of the low pressure system versus
the means.  

So far the ensemble means and ECMWF runs have been the most
consistent with the forecast of western Pacific low pressure whose
influence may extend into the western Aleutians by next weekend. 
The 06Z/12Z GFS runs are well suppressed compared to the ensemble
mean/ECMWF cluster.

Based on combined preferences by system/region, the forecast
started with a blend of the 00Z-12Z GFS/00Z ECMWF and to a lesser
extent 00Z CMC for day 4 Wed followed by a gradual increase of 06Z
GEFS/00Z ECMWF ensemble means with total ensemble mean weight
reaching about half by day 8 Sun.  The blend excluded the 12Z GFS
late in the period due to its departure from best model/mean
clustering over the Pacific and straying a bit southeast with its
Arctic upper trough axis.

Rausch


WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can be found at:

http://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif
www.wpc.ncep.noaa.gov/alaska/akmedr.shtml
http://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html