Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
221 PM EST Mon Feb 18 2019

Valid 12Z Fri Feb 22 2019 - 12Z Tue Feb 26 2019

Guidance continues to advertise the transition to a very amplified
pattern, featuring a strong ridge that extends from the
northeastern Pacific through Mainland Alaska/extreme northwestern
Canada into the Arctic and farther west a broad mean trough
anchored by an upper low near Kamchatka.  Flow between these two
features should bring at least a couple strong storms from the
Pacific into the central-western Bering along with associated
strong winds/high waves/enhanced precipitation across the North
Pacific/Aleutians/Bering.  Some of the wind/moisture may reach the
Alaska Peninsula/western mainland as fronts approach the mean
ridge aloft.  The upper ridge should promote a drier trend farther
eastward.

Early in the period (Fri-Sat) a blend of 06-12Z GFS and 00Z
ECMWF/CMC represents the best combination of detail/intermediate
position for significant features.  Low pressure forecast near the
southern Panhandle as of 12Z Fri has trended a bit faster/eastward
from continuity.  The system shown over the northern Bering at
that time has fluctuated widely in recent runs.  Signals were
inconsistent but generally northward 24-36 hours ago, then jumped
south in the 12Z/17 cycle, and have now reverted back to the
north.  The storm tracking across the western Aleutians Fri-Sat
has trended somewhat slower/eastward, with ensemble means
providing a good center point between the fast 00Z GFS (and much
faster 06Z FV3 GFS) and slow 00Z ECMWF.  The 06Z GFS is closer to
consensus for the northern Bering low (versus westward 12Z run)
while the 12Z GFS is closer to preferences for the western
Aleutians storm.

Then for days 6-8 Sun-Tue one ongoing uncertainty is whether the
amplifying ridge aloft ultimately closes off a high.  Operational
runs over recent days have been signaling at least moderate
potential for a closed high but poor agreement for location of a
high center.  Recent ensemble means have been showing more
potential though the 00-06Z cycles have trended a bit south of the
12Z/17 means.  The 12Z GFS has backed off from its 00-06Z runs and
the 00Z ECMWF does not close off a high either.  A blend of
model/ensemble mean solutions to yield just a subtle hint of an
upper high south of the mainland seems to provide the best single
starting point at this time.  Meanwhile there is a reasonable
degree of continuity in principle for the next storm that may
track across the western Aleutians during around Sun-Mon, with an
average of model/ensemble mean solutions a reasonable approach to
resolve various track/timing differences among individual runs. 
There is still considerable spread for potential waviness along
the fronts extending from the Pacific-Bering systems and a
possible low-mid latitude central Pacific feature that could be
drawn up ahead of a front during the weekend.  Thus far clustering
for these smaller features has not been great so prefer not to
represent them too strongly in the manual forecast.  The mid-late
period blend gradually increases 06Z GEFS/00Z ECMWF mean weight
from 40 percent to 70 percent while keeping a higher proportion of
the ECMWF mean due to its deeper surface low over the Bering
versus the GEFS mean.

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