396
FXAK02 KWNH 192352
PMDAK

Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
752 PM EDT Sat Apr 19 2025

Valid 12Z Wed 23 Apr 2025 - 12Z Sun 27 Apr 2025

...Overview...

A low forecast to be southeast of Kodiak Island as of early
Wednesday should continue dropping southeastward with time,
supporting a drier trend along the southern coast and Panhandle
after producing some rain and higher elevation snow around
midweek. Then a northwestern Pacific system should track into the
Aleutians/southern Bering Sea, spreading moisture across the
Aleutians and possibly as far eastward as the southern coast.
However latest guidance has been diverging for system details from
Friday onward, lowering forecast confidence. Additional North
Pacific/Aleutians waviness could come into the picture around next
weekend but again with low confidence. Meanwhile consensus still
shows an Arctic deep-layer ridge northwest of the mainland while a
somewhat ambiguous upper pattern prevails over the mainland. The
most common theme is for some degree of weakness aloft over the
central/northern mainland with perhaps a little ridging at times
to the south or southeast. The forecast pattern should keep the
mainland in the gradient between Arctic high pressure and North
Pacific/Aleutians low pressure.

...Guidance/Predictability Assessment...

There are still some noticeable differences in the handling of the
system just to the southeast of Kodiak Island early Wednesday.
Latest GFS runs are not as fast to pull the system southward as
they were 24 hours ago but the 12Z UKMET holds the upper low well
northward near the southern Gulf of Alaska into early Friday. A
couple recent ECMWF runs through the 00Z/19 cycle also showed this
idea, which led to a rebound in Panhandle precipitation as the
energy eventually lifted northeast. Ensemble means and recent
machine learning (ML) models do not support the UKMET/old ECMWF
scenario. Otherwise there are still low-predictability detail
differences that affect coverage/amounts of precipitation along
the southern coast/Panhandle. An operational model composite,
minus the UKMET by Friday, provides a reasonable starting point
for the mass fields/QPF with this system.

Overall the guidance shows more divergence today for the
combination of the system tracking into the western Aleutians by
around Thursday and any upstream waves. Latest ensemble means
(even adding in the ICONens along with the GEFS/ECens/CMCens) have
been the most stable so far, maintaining a coherent system along
or just north of the Aleutians from late week into the weekend and
with fairly consistent timing. GFS runs have tended to stray a bit
southward by next weekend but the 12Z and new 18Z runs have
adjusted closer to the means. However the 12Z ECMWF/UKMET in
particular have trended noticeably slower with the parent low. In
addition the 12Z ECMWF and ECMWF AIFS runs are quick with an
upstream North Pacific wave, ultimately merging it with the slower
leading low. The 12Z CMC is also fast to bring additional waves
into the picture while lifting the leading Aleutians low well into
the northern Bering Sea. 00Z/06Z ML models favored a solution
closer to the ensemble means in principle. Preference is to lean
closer to the ensemble mean evolution as model divergence
increases, with lingering input from the closest operational runs
late in the period (12Z GFS and 00Z ECMWF). This yields an
Aleutians/southern Bering track close to continuity, with low
pressure lingering near the eastern Aleutians/western Alaska
Peninsula into next Sunday. Trends are a bit weaker than
yesterday. Behind this system, there is enough of a signal for a
weak wave to depict such a feature near the western Aleutians by
next Sunday.

Exact details aloft over the mainland continue to have low
predictability, with both dynamical and ML models differing for
how elongated trough/closed low energy may evolve or retrograde
over central-northern areas. A blend approach reflects the
expected general weakness aloft.

Guidance considerations led to starting the Wednesday-Thursday
part of the forecast with an operational model composite (ECMWF
split between 00Z/12Z runs). By Friday the blend introduced 30
percent 12Z CMCens/00Z ECens means while reducing 12Z ECMWF
influence and eliminating the UKMET. Then Saturday-Sunday started
with 60 percent means (now including the GEFS due to becoming
acceptably close to the other means) with lingering input from the
12Z GFS/00Z ECMWF.

...Weather/Hazards Highlights...

The system forecast to track southeastward away from Kodiak Island
should continue to produce some rain and higher elevation snow
along the southern coast and Panhandle through Wednesday. Expect
most of this activity to be in the light to moderate range, with
the relatively higher totals over the northern-central Panhandle.
There is continued uncertainty over precise totals though. Upstream
Pacific low pressure tracking near the Aleutians should bring a
period of organized rainfall and possibly brisk winds to the
Aleutians from west to east during mid-late week. From Friday
onward the guidance increasingly diverges regarding how well
defined the surface low/front remain and how far eastward they
reach. Current preference closest to the ensemble means would
bring some leading flow and associated moisture into the Alaska
Peninsula and southern coast by the end of the week and the
weekend. An upstream North Pacific/Aleutians wave could bring some
additional precipitation into the Aleutians next weekend but with
below average confidence in details. Meanwhile, deep layer
ridging over higher latitudes of the Arctic will likely support
brisk easterly easterly winds and possibly episodes of blowing
snow across the North Slope through the period.

Expect temperatures to be mostly below normal over the northern
half to third of the mainland due to flow around the Arctic ridge.
Most southern areas will see above normal readings, though clouds
and precipitation should support below average daytime highs
along the Southcentral coast and Panhandle especially during
Wednesday. Anomalies should generally be warmer for morning lows
relative to those for daytime highs, while most of the state
should see a very gradual warmer trend over the course of the
period.

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

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