Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
336 PM EDT Sun May 13 2018

Valid 12Z Thu May 17 2018 - 12Z Mon May 21 2018

The strong upper ridge that becomes established over western
Canada and Mainland Alaska appears to be the one feature that has
reasonable confidence during the period.  GFS trends over the past
day have added to the consensus idea of keeping this ridge fairly
strong while the 00z CMC is now the one solution that strays from
the majority cluster.  On the other hand, already moderate to
lower confidence for upstream features has decreased over the past
day given run to run variability/spread in operational solutions
and broader ensemble spread over some areas.

Guidance continues to have difficulty in resolving individual
bundles of energy/one or more closed lows within an area of
general mean troughing over/near the eastern Bering Sea and
extreme western Mainland Alaska.  Further complicating the
forecast are individual Pacific/Aleutians systems that may or may
not interact with the aforementioned features.  There is some
loose agreement on a leading system near or south of the eastern
Aleutians/southwest Alaska Peninsula to start the period.  However
potential tracks diverge widely from early in the period, plus
ensembles have become more scattered with one or more trailing
North Pacific waves--with somewhat of a northward shift in some
members.  This spread for the initial system includes the 12z
GFS/ECMWF which show a relatively new scenario of a mostly
northward track.  Overall preferred a blend of model/ensemble mean
guidance (06z GFS/00z ECMWF and their means, 70 percent
operational model weight day 4 and 50 percent days 5-7) to add
some detail to the extent possible while also incorporating the
more stable solutions of the means.  ECMWF means have been
trending steadily stronger for Aleutians low pressure around
mid-period so its weight in the forecast was greater than that of
the GEFS mean.  As in past days the best signal for a period of
enhanced moisture flow extends across the Alaska Peninsula to
Kenai Peninsula but detail spread keeps confidence low for
extent/duration of this flow.

Toward the end of the period the most significant question is
how/to what extent flow splits.  By day 8 Mon the ECMWF mean
brings its Aleutians system somewhat farther east or north versus
other solutions--between the 00z CMC mean that holds it back over
the Alaska Peninsula and 00z ECMWF/00z-06z GFS that take it well
southward.  Persistence of the downstream ridge aloft seems to
favor a more even weight among the ensemble components (70 percent
total for day 8) by this part of the forecast with the lingering
operational input providing an extra nudge from the ECMWF mean.

Elsewhere, for the time being there is decent agreement for a
western Pacific system whose influence may extend into the western
Aleutians by day 8 Mon.  Before arrival of 12z guidance there was
a fairly strong consensus toward persistent surface high pressure
over the Arctic.  However some 12z solutions that bring up low
pressure along the eastern Bering and vicinity would disrupt this
surface ridge.


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

Rausch