Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
658 PM EST Thu Dec 31 2020
Valid 12Z Mon Jan 04 2021 - 12Z Fri Jan 08 2021
...Overview and Guidance/Uncertainty Assessment...
Systems carried along or influenced by progressive North Pacific
flow, and feeding into a general area of northeastern Pacific mean
troughing aloft, continue to provide the main uncertainties of the
forecast in terms of sensible weather effects. Farther north
there are ongoing detail differences but reasonable agreement that
heights over the mainland should gradually rise as initial upper
low/shortwave energy retrogrades and merges into the circulation
around an Arctic upper low that drops into and retrogrades over
Siberia. Beyond midweek some residual upper troughing will likely
persist over the eastern Bering and southwestern coast of the
mainland.
The early/mid period emphasis on operational guidance used
somewhat lower weighting of recent ECMWF runs than average. This
is due in part to the average of other guidance generally holding
onto yesterday's ideas for a deepening system expected to track
south of the eastern Aleutians and Alaska Peninsula Mon into early
Tue followed by a track that approaches the eastern side of Kodiak
Island--an evolution somewhat slower/westward of the 12Z ECMWF.
The 00Z run was closer in principle. Confidence is still not very
high given potential dependence on interaction of separate
features over the North Pacific and Aleutians as of early Mon.
Eastward timing of the low over the southern Bering/Aleutians on
Mon (a much-weakened form of today's low analyzed at 921mb)
continues to be troublesome with a range of possibilities
depending on the specifics of the northern fringe of North Pacific
flow. The ECMWF remains on the eastern side of the envelope
initially with the GFS eventually catching up, while the 12Z
UKMET/CMC are slower. Current forecast depicts it becoming
absorbed into the mean flow by early Wed but there is potential it
could still be better defined at that time.
Increasing spread for some details within a fairly agreeable large
scale pattern favored a rapid increase of 12Z GEFS/00Z ECMWF mean
input by days 7-8 Thu-Fri. Of particular note there is not yet
much support for the low near Kodiak Island by midweek continuing
northwest into Bristol Bay per the 06Z/12Z GFS. There is still a
loose signal for another system farther to the south after midweek
but with poor agreement from operational runs. The last couple
ECMWF mean runs are the most coherent with this system. Farther
west the trends from the past day have extended some early
indications 24 hours ago that low pressure reaching the central
Pacific by Thu will track far enough south to have little
influence on the Aleutians. However models and ensembles
currently show better than average clustering with the next
western Pacific storm that could extend its influence into the
Aleutians after early next Fri.
...Weather/Hazards Highlights...
The primary focus for meaningful precipitation through much of
next week will be along the southern coast and Panhandle.
Coverage and intensity of rain/snow will continue to be dependent
on lower confidence track and depth specifics of individual
surface lows reaching the northeastern Pacific/Gulf of Alaska.
Currently the best consensus of guidance suggests some of the
locally enhanced precip should reach at least as far west as the
Kenai Peninsula. The system expected to track somewhat south/east
of the Alaska Peninsula around Tue-Wed may have the best potential
to produce areas of strong winds. Gradually weakening southern
Bering/Aleutians low pressure early-mid week will likely maintain
periods of precipitation over the Aleutians through the first half
of the week. After a western Pacific system likely passes well
south of the Aleutians after midweek, the next in the series may
bring increasing winds/precipitation into the western Aleutians
next Fri.
At the start of the period on Mon expect most of the mainland to
see below to well below normal temperatures (especially for
highs), with the southeast corner of the mainland and Panhandle
likely to be the exception with above normal readings. Pattern
evolution aloft will support a steady warming trend from east to
west, leaving only the far western part of the mainland below
normal by late week.
Rausch
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
No significant hazards are expected over Alaska during this
forecast period.
WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can also be
found at:
https://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml
https://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html