Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
256 PM EDT Wed Mar 28 2018

Valid 12Z Sun Apr 01 2018 - 12Z Thu Apr 05 2018

The primary focus of today's forecast will be on a deepening
trough (likely to contain an embedded closed low eventually) that
most guidance shows developing over the mainland in response to
the retrogression of a strong upper high initially over the Bering
Sea.  The full array of model and ensemble guidance seems to
suggest better than average agreement in the large scale pattern
evolution.  However there are significant differences for one or
more pieces of energy flowing around the upper ridge/high--and to
a lesser degree path of the upper high--so there is still
meaningful uncertainty in some forecast details at the surface and
aloft.

Primary emphasis on operational guidance (06-12z GFS/00z ECMWF/00z
CMC) versus a small weight of ensemble means represents consensus
well for days 4-5 Sun-Mon.  By Mon most of the means start to
become a tad weaker with the upper high, with the operational
solutions likely better given stronger trends in the means over
the past day.  Prior to the 12z/27 cycle the GEFS means were
earlier to latch onto the upper high's strength versus the ECMWF
mean.  Shortwave differences are already becoming apparent to the
north/northeast of the upper high as well as over/near the Gulf of
Alaska, with the preferred blend providing a good intermediate
starting point given the low predictability of these details.

Days 6-8 Tue-Thu steadily increase 06z GEFS and 00z NAEFS/ECMWF
mean weight such that the end of the forecast reflects 80 percent
ensemble input.  Thus far there is at most 10-20 percent 00z
ensemble support for the past couple of ECMWF runs that bring
enough shortwave energy into the Bering Sea to generate a defined
surface wave to the west of the mainland, so prefer to downplay
that aspect of the ECMWF.  Meanwhile there is decent
model/ensemble support for a surface wave over the Gulf of Alaska
by around day 7 Wed in response to the mainland troughing/closed
low aloft.  Recent GEFS means are the main exception to that idea
with a much more ill-defined surface pattern.  There is plenty of
guidance spread for the finer details of the mainland upper
trough/closed low by Tue-Thu, favoring the fairly agreeable means
as the primary anchor of the forecast.

A significant question mark by the end of the period is how much
interaction there may be between the mainland into Bering upper
trough/low and energy that may track out of the north-central
Pacific.  While the ensemble means provide the best starting point
for the manual forecast, it is possible that they could be a
little too eager to phase the two streams.  Operational model
signals are mixed regarding how much stream interaction may occur
and when.  This issue will require monitoring over coming days.

Rausch