Storm Warning 223, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
444 PM AKST Sat Dec 12 2020

.ANALYSIS AND UPPER LEVELS...

A high amplitude upper level ridge was present extending northward
from western North America into Southeast Alaska including over
the Copper River Basin. To the west of this upper level ridge was
a potent upper level low over the North Pacific. This upper level
low has a corresponding surface low with a front extending south
to Hawaii. Looking at CIMSS Precipitable water, we see a moisture
feed extending from Hawaii northward to where the center of the
low is. An upper level low/trough was present across the North
Slope of Alaska. Another larger upper level low/trough was present
across the Bering Sea.

Across the Bering Sea, a broad upper level low and associated
surface low is keeping the region rather unsettled, especially the
eastern to northern Bering Sea. Latest ASCAT Scatterometer data
shows gale force winds occurring north of Nunivak Island. The
Aleutian Chain remains fairly quiet with some breezy conditions
for the western Aleutians.

&&

.MODEL DISCUSSION...

Models are in fairly good agreement out through Monday night/Tuesday
morning before diverging significantly. All models are on board
with an elongated storm system lifting northward from the North
Pacific with 2 low pressure centers. One low center is expected to
pass over southern Kodiak Island with the other low moving
northward towards the north Gulf Coast. All models agree on
another low moving northwestward through the northern Bering
towards east Russia before dissipating Sunday night. Attention
then turns to another low pressure moving northward from the North
Pacific toward Southeast Alaska Monday night into Tuesday with
most impacts expected there. A front moves into the western
Aleutians/Bering Sunday afternoon with model guidance showing a
new area of low pressure forming along this front Sunday morning
and then moving eastward staying south of the Aleutian Chain
Monday across the North Pacific. Models then form another low
Sunday night southwest of the Aleutian Chain before bringing it
northward towards the western Chain Monday night into Tuesday.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist through the TAF
package.

&&

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

A complex low pressure system will continue moving northwest
across the Gulf of Alaska through Monday afternoon. Gale to storm
force winds and high seas will persist across the Gulf through
Monday morning, as the system weakens and moves inland across the
north Gulf coast. Offshore flow will continue for the duration, as
the pressure gradient remains between the low in the Gulf and high
pressure over the mainland. Precipitation will continue as rain
through Monday for coastal locations and Kodiak Island as warm
air approaches with the low. Conditions will dry out by Tuesday
afternoon as an upper level ridge sets up across the Gulf.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Sunday
through Tuesday)...

The combination of three upper level lows will bring snow to
Southwest Alaska through Tuesday. The moisture and upper level
energy associated with an oblong low over Gulf will bring
continuous light snow to Bristol Bay on Sunday. The heavier snow
should be over and to the northwest of Dillingham with totals up
to 6 inches by Sunday night. Localized areas of higher
accumulations are possible over areas of high terrain. Winds look
to be between 15 and 20 mph, which may create some areas of
blowing snow. However, due to warmer temperatures aloft the snow
is expected to be on the denser side, so hazardous visibility
reductions are unlikely. Any remaining moisture making it over the
Kilbuck Mountains will produce some light snowfall for the
Kuskokwim Delta with totals up to 2 inches by Sunday evening,
primarily near the coast. The remnants of an upper level ridge
over the interior should keep the Lower Kuskokwim fairly dry.
Downsloping is likely to occur adjacent to the western Alaska
Range and Aleutian Mountains resulting in mixed precipitation and
warmer temperatures from Iliamna south.

The transition to all snow throughout Bristol Bay and the northern
AKPen should occur Sunday night as an upper level low over the
eastern Bering brings in colder air aloft. This low will also absorb
the energy from the Gulf low to create a new low over the eastern
Bering. Thus, snow is expected to continue throughout the day on
Monday with the higher totals once again favoring the northern
half of Bristol Bay. An additional 2-4 inches is possible by
Monday night. On Tuesday, onshore flow will begin to develop over
Bristol Bay, thus bringing another chance for snow to the region.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3: Sunday
through Tuesday)...

A broad region of low pressure is expected to encompass the
central Gulf on Sunday leading to mostly calm conditions
throughout the region. Cold westerly winds should continue over
the Aleutian Chain through Sunday night, producing rain and snow
showers. Active weather looks to return to the western Bering
Monday night as a compact, storm force low moves over Shemya.
There is some uncertainty in the track of the low, with some
guidance continuing an eastward propagation while others re-curve
the storm back west towards the Kamchatka Peninsula. The front
associated with this system should reach Saint Paul and Dutch
Harbor by Tue evening. By that time it will have lost most of its
energy, leaving behind winds around 25 mph and rain.

&&

.MARINE (Days 3 through 5/Tuesday through Thursday)...

Gulf of Alaska: Confidence is average to high for a storm force
low to move northeastward from the North Pacific towards Southeast
Alaska Monday night through Tuesday. This system may graze the
central Gulf with small crafts but any gales and storms are
expected to remain across the eastern areas of the Gulf.
Confidence is low beyond Wednesday afternoon in regards to where
the next potential gale force system tracks.

Bering Sea: Confidence is average for a gale force system to
impact the western and central Bering Sea Monday night through
Tuesday. The potential is there that this storm may be a storm
force low but there remain some intensity discrepancies. Beyond
Tuesday night, confidence is low on whether or not another
potential gale force system develops and affects the eastern
Bering Sea or rather benign conditions will prevail with sub small
crafts.

&&

.LONG TERM FORECAST (Days 4 through 7/Wednesday through
Saturday)...

The weather pattern from mid week through the first half of next
weekend continues to look quite dynamic. The subtropical jet
stream is expected to remain strong over the North Pacific with a
deep Arctic low/trough anchored somewhere over the Yukon and
Mainland Alaska. More often than not, the subtropical jet will
phase with the northern polar jet stream energy. The combination
of these two will continue to spawn storm systems anywhere from
the western Bering/North Pacific to the Gulf of Alaska with the
details of each of these being entirely uncertain (intensity/track
of each one). Bottom line for this period is expect periods of
sensible weather impacts with occasionally quiet conditions
between any systems.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale 119 120 131-139 150 177 181 185.
		 Storm 130.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...MV
SOUTHCENTRAL ALASKA...CK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CJ
MARINE/LONG TERM...MV