Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
751 PM EDT Fri Apr 18 2025

Valid 12Z Tue 22 Apr 2025 - 12Z Sat 26 Apr 2025

...Overview...

The Tuesday-Saturday period will feature two primary features of
interest. The first is an eastern Bering Sea and vicinity upper
trough/low that likely drops into the northeastern Pacific and
away from the mainland, with associated low pressure bringing a
period of rain and higher elevation snow primarily to areas along
the southern coast. Then a northwestern Pacific system should
track along or north of the Aleutians with some enhanced
precipitation and brisk winds. Meanwhile, there is reasonable
consensus for a high latitude deep-layer ridge that will maintain
fairly strong easterly winds across the North Slope. Guidance
shows a lot of detail differences for upper flow details over the
mainland though. Ensemble means suggest a transition from weak 
ridging to development of a col region while operational model 
runs vary with how much ridging or weakness/closed low may exist
from day to day. Even with these differences, there is at least
better agreement upon most of the mainland staying within the
gradient between high pressure over the Arctic and low pressure
systems to the south.

...Guidance/Predictability Assessment...

For the system forecast to track from the eastern Bering into the
northeastern Pacific, latest guidance clusters fairly well as of
early Tuesday but then diverges with how quickly the upper low
energy drops southeastward and at what longitude. This affects 
the surface low track along with duration and eastward extent of 
associated precipitation along the southern coast. Among the 
dynamical guidance, the 12Z GFS looks particularly quick to pull 
the system southward and trend drier along the southern coast. 
Other solutions still vary (12Z UKMET/CMC extending farthest
eastward with the front and bringing the most precipitation into
the Panhandle--perhaps overdone), while some recent machine 
learning (ML) models suggest that low pressure could end up 
farther north in the Gulf versus most dynamical solutions. The 12Z
UKMET would be the closest dynamical model to that idea. Initial 
preference follows a multi-model composite that downplays 12Z GFS
influence and otherwise follows an intermediate approach. This 
leads to only moderate refinement from continuity while monitoring
for possible trends in future cycles.

There are also some noticeable differences for the handling of 
the system that may track near the Aleutians during the latter 
half of the week. In principle the ECMWF/ECens mean have been more
consistent with the evolution than GFS/GEFS runs over the past 
couple days. As the system comes into the picture around 
Wednesday-Thursday the latest GFS runs lift the low/surface front 
somewhat north of most other dynamical guidance but some ML models
offer potential for a low track that could be a little north of 
the dynamical majority. This ML model tendency persists into 
Saturday, supporting exclusion of the GFS/GEFS especially by that 
time as those solutions stray farther south. The 12Z CMCens and 
00Z ECens (with confirmation from the new 12Z ECens run after 
forecast preparation) are somewhat deeper than the GEFS mean and 
favor a track along or a little north of the Aleutians, as 
recommended by the ML models. Meanwhile ECMWF timing differences 
later in the period favored averaging their two runs for that 
model's input and the 12Z CMC strays a bit north late but remains 
close to the northern edge of the ML model spread.

There is decent agreement among the means for deep-layer Arctic
ridging between 75-80 north latitude, with other models displaying
typical spread. 06Z/12Z GFS runs and latest ECMWF runs compare
well to the means but the new 18Z GFS displaces the ridge
east/south in favor of an upper low lifting up from Siberia. The
12Z CMC becomes elongated/south with the ridge. Meanwhile,
confidence is well below average for the fine-scale details aloft
over the mainland. Based on other solutions the 18Z GFS has even
relatively lower confidence for its upper low that forms over the
central/southwestern mainland by the end of the week.

Putting together the most common/favored themes of guidance, the
first half or so of the forecast consisted of 40 percent 12Z ECMWF
and the rest split evenly among the 12Z GFS/CMC/UKMET. After early
Thursday the blend rapidly incorporated more 12Z CMCens/00Z ECens
input, reaching 60 percent total by Saturday, while also splitting
operational ECMWF input between the 12Z and 00Z runs.

...Weather/Hazards Highlights...

The system dropping southeast from the eastern Bering Sea and
vicinity will help to focus rain and higher elevation snow over
the far southwestern mainland/Alaska Peninsula, and then with
somewhat higher totals extending eastward along the southern coast
and possibly the Panhandle as low pressure reaches the
northeastern Pacific. Currently expect potential for some
localized enhancement but amounts should remain below hazardous
thresholds. Specifics are still very sensitive to the exact track
of the low with no clear consensus yet. Some lighter activity
could linger along the southern coast into late next week. 
Upstream Pacific low pressure should bring a period of organized 
rainfall and brisk winds to the Aleutians from west to east during
mid-late next week. Leading flow may bring this precipitation 
into the Alaska Peninsula and vicinity toward the end of the week,
but with decreasing confidence given the guidance spread for low 
pressure evolution and track. Deep layer ridging over higher
latitudes of the Arctic will likely support strong easterly flow 
across the North Slope through the period. Wind speeds should not
reach hazardous criteria but may lead to periods of blowing snow.

Expect a fairly stable temperature regime during the period.
Northern areas will tend to be below normal due to the proximity
of Arctic ridging, while most areas to the south will see above
normal readings--especially for morning lows. As for one
exception, clouds and precipitation should support below average 
daytime highs along the Southcentral coast and Panhandle 
especially during Tuesday-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