Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
657 PM EST Fri Jan 06 2023
Valid 12Z Tue Jan 10 2023 - 12Z Sat Jan 14 2023
...Overview and Guidance/Predictability Assessment...
Through a combination of trends and refinements, latest guidance
valid through next Thursday-Friday shows somewhat more similar
ideas for most aspects of the forecast compared to 24 hours ago.
The key trend relating to the mainland pattern aloft involves a
westward shift of any upper troughing/embedded low near the
western coast at the start of the period early Tuesday, with the
western portion of the persistent western Canada mean ridge now
extending its influence farther across the mainland. There is
some continuation of the ongoing pattern of small-scale (thus
having low predictability) northeastern Pacific waves tracking
around the upper ridge, with one reaching near the Alaska
Peninsula by the start of the period and then one or more waves
farther east/southeast later in the forecast. Improved clustering
for the mid-late week Aleutians storm increases confidence that it
should be fairly deep. The better consensus for flow aloft over
the mainland falls apart by late in the forecast, especially day 8
Saturday.
Most of the first half of the forecast emphasized a composite of
the 00Z/12Z ECMWF runs and 12Z GFS/UKMET/CMC. This provided a
good intermediate solution for the initial Alaska Peninsula wave,
for which the 12Z ECMWF was on the slow side and the 12Z GFS on
the fast side. An average of the 12Z UKMET/CMC and 00Z ECMWF was
closest to the preferred scenario and the new 18Z GFS has trended
slower from its 12Z run (close to the UKMET, a bit faster than the
00Z ECMWF/12Z CMC). Thereafter the blend reflected latest trends
toward higher heights over the mainland, with a little quicker
retrogression of the west coast upper trough/low versus the means,
along with a deeper trend for the Aleutians storm. Latest
guidance suggests this storm may be strongest around Thursday as
it reaches near the islands, with the current manual forecast in
the lower half of the 960's mb with some individual GFS/ECMWF runs
down into the 950's. Meanwhile, the succession of small-scale
northeastern Pacific waves may get dampened for a time as a
mid-latitude Pacific system encompasses an increasing portion of
the eastern third to half of the ocean.
The mid-late part of the forecast incorporated increasing weight
of the 12Z GEFS/00Z ECMWF means such that day 8 Saturday reached
an even blend of the means and operational model runs. This
partially blunted some of the model trends toward stronger ridging
over the mainland, given the model inconsistencies of recent days.
Favorably, this approach ended up gaining support from the later
arriving 12Z ECMWF mean as of early day 7 Friday. By day 8
solutions go astray, with various ways flow over the mainland
could become dominated by a trough or upper low (12Z ECMWF/18Z
GFS) or ridging (12Z GFS/CMC) with the latter better supported by
the ensemble means. There is also a question as to what extent
cyclonic flow over the Arctic could reach into the northeastern
mainland toward the end of the period, again with a lot of spread
and run-to-run variability. Some northeastern Pacific waviness
shows up again off of Haida Gwaii and the Panhandle by late in the
week, with the ensemble mean input helping to keep these features
farther offshore than in some individual operational model runs.
The Aleutians storm should linger over the Aleutians/southern
Bering Sea into Friday-Saturday. There is a general signal that
energy dropping into the mean trough could support another feature
coming into the picture from the south by next Saturday.
...Weather/Hazards Highlights...
Moisture/easterly low level flow initially associated with a wave
forecast to reach near the Alaska Peninsula as of early Tuesday
should help to maintain some precipitation focused along the Kenai
Peninsula and far eastern Alaska Peninsula early in the period.
Activity along the southern coast should trend lighter with time
while one or more eastern Pacific waves could bring some moisture
northward into the Panhandle during the latter half of the week.
The latest consensus for the upper pattern would support some
light snow scattered over the western mainland through midweek or
so followed by a drier regime. The deep system affecting the
Aleutians by mid-late week will bring a period of focused
precipitation and strong winds. Some of this system's moisture
may reach as far east as the Alaska Peninsula by late week.
Temperatures will depend on the upper pattern which has been
somewhat inconsistent over recent days. Latest trends suggest
most areas may remain above normal, with somewhat greater
anomalies for lows. The best potential for moderately below
normal highs would be near the western coast through midweek, near
the axis of the expected weak upper trough before it retrogrades,
and perhaps over localized parts of the interior.
Rausch
No significant hazards are expected over Alaska during this
forecast period.
Additional 3-7 Day Hazard Information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
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