Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
227 PM EST Thu Feb 14 2019

Valid 12Z Mon Feb 18 2019 - 12Z Fri Feb 22 2019

Today's guidance provides similar ideas for the general pattern
aloft, with fairly progressive flow between Siberia/Kamchatka mean
troughing and east-central Pacific mean ridging.  This pattern
should carry a series of wind/precipitation-enhancing systems
across the Bering Sea into Mainland Alaska with some of the
shortwave energy helping to support triple point waves over the
Gulf of Alaska.  There are discrepancies over location and
magnitude of highest 5-day precipitation totals but best potential
exists over the western/southern mainland and from the eastern
Aleutians to the Panhandle.

To start the forecast, the 12Z GFS and 00Z ECMWF/CMC provide the
best clustering to represent operational model detail for the
leading Bering system expected to track into the mainland by day 5
Tue as well as the triple point wave quickly crossing the Gulf. 
Models and ensemble members as a whole have trended noticeably
farther northward for Bering low track compared to forecasts from
24 hours ago.

Details behind this first system become more uncertain.  There is
a reasonable signal for the next system(s) to reach the Bering Sea
during Tue-Wed but as a whole operational models are suggesting
two waves--a leading wave tracking northeast from the mid-latitude
Pacific (with very uncertain latitude, GFS runs generally farthest
south) and another crossing the western Bering--versus the
ensemble means that depict either a more consolidated system or
Pacific wave that is slower than the operational models.  As a
transition from the model-focused Mon-Tue forecast and to account
for the various possibilities by Wed, forecast preferences trend
to starting with a model/ensemble mean blend for midweek.  This
solution is ultimately faster than continuity but seemingly
reasonable based on the progression of mean flow.

Toward days 7-8 Thu-Fri there is a decent level of model/ensemble
mean agreement toward an amplifying western Pacific upper trough
that will support low pressure eventually tracking into the
extreme western Bering Sea, an upper ridge building toward the
mainland, and southeastward progression of another Gulf wave. 
During this time frame the 06Z GFS looks like the most
questionable solution with a Bering/Aleutians shortwave that is on
the deep side relative to most guidance.  Very good clustering
among the ensemble means by the end of the week favors primary use
of the means to represent the expected pattern at that time. 

Over the course of the week there is a general theme of
progression across the Arctic but poor agreement on the details. 
At the very least the 00Z ECMWF seems to have the least-supported
solution, depicting a deeper/more amplified mid-late week upper
feature tracking north of the mainland versus a large majority of
guidance.   

Based on forecast considerations the forecast starts with a blend
of the 12Z GFS and 00Z ECMWF/CMC Mon into Tue and then rapidly
increases ensemble mean weight thereafter so that the means
comprise 80-100 percent of the blend by Wed-Fri.  The ECMWF mean
has been somewhat more consistent than the GEFS mean over the past
1-2 days so the former has about twice the weight of the latter.

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