Storm Warning 198, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
533 AM AKST Sat Dec 14 2024

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...

Discussion:

A much colder airmass and widespread light snow have begun to
push into the portions of the northern Copper River Basin and the
Susitna Valley this morning as a low begins to push into Interior
Alaska. Satellite imagery shows that the center of this low has
reached the ALCAN and will continue to slowly drop southwards
towards the Alaska Range through tomorrow morning. The Copper
Basin will see accumulating snowfall today with areas north of
Glennallen along the Richardson Highway and Tok Cutoff seeing
accumulations of 3 to 6 inches. The mid and long range models keep
the center of the upper low on the north side of the upper low,
but ridging over the Gulf combining with colder air wrapping
around the south side of the low will enhance lift over the Prince
William Sound Region and coastal areas along the northern Gulf.
The NAM, GFS, ECMWF, and UKMET are all in agreement with
increasing isentropic lift through a nearly saturated 285 to 290K
layer beginning tonight and continuing through at least Sunday
afternoon. While this set-up doesn't have significant forcing
associated with it, low-level southwesterly flow off the Gulf will
provide ample moisture, and upglide along with terrain forcing
should result in decent snowfall amounts for Valdez and Cordova.
Confidence is high enough that a Winter Storm Watch has been
issued beginning tonight and continuing through tomorrow...but
given the set-up, changes to the forecast are possible as this
event starts to unfold.

Further west over the Mat-Su Valley, Anchorage Bowl, and the western
Kenai... Cloud cover has started to increase as high clouds spill
over the ridge to the west, and lower cloud cover working in from
the north as colder air begins to work into the region. This colder
airmass has allowed for light snow to push southwards through the
Susitna Valley last night. The question for here will be how far
south this band of light snow will go. For now, have light
accumulations for Talkeetna down towards Palmer, and minor
accumulations of a half inch or less for Chugiak down towards the
Anchorage Bowl.

The evolution of the Arctic low becomes quite nebulous past Monday,
with it either shearing out or pushed off towards the Seward
Peninsula as upper level ridging builds over southern Alaska.

- PP

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA/BERING SEA/ALEUTIANS (Days
1 through 3: Today through Monday)...

Unsettled weather continues through Monday as various lows and
frontal boundaries move across the region. Confidence is
increasing that winds will stay below High Wind criteria (75 mph)
for the Western and Central Aleutians as a strong low moves
through this afternoon and tomorrow. However, this impressive
storm will still bring gusts 60-70 mph and seas 35-45 ft to the
Aleutian Islands and Bering Sea. For Southwest Alaska, conditions
are trending colder, with breezy winds contributing to wind chills
below zero. Precipitation chances and intensity increase as we
head towards early next week.

Diving into the details... the forecast for the Southwest coast
continues to be quite tricky, if not impactful. The incoming
front looks to weaken; however, guidance is suggesting the
development of a compact surface low somewhere along the front.
Guidance is also hinting that the weakening front and surface low
will interact to some degree with an inverted trough approaching
the AKPen from the North Pacific. The degree of interaction will
determine the extent of moisture advection and snowfall along the
immediate coast today. This feature then looks to stall and
elongate just off the coast for Sunday. Again, just how close to
the coast this feature lingers will determine snow amounts and
intensity of wind. Although it appears that locations from Togiak
to Toksook Bay will see 1 to 3 inches of snow through Sunday, any
increase in wind along the coast could also result in periods of
blowing snow and reduced visibility.

For the Aleutians and Bering, the aforementioned inverted trough
will bring a round of mixed rain and snow to the southern AKPen
and parts of the eastern Aleutians through Sunday morning. Any
prolonged snow in colder locations should not amount to much given
colder locations will still see temperatures in the mid 30s. This
trough will help to increase winds for places like Cold Bay and
Dutch Harbor.

The bigger weather feature will be a large storm-force low moving
toward the Western Aleutians late today through Sunday. This will
be a rather impressive system, with central pressure dropping as
low as 940 mb as the system approaches the Western Aleutians.
Given the strength of this system, confidence is very high that
there will be widespread storm force winds both along the front
and wrapping around the low. There is moderate confidence of
hurricane-force wind gusts for the coastal and open waters of the
Western Bering and for the Western Aleutians, including Shemya. The
occluded front associated with the low continues its eastward
progression, reaching the Pribilofs and Eastern Aleutians by
Monday morning.

-TM/Chen

&&
.LONG TERM FORECAST (Days 4 through 7: Tuesday through
Friday)...

Our small taste of arctic air over Southcentral will be short
lived as temperatures begin to moderate already by Tuesday. A low
developing along the front of a strong low pressure system over
the Aleutians will bring warmer southeasterly flow to the Gulf of
Alaska. Terrain enhanced rain at low elevations and mountain snows
are possible for coastal regions, while downsloping should keep
lee-side locations drier. Unsettled weather will persist across
the Bering Sea and Aleutians as the system stalls out there.

There could be a brief break in unsettled weather on Wednesday,
after which poor model agreement gives us less confidence in the
exact placement of features. However, it is looking increasingly
likely another system will spin up from the parent Bering low and
bring enhanced winds and precipitation to the Gulf Coast on
Thursday. There are hints of especially strong easterly winds from
Kodiak Island up to the southern Kenai Peninsula, although this
too will be dependent on where features set up.

&&.AVIATION...

PANC...Dense freezing fog this morning appears to have become
less widespread, with higher clouds moving in over the area and
the fog mixing out. Winds this morning expected to be light and
variable, shifting northerly by late this afternoon as a trough
approaches the area. Light snow possible over the terminal late
today into Sunday, with highest likelihood expected this
afternoon.

&&

Preliminary Local Storm Report
National Weather Service Anchorage AK
1136 AM AKST Sat Dec 14 2024

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1050 AM     Snow             Valdez                  61.11N 146.22W
12/14/2024  M1.0 Inch        Chugach            AK   Trained Spotter

            0.02 in liquid equivalent. Lightly snowing.

&&