Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
630 PM EST Sat Dec 21 2024
Valid 12Z Wed Dec 25 2024 - 12Z Sun Dec 29 2024
...Enhanced precipitation from Southcentral to Southeast Alaska
and maritime winds over the Northeast Pacific gradually decrease
later next week...
...Coldest air of the season thus far coming into the
western/central Mainland late week/next weekend...
...Overview...
Spokes of upper troughing across the northern Pacific and vicinity
will support a handful of reasonably deep surface lows through
midweek, and lows look to settle near the Gulf of Alaska while
weakening later next week. Southerly mean flow is forecast to
bring rounds of precipitation to southern coastal areas to the
east of these systems and behind a weak ridge coming into eastern
Alaska from western Canada. Strong winds are possible in the
northeast Pacific Wednesday/Christmas Day with the leading front
of a deep low, perhaps reaching southwestern parts of the
Panhandle. During the latter part of the week, an Arctic upper
low is forecast to drop toward the Chukchi Sea, and its associated
troughing is forecast to phase with the lingering trough and lead
to a much colder patter across western and central Alaska.
...Guidance Evaluation and Preferences...
Model guidance shows rather good agreement through the forecast
period on the large scale. The first smaller scale issue of note
was that the 12Z UKMET was stronger and closer to Southeast Alaska
with its upper and surface lows into Thursday, and a compromise
solution of other deterministic models seemed more reasonable.
Models over the past couple of days have trended toward showing
multiple low centers with central pressures in the 960s or perhaps
950s (mb) at the start of the period Wednesday, with some
lingering spread on the specifics of the lows combining and
weakening, but a blended approach seemed reasonable.
Into later week, the Arctic upper low is forecast to extend its
troughing south across the western Mainland, combining with
preexisting energies and tracking across the state into the
weekend, with fairly good support from the deterministic guidance
for a mid-late period forecast. Another large upper/surface low
looks to slowly approach the Aleutians from the Kamchatka
Peninsula by Friday into the weekend, with reasonable agreement on
that and with a brief period of upper ridging traversing the
Aleutians west to east ahead of it.
The WPC forecast used a model blend favoring the 12Z ECMWF along
with the 06Z/12Z GFS and some 12Z CMC for the early-mid period.
Better than normal model consensus meant a majority of
deterministic models could be maintained throughout the extended
period, with some incorporation of the GEFS and EC ensemble means.
...Weather/Hazards Highlights...
A few deep surface lows in the northern Pacific along with mean
Pacific flow into the southern coast will bring light to moderate
precipitation into the Aleutians to Alaska Peninsula and moderate
to locally heavy amounts into Southcentral and Southeast Alaska
mid- to late week, with multiple rounds through the period. Since
there will be multiple low centers rather than one dominant
feature by the extended period, this does not promote a pattern of
strong winds given a weaker pressure gradient. However, the
exception may be on the eastern side of the easternmost low with
the leading front, where high winds could affect parts of
Southeast Alaska on Wednesday/Christmas Day. The surface lows
should weaken fairly quickly as they stall in the Gulf late week
into the weekend, with gradually weakening winds and lowering
precipitation amounts.
Farther north, relatively mild and moist conditions for some of
the shortest days of the year will lead to potential for snow in
portions of Interior Alaska continuing from Tuesday into
Wednesday. Snow totals do not look to be too heavy on the broad
scale, but some localized totals up to a foot could accumulate
gradually over a few days. The main areas affected are forecast to
be north and west of Fairbanks but there is some spread in the
snowfall axis, and today's forecast shifted a bit more northwest
of the previous one. Parts of the Dalton Highway could see snow,
along with communities like Bettles and Galena to Grayling. Any
snow chances will get pushed eastward later in the week as the
upper trough brings in much colder and drier air to western
Alaska. Gap winds may be a concern in favored areas of the eastern
Aleutians/Alaska Peninsula as winds turn to northerly.
Expect above normal temperatures over the southern two-thirds of
the state into Wednesday while parts of the North Slope see below
normal readings. In general anomalies for low temperatures should
be warmer than those for highs. But as the period progresses, much
colder temperatures will gradually push across western Alaska
Thursday-Friday, central Alaska (especially the Interior) by
Saturday, and across much of the Mainland next Sunday.
Temperatures are forecast to be around 20-30 degrees below
average, with actual temperatures dipping into the -30s in the
northwestern Interior and -10s and -20s as far south as the Lower
Yukon and Kuskokwim. Into Southeast Alaska, temperatures look to
gradually cool from above normal to near normal.
Tate
Additional 3-7 Day Hazard Information can be found on the WPC
medium range hazards outlook chart at: The WPC Hazards Outline
depicts these threats.
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