Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
655 PM EST Thu Nov 25 2021
Valid 12Z Mon Nov 29 2021 - 12Z Fri Dec 03 2021
...Well below normal temperatures to start the week, then
persisting over the west and moderating some over the east...
...Potential for heavy precipitation along the southern coast and
Panhandle early-mid week...
...Overview and Guidance Evaluation/Preferences...
The most prominent feature during the forecast period continues to
be deep low pressure expected to track from south of the eastern
Aleutians/Alaska Peninsula into the Gulf of Alaska as initially
elongated upper troughing from the mainland into the eastern
Bering Sea interacts with North Pacific dynamics. The surface low
should slowly weaken in place during the latter half of the week
as the upper trough aligned near the western coast around
Wednesday persists, drifting only gradually to the east with time.
Farther west a compact system should track across the western
half of the Aleutians around Tuesday and drop into the Pacific
thereafter. Then an amplified upper ridge will likely build
across the Aleutians/Bering Sea, followed by a frontal system
anchored by low pressure expected to track over or near southern
Siberia.
After showing some progress in clustering for North Pacific into
Gulf low pressure yesterday, latest guidance has maintained some
noticeable detail differences for one or more individual surface
lows. These include waves along the general primary low path as
well as frontal waves that could reach into the Panhandle. The
relatively small scale of some of these waves will probably keep
predictability below average until the short-range period.
Starting the Monday through early Wednesday part of the forecast
with a 12Z model composite (though splitting ECMWF input between
its 12Z and 00Z runs) provided a consolidated system that fit in
well with the ensemble means. This solution downplayed any
prominent frontal waves farther to the east given the lack of
agreement, but their existence is still plausible. For the
primary low the new 12Z ECMWF mean adjusted to a solution fairly
close to this blend, which reflects a second day in a row for
guidance consensus nudging the low track northwestward--bringing
it closer to Kodiak Island by early Tuesday. The preferred
forecast also reflected the steadily improving definition and
clustering for the system that should cross the western Aleutians
on Tuesday.
In varying ways the guidance maintains the idea of a mean trough
aloft persisting over the mainland with only very slow eastward
movement after being aligned near the west coast of the mainland
around midweek. By day 8 Friday a noticeable divergence arises
for flow within the overall upper trough that reaches the
east-central Pacific. Latest GFS/CMC runs split the flow while
the past two ECMWF runs develop a sharper and more phased trough.
The ECMWF scenario would bring more moisture into the Panhandle
around and after the end of the forecast period versus the GFS
cluster. Ensemble means show a phased trough (as would be
expected given the averaging of diverse members) but enough
broadness to keep the moisture axis a little farther east. Lack
of confidence in specifics from the mainland southward after early
Wednesday led to transitioning the forecast toward an even mix of
the operational models and 12Z GEFS/00Z ECMWF means by day 8
Friday.
With the upper ridge that builds across the Bering Sea/Aleutians
Thursday-Friday and the trailing frontal system, the 12Z GFS
trended slower than its 06Z/00Z runs. This still left the GFS a
bit on the faster side of the spread (including the GEFS mean) by
Friday but sufficiently within typical guidance spread/error to
maintain as part of the model-mean blend.
...Weather/Hazard Highlights...
Expect very cold temperatures to remain over much of the mainland
into Monday with some areas (especially the west and south-central
Interior) 20F or more below normal. Transition of the upper
pattern toward a north-south trough aligned over the western
mainland by Wednesday should support a moderating trend over
eastern areas (with varying magnitude of above or below normal
anomalies) while keeping the west/southwest well below normal.
Southwestern areas could see a brief warming trend (while staying
below normal) as the storm passes south of the Alaska
Peninsula/Kodiak Island but then trend colder again as the upper
trough axis drifts overhead.
The strong storm forecast to track from south of the eastern
Aleutians into the Gulf should bring enhanced precipitation and
brisk to strong winds from the eastern Aleutians into the Alaska
Peninsula/Kodiak Island early in the week. Recent trends to nudge
the storm track northwestward have led to some increase of winds
over these areas. Flow behind the storm will then bring a period
of fairly strong northerly to northwesterly winds to the Aleutians
and nearby areas. Meanwhile precipitation will increase from the
southern coast through the Panhandle Tuesday-Wednesday, with
highest totals likely to be from eastern Kenai Peninsula into the
Panhandle. Somewhat lighter precipitation should linger over this
area through the rest of the week as low pressure lingers over the
Gulf. The system tracking through the western half of the
Aleutians during the first half of the week will be accompanied by
light to locally moderate precipitation. Then precipitation and
strengthening winds will likely accompany a frontal system pushing
into the Aleutians/Bering Sea late in the week.
Rausch
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Tue-Wed, Nov 30-Dec 1.
- Heavy precipitation across portions of mainland Alaska, Mon, Nov
29.
- Much below normal temperatures across portions of mainland
Alaska, Sun-Wed, Nov 28-Dec 02.
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