Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
238 PM EST Sun Feb 17 2019

Valid 12Z Thu Feb 21 2019 - 12Z Mon Feb 25 2019

The majority of guidance still indicates that the mean pattern
will transition from a progressive regime to a more amplified and
slower evolving one, featuring a mean trough/low aloft anchored
over Siberia/Kamchatka and ridge that extends from the
northeastern Pacific through Mainland Alaska and into the Arctic. 
This overall evolution should favor a drier trend over the
mainland along with high pressure becoming established over the
mainland/northwestern Canada while storms farther west over the
Bering (and associated fronts) promote periods of strong winds and
enhanced precipitation over the Aleutians/Bering Sea.  The eastern
fringe of moisture could extend into the western mainland.

The combination of guidance changes over the past day and in some
cases increased spread have made the forecast more difficult for
resolving individual features within the more agreeable pattern
evolution.  Already early in the forecast there are significant
differences for separate systems over the Gulf of Alaska and
southwestern Bering.  The Gulf system is more a matter of the past
two ECMWF runs becoming more separated from the westerlies than
nearly all other guidance except for about 10-20 percent of
ensemble members.  The 00Z ECMWF mean agrees well with other
models/means which keep the system much closer to the
coast/Panhandle.  Meanwhile through the 00-06Z ensemble cycles
there is at best minimal ensemble support for the 06-12Z GFS
southern track of the southwestern Bering system.  Outside of
suggesting a farther north/west track, ensembles are all over the
place and ultimately weaken the system over the northwestern
Bering a trailing stronger system approaches from the south.  In
spite of this lack of prior support, a quick review of new 12Z
guidance coming in actually strengthens the case for a 12Z GFS
type of solution.  Aloft the means have shown a 24-hour trend
toward somewhat more shortwave energy rounding the mean ridge in
the Fri-Sat time frame so the new 12Z clustering could be
plausible from those recent trends aloft.

Behind the initial southwestern Bering wave, there is some degree
of continuity with the idea of a strong trailing system reaching
the western Aleutians/Bering by days 5-6 Fri-Sat but then
considerable divergence on track thereafter.  Also there is still
a signal for some possible waviness along the leading front but
poor agreement on specifics.  Prefer to lean increasingly toward
the ensemble means with time as solutions differ on track
longitude and northward timing of the parent low as well as
frontal wave details.  The end result is a parent low center that
is a little eastward of yesterday's continuity.  Behind this
system the GEFS/ECMWF means suggest yet another storm taking a
similar track into the western Aleutians/Bering during days 7-8
Sun-Mon.

As for the upper ridge building into the mainland during the
latter half of the period, a blend dominated by the ensemble means
would provide good continuity from yesterday.  Operational model
runs continue to provide mixed signals regarding the possibility
that an upper high could close off within the overall ridge, with
the means providing the best conservative starting point (on open
ridge) until there is better agreement on when/where an upper high
closes off.  

Based on the array of guidance available through arrival of the
12Z GFS, the forecast started with a blend of ideas from recent
GFS runs/06Z GEFS mean and 00Z ECMWF/ECMWF mean/CMC days 4-5
Thu-Fri (leaning away from the 00Z ECMWF mean over the Gulf). 
Then the blend steadily trended more toward the ensemble means. 
Toward days 7-8 Sun-Mon the 12Z GFS pattern was close enough to
the means to allow for keeping a very small component of its
forecast in the blend.  Where appropriate winds were enhanced
relative to the initial blend.

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