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FXAK02 KWNH 012352
PMDAK

Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
752 PM EDT Tue Apr 1 2025

Valid 12Z Sat 05 Apr 2025 - 12Z Wed 09 Apr 2025

...Heavy coastal rain and heavy mountain snow Friday into the
weekend, moderating somewhat next week...

...Overview...

An upper ridge extending from western Canada through most of the
mainland as of early Saturday should steadily give way to an upper
trough (with a potential embedded low) amplifying over the
western half of the mainland into early next week and persisting
into Wednesday. At the same time a mean upper low should wobble to
the south/southeast of the Alaska Peninsula/Kodiak Island through
the period. The initial surface system associated with the
Pacific upper low will bring strong winds to the eastern Aleutians
and heavy precipitation to the southern coast and Panhandle late
this week into the weekend, with some rain/snow likely continuing
early-mid next week as another system may track into the mean low
position and ultimately reach closer to the southern coast. A
broad area of lighter precipitation should extend farther north
across the mainland, especially during the weekend, as moisture
advects inland.

...Guidance/Predictability Assessment...

Today`s guidance agrees fairly well with the overall pattern
evolution through the period but there are embedded differences
for the developing western upper trough/embedded low (affecting
temperatures over that part of the state) and northeastern Pacific
surface low specifics (influencing precipitation coverage and
totals along the southern coast).

Models/ensembles have varied noticeably for the depth and
position of the potential upper low within the western mainland
trough. Especially by later in the period the machine learning
(ML) models through the 00Z/06Z cycles favored keeping the upper
low farther north than latest ECMWF runs and particularly the 12Z
CMC. The ML tendency would favor a tilt toward the ensemble means
and 12Z GFS/UKMET/ICON. Some solutions arriving after forecast
preparation have raised the potential for a farther south upper
low. The new 18Z GFS has adjusted to a compromise between the 12Z
ECMWF and 12Z ECens mean (a little deeper than the old run and
still north of the operational run) while some 12Z ML models have
shifted southward, one or more reaching down to/near the Seward
Peninsula.

The means agree quite well for most of the period with respect to
the area of low pressure over the northeastern Pacific while
individual models diverge for some details. By Sunday the 12Z
UKMET becomes the most suspect, lifting the low well
northwest/west in response to digging Bering Sea energy. 06Z and
12Z GFS runs stray a little west early next week but the 12Z run
is acceptable as part of a compromise. Latest dynamical/ML model
runs are starting to show more definition for a wave expected to
track south of the Aleutians during the weekend and then
underneath the initial low. Then by Tuesday-Wednesday questions
arise for the longitude of this wave as it curls northward, along
with its potential interaction with the initial low. Specifics
have fairly low predictability at that time frame so initially
prefer a model/mean blend that hints at some eastward elongation
of the overall low pressure area without explicitly showing the
more eastern wave track of the 12Z GFS/ECMWF/CMC.

Late in the period the 12Z CMC becomes a very pronounced extreme
with tracking western Pacific low pressure into the Bering Sea,
affecting the downstream pattern as well. There is little to no
support for the CMC from other dynamical and ML models. The CMCens
mean is a little faster than the other means but at least is much
slower than the operational run.

Above comparisons led to leaning a little more in the GFS
direction over the mainland and Arctic but a little more to the
past couple ECMWF over the northeastern Pacific, while various CMC
issues and UKMET questions over the northeast Pacific favored
excluding those models from the forecast. Thus the first half of
the period started with a blend of half 12Z GFS and half ECMWF
(split among the 00Z/12Z runs) while the rest of the forecast
added some 12Z GEFS/00Z ECens means. Ensemble weight reached 50
percent total by Day 8 Wednesday.

...Weather/Hazards Highlights...

Before the start of the extended period, strong North Pacific low
pressure will already be producing heavy precipitation from parts
of the eastern Alaska Peninsula and Kodiak Island to the Prince
William Sound on Friday while the tight gradient should generate
strong winds over the eastern Aleutians, and to a lesser extreme
over the Alaska Peninsula. Expect the coastal rain and heavy
mountain snow to persist along the southern coast and at least the
northern half of the Panhandle through the weekend. The 3-7 Day
Hazards Outlook depicts these wind and precipitation areas. Strong
winds are also likely over open waters of the North Pacific/Gulf,
especially into Saturday. Another wave that may lift into the
northeastern Pacific and possibly the Gulf early-mid next week
would maintain the potential for precipitation along the southern
coast, but with somewhat lesser amounts and a trimming of the
western side of the moisture shield. Some moisture should flow
northward ahead of the upper trough developing over the western
mainland, spreading lighter northern/western and mountain snow and
valley/Interior rain across the mainland. Greatest coverage of
this activity should be during the weekend, followed by a drier
trend from west to east. A persistent surface front, reinforced by
a cold front arriving from the West, may also provide some focus
for the rain/snow.

The weekend should start with above normal temperatures over most
areas, with near to slightly below normal readings along the
western coast. Warmest anomalies should be over eastern locations.
The upper trough developing over the western mainland and leading
frontal boundary starting to push eastward will promote an
eastward expansion of near to below normal temperatures next week,
ultimately confining most of the above normal anomalies to the
southeastern part of the state by the middle of next week.

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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