Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
301 PM EDT Wed Apr 25 2018

Valid 12Z Sun Apr 29 2018 - 12Z Thu May 03 2018

A reasonable majority of guidance continues to show a tendency for
mean ridging from the northeastern Pacific into Mainland
Alaska/Yukon with mean troughing upstream.  However there are
still significant differences for some aspects of the forecast,
including systems crossing the Bering Sea toward and possibly into
the mainland as well as the extent to which North
Pacific/Aleutians/southern Bering flow may at least temporarily
dampen the downstream ridge.  Today there is somewhat better
clustering/definition of an upper low over/near the Chukchi Sea
that will support a surface low near the northwest corner of the
mainland, with a gradual northwestward drift of the overall
feature during the period.  Consensus maintains a stationary front
near the northern coast of the mainland.

In general for forecasts valid from Sun onward recent GFS runs
have been more eager to bring North Pacific to southern Bering
energy into the downstream ridge relative to other guidance.  00z
ECMWF/CMC runs and their ensemble means are similar in principle
while the 06z GEFS mean trended somewhat faster than the 00z
version to reflect a hedge toward the even faster operational GFS
runs (whose evolutions aloft lead to a Gulf wave by Wed not
reflected in most ensemble members).  The 12z GFS has adjusted a
little away from 00z/06z GFS traits with an upper trough position
close to the 160-165W location seen in the 06z GEFS mean as of 12z
Tue.  After that time 12z GFS trough amplitude becomes greater
than a majority of ensemble members.  For the first half of the
forecast period prefer a blend among the 12z GFS and 00z ECMWF/CMC
ideas (with small input of 06z GEFS/00z ECMWF mean), as trends
have supported some of the more progressive GFS concepts for
shortwaves now in the short range time frame but the full array of
guidance still leads to some skepticism of the GFS scenario by day
4 Sun and beyond.  This blend reflects the consensus ideas over
the northern mainland and Chukchi Sea.

Later in the period there has been a signal from some model runs
and a decent number of ensemble means toward the idea of another
well-defined low tracking into/through the Bering around days 7-8
Wed-Thu.  At the same time the means offer decent agreement on the
downstream ridge while operational runs continue to show various
ideas for flow details.  Thus for the latter part of the forecast
the blend leans mostly toward the 00z ECMWF/NAEFS means and 06z
GEFS mean with a very small 00z ECMWF/CMC component maintained
given some similarity to the overall pattern of the means.


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