Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
325 PM EDT Fri Apr 06 2018

Valid 12Z Tue Apr 10 2018 - 12Z Sat Apr 14 2018

Guidance continues to show two general regimes--a North
Pacific/southern Bering stream containing a series of systems,
with some potential interaction among features south/east of the
Alaska Peninsula, and a mainland/Arctic evolution that becomes
less defined after the start of the period that features an Arctic
ridge and weak energy over the north-central mainland.  As has
been the case over the past couple days or so, both operational
model and ensemble guidance seems to provide useful information
for different aspects of the forecast.

Across the mainland/Arctic, early period trends toward a farther
eastward Arctic ridge aloft represent a continuation of trends
seen 24 hours ago.  South of the ridge there is still weak
northern mainland energy that varies in guidance depiction for
shape/motion--leading to continued low confidence in any
particular solution.  The most common idea in today's guidance is
for the Arctic ridge to stretch/split in some fashion by around
Wed-Thu.  From late week into the weekend operational model
forecasts have been varying widely for details of flow aloft but
the most common idea is for the mainland to be under a col region.
 The ensemble means have been the most consistent with this theme
over the past couple days.

Looking at the North Pacific/Bering stream, notable trends include
a slightly longer-lasting/farther northwest low over the Bering as
of the start of the forecast early Tue and faster timing for the
system passing near the Aleutians around midweek.  To the
south/east of the Alaska Peninsula the primary uncertainty
involves how the succession of features will interact with leading
northeastern Pacific low pressure.  Some individual model
solutions over the past couple days have been suggesting potential
for one or two waves to track northwestward toward or near the
Peninsula/Kodiak Island with corresponding enhancement of
wind/precip.  Thus far there remains too much spread/variability
to follow a particular operational scenario over this area,
favoring a blended model/ensemble mean approach to emphasize the
larger scale patter that has better predictability.  For the
midweek Aleutians system there is good model support for a deeper
depiction than the ensemble means into early Wed but detail spread
becomes more apparent thereafter as it interacts with low pressure
to the east.  The resulting area low pressure should drift off to
the southeast by the end of the period.  Finally, relative to the
time frame involved there is still better than average clustering
for the next Aleutians system that comes into the picture by days
7-8 Fri-Sat.

In order to reflect the best combination of model/ensemble mean
ideas today's forecast incorporates the 06z-12z GFS/00z ECMWF and
the 06z GEFS/00z ECMWF means.  Operational weight starts at 70
percent early and decreases to near even weight with the means by
the latter half of the period.  The blend helps to enhance the
means for Pacific/Bering features while operational model
differences over the mainland/Arctic mid-late period generally
cancel out to keep the forecast closer to the means over those
areas.

Rausch