425
FXAK02 KWNH 212355
PMDAK

Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
654 PM EST Tue Jan 21 2025

Valid 12Z Sat Jan 25 2025 - 12Z Wed Jan 29 2025

...Heavy precipitation shifting from the southern coast into the
Panhandle this weekend into early next week...
...Heavy snow expected from the Alaska to Brooks Ranges and
westward to/near the Seward Peninsula through the weekend...
...Trending much colder next week...


...Overview...

At the start of the period early Saturday the guidance shows a
very strong eastern Pacific through northwestern North America
upper ridge with a mean trough from the eastern tip of Siberia
southward into the North Pacific.  With time the pattern should
gradually shift eastward, while flow between the two features
likely carries a North Pacific system through the southwestern
into eastern mainland during the weekend and the start of next
week.  Expect mean troughing to prevail over the mainland by next
Tuesday-Wednesday, leading to much colder temperatures compared to
the weekend along with a drier trend over most areas.  On the
western side of the overall upper trough, a surface low may track
east-southeast across the Bering Sea and into the North Pacific
early next week with primary effects limited to the Aleutians and
Alaska Peninsula.

...Guidance Evaluation and Preferences...

Recent guidance has displayed a fair degree of spread and
variability for the North Pacific through mainland system which
should take a path close to that initially defined by a
southwest-northeast front draped across the state.  The most
pronounced differences involve timing and strength, reflecting
differences in how North Pacific shortwave dynamics
evolve/progress and then how this feature is incorporated into the
larger scale flow.  00Z/06Z machine learning (ML) models and the
00Z AIFS mean offered a decent signal for the Pacific shortwave to
take on a negative tilt while south of the Alaska Peninsula into
Sunday, leading to timing in the middle to slower half of the
spread.  However the ML models then reflected sufficient phasing
to increase progression beyond the slow side of the envelope by
early Monday (reaching the southeast or east-central mainland).
Based on guidance available through the 12Z ECMWF, slightly faster
12Z ECMWF trend and a slower/deeper 12Z GFS versus prior runs led
to a 60 ECMWF/40 GFS weight offering a good reflection of ML model
signals.  The new 18Z GFS has actually adjusted very close to the
12Z ECMWF.  Comparing other guidance, the 12Z GEFS/CMCens means
are questionably fast while the 12Z UKMET is likewise suspect on
the slow side.  The 12Z CMC strays to the west of other guidance
by Sunday-Monday.  As for depth, the primary ML model cluster
shows a central pressure averaging in the mid-upper 990s mb during
Sunday-Monday.  The preferred dynamical model solution is somewhat
deeper, as ML guidance can lag in reflecting sufficient depth for
some features.  However the similarity among the ML models also
tempers confidence in solutions any deeper than the 12Z ECMWF/18Z
GFS.

Latest guidance runs are improving their signal for a wave that
may emerge from the western Bering Sea on Monday and drop
east-southeast across the eastern Aleutians/southwest Alaska
Peninsula on the way into the North Pacific.  ECMWF runs have been
showing this wave for a couple days and the GFS from the 06Z run
onward have been reflecting it as well. The signal is still fairly
weak/diffuse in the ensemble means and ML models.  ECMWF
continuity/GFS trends offer support for improving the definition
of the wave in today`s forecast, with some ECMWF/GFS input
continuing from the preferences for the leading system providing a
good starting point.

By late in the period, dynamical/ML guidance spread increases for
the details of mean troughing aloft that should push into the
mainland.  These differences also influence the northeastern
Pacific/Gulf of Alaska surface pattern, with increasing divergence
after some clustering toward low pressure settling over the
northern Gulf by early Tuesday. The differences at the surface and
aloft favored a fairly rapid increase of ensemble mean input (12Z
GEFS/CMCens and 00Z ECens) by next Tuesday-Wednesday, reaching 60
percent total by midweek.

...Weather/Hazards Highlights...

On Friday, before the start of the extended period, the
combination of a storm tracking northward just west of the
mainland and a strong northeastern Pacific surface high will
produce an area of high winds from the Aleutians through parts of
the western and northern mainland.  The system may also generate
areas of heavy snow between parts of the southern mainland and the
Brooks Range.  Strong winds could linger into Saturday over some
locations.  The North Pacific system forecast to track through the
southwestern and then eastern mainland by late Sunday-Monday will
have a long fetch of southerly moisture ahead of it, supporting a
period of heavy precipitation that should initially focus along
the southern coast and then gradually push more into the Panhandle
by early next week.  Meanwhile heavy snow will likely continue
farther north over parts of the mainland.  Winds may be brisk with
this system but should not be as strong as with the Friday storm.
The Days 3-7 Hazards Outlook depicts the above weather threats.
Toward midweek the majority of the mainland should trend drier
while some precipitation may linger over the Panhandle, with
specifics determined by upper trough/surface details.  Farther
west, there is emerging potential for a Bering Sea wave to bring a
period of brisk winds and light to moderate precipitation to the
Aleutians and possibly some of the Alaska Peninsula early-mid week.

The forecast pattern transition will bring a pronounced colder
trend.  Central/eastern parts of the mainland will be particularly
warm versus normal during the weekend with some plus 20-30F or
greater anomalies.  The approach and arrival of upper troughing
from the west will bring steadily colder temperatures, with nearly
the entire state seeing below to well below normal readings by
next Wednesday.  The North Slope and southern Panhandle could hold
onto slightly above normal anomalies into early Wednesday.

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

$$