Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
659 PM EST Fri Dec 20 2024
Valid 12Z Tue Dec 24 2024 - 12Z Sat Dec 28 2024
...Stormy pattern continues next week with periods of
precipitation from Southcentral to Southeast Alaska and hazardous
maritime conditions over the North Pacific/Gulf of Alaska...
...Overview...
A large and strong mean upper/surface low will track across the
North Pacific/Aleutians Tuesday-Wednesday and settle over the
Alaska Peninsula or western Gulf of Alaska 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 at times, especially in maritime areas,
but could reach the southwestern parts of the Panhandle with a
smaller initial surface low near Haida Gwaii on Tuesday and then
with the leading front of the North Pacific storm on
Wednesday/Christmas Day. During the latter part of the week, an
Arctic upper low is forecast to drop toward the Chukchi Sea, and
its associated troughing could phase with the lingering trough and
lead to a much colder patter across western Alaska.
...Guidance Evaluation and Preferences...
The first forecast feature of note is on the smaller scale but
could have implications for winds over Southeast Alaska--an upper
and surface low near Haida Gwaii Tuesday. Models vary with the low
track and how close it is to Alaska, with recent GFS runs on the
northern side and the CMC and UKMET farther south. The ECMWF
seemed like a reasonable middle ground. If the southern track like
the CMC/UKMET were to verify, wind impacts in Southeast could be
minimal on Tuesday.
On the larger scale, models have been agreeable for the past few
days with the North Pacific/Aleutians storm in terms of track and
depth. The consensus path is just a bit south of the Aleutians
into midweek, reaching the Alaska Peninsula or western Gulf of
Alaska by late week, with the central pressure in the 950s or so.
Today's deterministic models show more agreement for a couple of
embedded surface low centers on Tuesday-Wednesday, as what may
start as a triple point low to the east of the initial low becomes
dominant for the latter part of the week while gradually
weakening. There is lingering spread on the specifics but a
blended approach seemed reasonable, excluding the 12Z GFS as it
appeared a bit more suspect with the farthest south position of
the low early Thursday.
Into later week, the Arctic upper low is forecast to extend its
troughing south across the western Mainland at least, with fairly
good support from the deterministic guidance for a Days 6-8
forecast. Another large surface low looks to approach the
Aleutians from the Kamchatka Peninsula by next Friday-Saturday,
and its forecast will continue to be monitored.
The WPC forecast used a model blend favoring the 12Z ECMWF with
lesser amounts of the CMC and 06Z and 12Z GFS for the early
period, and with time eliminated the 12Z GFS and slightly
decreased the proportion of other models in favor of the GEFS and
EC ensemble means.
...Weather/Hazards Highlights...
Near a Haida Gwaii or vicinity low pressure system, high winds may
affect southwestern portions of the Panhandle on Tuesday if the
low track is close enough. The system will also bring also bring
some modest precipitation to the Panhandle, while southerly flow
increasing ahead of the large low south of the Aleutians brings
light to moderate precipitation into the Aleutians to Alaska
Peninsula and generally moderate amounts into Southcentral, with
potential for multiple rounds of precipitation through the period.
There may also be a band of strong winds ahead of the storm's
leading front reaching into the northeastern Pacific on
Wednesday/Christmas Day, and parts of the southern Panhandle have
an area delineated in the Hazards Outlook for both Tuesday and
Wednesday to account for this.
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 Monday into
Tuesday-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, which would be unusually
heavy for this time of year in some areas. The main areas affected
are forecast to be north and west of Fairbanks including parts of
the Dalton Highway and from the communities of Fort Yukon to
Galena to Grayling. Any snow chances will get pushed eastward
later in the week as the upper trough brings in colder and drier
air to western Alaska. Gap winds may be a concern as winds turn to
northerly.
Early in the week expect above normal temperatures over the
southern two-thirds of the state while parts of the North Slope
see below normal readings. In general anomalies for min
temperatures should be warmer than those for highs. By mid-late
week some North Slope locations will remain below normal and the
western mainland will likely trend below normal as upper troughing
deepens near or just west of the western coast. If the trough is
on the deep side of the guidance, regions like the Lower Kuskokwim
could see extreme cold. Southern and eastern areas should remain
above normal into late week.
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