Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
247 PM EDT Tue May 29 2018

Valid 12Z Sat Jun 02 2018 - 12Z Wed Jun 06 2018

During the days 4-8 (Sat-Wed) period most guidance suggests that
the overall flow aloft will be fairly blocked as a mean ridge
prevails over much of northwestern Canada/Mainland Alaska into
portions of the Arctic while an upper low and surrounding trough
that reach the Aleutians/Bering Sea early in the period lingers
for multiple days.  This circulation may display some
east-southeastward elongation with time.

The primary focus and greatest uncertainty in the forecast involve
the strong system likely reaching the western Aleutians/Bering Sea
on Sat.  Early in the period there is decent ensemble mean support
for a track into the west-central Bering, close to an average
among the 00Z ECMWF/CMC and recent GFS runs.  Thus for a single
deterministic forecast an operational model blend among those
solutions will provide the best representation of system detail. 
However it is important to note that even for the weekend portion
of the forecast the past couple UKMET runs and some individual
ensemble members offer potential for a farther south track that
could even be south of the Aleutians.  After the weekend there
continue to be various possibilities for low evolution.  The
preferred ensemble mean approach would have the Bering low curl to
the southeast over or near the eastern Aleutians.  Recent GEFS
means become more progressive in a nod to some operational model
runs that develop one or more leading frontal waves while the
parent low weakens or retrogrades/becomes absorbed in an upstream
feature.  Tilting the forecast more toward the 00Z ECMWF mean
relative to the 06Z GEFS mean by Tue-Wed accounts for the general
trend in the means toward a farther southeast track at that time
but with a more moderate change in continuity.

The above preferred blends (operational runs early followed by a
fairly rapid transition to the means) represent consensus well for
the mean ridge aloft over and northwest of the Yukon while
downplaying low confidence smaller scale detail differences
within/around the ridge.  The forecast also depicts the model/mean
consensus for a modest system tracking south of the Alaska
Panhandle during the first half of the period.

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