Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
654 PM EST Sat Jan 07 2023
Valid 12Z Wed Jan 11 2023 - 12Z Sun Jan 15 2023
...Overview...
Most of today's guidance provides fairly good continuity with the
overall forecast across the mainland from Wednesday into the
weekend, with a couple weak shortwaves passing through followed by
stronger ridging for Friday (a combination of westward extension
of a long-term Canada ridge and an approaching ridge downstream
from a deep storm tracking into the Aleutians). Recent runs have
been hinting at cyclonic Arctic flow nudging into the
northern/northeastern mainland by next weekend. The large scale
pattern may direct another wave or so north/west across the
northeastern Pacific, bringing some moisture to the Panhandle and
then possibly back to Kodiak Island and vicinity. Meanwhile the
Aleutians storm will likely arrive by Thursday and linger into the
weekend, spreading moisture and a band of enhanced winds across
the islands, with some moisture likely reaching into the Alaska
Peninsula as well.
...Guidance/Predictability Assessment...
The 12Z GFS/ECMWF/UKMET all agree fairly well in principle among
each other and with continuity across the mainland, with a
strengthening east-west upper ridge by Friday after the passage of
a couple week shortwaves. This evolution fits the general
ensemble mean pattern, though for the second day in a row the
model consensus is somewhat stronger than the means, providing
some added confidence in the model cluster. The 12Z CMC is on its
own with a trough reaching the eastern mainland by Friday,
followed by deeper troughing from Arctic input over the weekend.
Meanwhile other guidance has been waffling around some with
respect to how much troughing may dip into the far north/northeast
during the weekend. Latest GFS/ECMWF runs and the ensemble means
on average appear to be centering the possible upper trough axis
near the northern coast of the mainland at that time.
After a brief lull as a strong/massive system dominates over the
eastern Pacific around midweek, there may be another episode of
one or more compact and thus low-predictability waves tracking
north/west over the northeastern Pacific. The 12Z operational
models appear to be starting to show a better signal for a defined
system tracking to the south and then west of the Panhandle around
Friday, becoming more diffuse by Saturday. A general average of
these model solutions would provide a good starting point given
the uncertainty that still exists.
Continuity has held on reasonably well for the deep Aleutians
storm, with guidance still suggesting it should be deepest (around
or a little under 960 mb) around Thursday but perhaps with a
similar depth anytime between late Wednesday and into early
Friday. Consensus today shows a track just a tad south of
yesterday's forecast, hanging around the Aleutians a little longer
before drifting into the southern Bering Sea by the weekend. At
that time, individual operational model runs suggest that low
pressure depicted over the area could be either persistence of the
initial low (12Z ECMWF) or arrival of another strong low (12Z/18Z
GFS). As the upper pattern elongates in response to strong
Pacific flow during the weekend, the overall area of low pressure
should likewise stretch out with some indication that another
focus for a system could be close to the southwestern Alaska
Peninsula by day 8 next Sunday, even if not as strong as the past
couple ECMWF runs.
Based on the above considerations, today's forecast started with a
12Z operational model composite for days 4-5 Wednesday-Thursday,
removed the CMC thereafter, and also incorporated gradually more
12Z GEFS/00Z ECMWF mean input so that the day 8 Sunday blend was
half models and half means.
...Weather/Hazards Highlights...
Prevailing flow over the Kenai Peninsula and Kodiak Island/Alaska
Peninsula should help maintain periods of precipitation over the
region mid-late week. A wave that may lift north/west over the
northeastern Pacific late in the week would spread precipitation
of varying intensity across the Panhandle, with some of this
moisture than possibly reaching the southern coast. Confidence is
still moderate to low for specifics since guidance appears to be
just starting to key in on this system a little better. Meanwhile
initial shortwave energy may produce light snow over parts of the
western mainland around midweek or so, with a little snow possibly
extending into the northeast. Otherwise most of the mainland away
from the southern coast should stay fairly dry. The deep storm
affecting the Aleutians from midweek onward will bring a period of
focused precipitation and strong winds as the associated front
progresses from west to east. The initial front will ultimately
dissipate but a combination of moisture and southeasterly flow
(whether focused by a mere surface trough or another low) may
extend into the Alaska Peninsula late week through the weekend.
Expect the majority of the state to see above normal temperatures
during the period, with forecasts consistent in showing greater
anomalies for lows over most areas. A few pockets of below normal
highs could develop over the interior while the degree of cyclonic
flow aloft over the far northern mainland by the weekend will
influence how much of a cooling trend may occur in its vicinity.
Rausch
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