991
FXAK02 KWNH 202344
PMDAK

Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
643 PM EST Thu Feb 20 2025

Valid 12Z Mon Feb 24 2025 - 12Z Fri Feb 28 2025

...Overview...

Today`s models and ensembles continue the theme of a mean ridge
aloft extending from the Panhandle and western Canada across the
mainland, likely strengthening a bit over the course of the week.
Multiple Pacific surface lows will support episodes of
precipitation along the southern coast and Panhandle, with the
most pronounced focus expected to be over the Panhandle in
association with a system tracking into the eastern Gulf of Alaska
around Wednesday/early Thursday.  A central Pacific low should
reach a position south of the Alaska Peninsula by late week and
may bring some brisk winds to parts of the Aleutians and southern
coast in addition to the precipitation.  Most of the mainland will
lie in the gradient between these lows and Arctic high pressure.
Parts of the North Slope may see brisk winds as a result.  The
forecast pattern will promote above normal temperatures over much
of the state.


...Guidance Evaluation and Preferences...

Early in the week, both the leading northeastern Pacific/southern
Gulf of Alaska low pressure and a separate area of low pressure
over the Bering Sea have been exhibiting somewhat random scatter
due to the relatively ill-defined nature of supporting upper
dynamics.  There is at least a little better signal for a possible
upper low (with sufficiently close contour intervals) with the
southern Gulf feature.  A general model blend provides a
reasonable starting point for these systems.  Note that the 12Z
GFS strays to the deep side of the guidance over the southern Gulf
after Monday but is acceptable as a minority component of a blend
to maintain some detail.  The new 18Z GFS trended favorably weaker.

Guidance has been consistent in principle with bringing a trailing
mid-latitude Pacific low northward into the Gulf of Alaska by
around Wednesday/early Thursday, but with a lot of spread and
variability for the track.  Over the past couple days the envelope
has generally shifted somewhat eastward, with the western side of
earlier spread (represented by a number of GFS runs) in particular
getting trimmed away.  While the 12Z/18Z GFS that have shifted to
the eastern side are within the broad envelope--which spans from
Prince William Sound to the far southern Panhandle even in the
latest machine learning models--those GFS runs look suspicious as
they split the surface system into two waves earlier in the period
over the central Pacific.  Other solutions do not show this
splitting.  Overall prefer the latest ECMWF runs and ECens/CMCens
means that provide the best intermediate track close to 140W
longitude.  For definition the manual forecast makes use of the
12Z ECMWF run, though ML models suggest that it could end up being
slightly slow--but not nearly as held back as the 12Z CMC.

The deep storm reaching the central Pacific by Wednesday should
start weakening gradually by late in the week as it reaches a most
probable position somewhat south of the Alaska Peninsula by early
Friday.  Multi-day trends have been somewhat slower/southward by
late week, though today`s runs do not seem to be extending this
recent trend.  Latest ECMWF runs and the ensemble means represent
the best cluster with general support from the ML models that are
near or slightly north of 50N between 150-160W as of early Friday.
 The 00Z GFS was comparable to this cluster but the 06Z run
(north) and 12Z/18Z runs (west) have strayed somewhat.

The composite of forecast preferences led to the early part of the
forecast starting with a 12Z model blend of the
ECMWF/GFS/CMC/UKMET in order of more to less weight.  Favored
representation of the eastern Gulf system by Wednesday resulted in
replacing the GFS component with the 12Z CMCens mean (deeper than
the 00Z ECens).  Then with the strong central/northern Pacific
system becoming the dominant feature after midweek,
Thursday-Friday consisted of half means (12Z CMCens/00Z ECens) and
half ECMWF (12Z run on Thursday, and split between 00Z/12Z runs on
Friday).


...Weather/Hazards Highlights...

The southern coast and Panhandle should see multiple episodes of
precipitation during the period, with mostly rain over the
southern Panhandle and along the coast while snow prevails over
higher elevations.  Initially a general area of low pressure (one
or more centers) over the northeastern Pacific into southern Gulf
of Alaska should support areas of mostly light to moderate
activity during the first part of the week.  A system tracking
northward, most likely into the eastern Gulf, should bring a brief
surge of enhanced moisture into the Panhandle around Wednesday and
perhaps into early Thursday.  Some operational models show
precipitable water values up to plus 2-3 standard deviations above
climatology.  Significant east-west spread in the guidance for the
system`s track tempers confidence in the westward extent of heavy
precipitation.  However there is a sufficient signal for the Days
3-7 Hazards Outlook to depict a heavy precipitation area for the
southern two-thirds or so of the Panhandle.  Continue to monitor
forecasts as guidance refines the surface low track.  Then a deep
but eventually slowly weakening central Pacific system to bring
brisk winds along with increasing precipitation to the eastern
Aleutians and spreading eastward along the southern coast.
Current guidance does not suggest either weather element will
reach hazardous criteria but some refinements may be possible over
the coming days.  Expect fairly dry conditions farther north over
the rest of the mainland.  The gradient between persistent high
pressure over the Arctic and the surface lows over the
Pacific/Gulf will produce some brisk winds, in particular across
parts of the North Slope.  While wind speeds should not reach
hazardous criteria, these winds could still be strong enough to
cause issues with blowing snow.

The mean upper ridge across most of the state will keep
temperatures well above normal through the period over most areas,
with southern parts of the state tending to see the highest
anomalies of plus 10-20F and locally higher.  The North Slope
should be closer to normal and there may be a few pockets of
slightly below normal readings.

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

$$