Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
251 PM EST Wed Dec 26 2018

Valid 12Z Sun Dec 30 2018 - 12Z Thu Jan 03 2019

Ensemble means continue to be fairly similar and consistent for
the expected flow evolution in the days 4-8 period (Sun-Thu). 
They show an upper low near the western Aleutians as of early Sun
drifting into the Bering Sea while downstream ridging builds into
Mainland Alaska and western Canada.  By mid-late period there may
be some broadening of the pattern followed by a retrogression of
the mean trough axis to 160-170E longitude as upstream energy
emerges into the western Pacific.  Within this pattern--likely
favorable for one or more episodes of significant precipitation
from the Alaska Peninsula to the Panhandle--individual
models/ensemble members still differ considerably over individual
storm systems that would produce strong winds and enhanced
precipitation.

With meaningful model differences already evident at the start of
day 4 Sun, the forecast again incorporated more than the typical
weight of 06Z GEFS/00Z ECMWF ensemble means during the first half
of the period (50 percent total today) and then trended toward all
ensemble mean input by day 8 Thu.  In most respects the 00Z
ECMWF/CMC provided the most compatible operational model guidance
for refining the ensemble mean base.

The forecast is still very uncertain for individual systems
carried along by fast flow around the southern/eastern side of the
Aleutians/Bering Sea mean low aloft.  Through Sun-Mon GFS runs
have been farthest east with their low tracks that extend to the
Alaska Peninsula/Gulf of Alaska, which is on the eastern fringe of
the full ensemble envelope for that time frame.  The ECMWF has
shown an eastward trend over the past day by Tue, joining 00Z/06Z
GFS runs for another Gulf low.  12Z GFS wave timing is completely
different.  After Mon the ensemble means have displayed a modest
trend toward faster erosion of the downstream ridge aloft so there
is somewhat more potential for a wave to reach the Gulf of Alaska.
 However at this point not enough ensemble members have such a
wave to pull the means in that direction, as they generally have
low pressure simply settling over the vicinity of Prince William
Sound by days 7-8 Wed-Thu.  00Z ECMWF weight was sufficiently low
to downplay its Gulf wave on Tue, while using somewhat more ECMWF
mean relative to the GEFS mean on day 7 Wed helped to keep the
pattern over and near the mainland fairly close to continuity.  To
varying degrees the ECMWF/CMC ensemble means continue to show
lowest surface pressures farther northwest than the GEFS mean for
a majority of the period.  Note that the ensemble mean trends
would imply a shorter duration of heavy precipitation potential
for locations west of the Kenai Peninsula.

Meanwhile there is also a lot of spread for the ultimate track of
the very deep low likely to be near the western Aleutians at the
start of the period.  Seeing individual model solutions ranging
from a long period of wobbling in place or even retrograding, to
an eastward drift toward the mainland, leads to negligible
confidence and furthers the need to rely on the ensemble means to
anchor the forecast.  By day 8 Thu another system may come into
the picture depending on timing/evolution of western Pacific
energy.  The 06Z GEFS/00Z ECMWF mean blend used for the forecast
yields a low track a little south of the Aleutians.

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