Small Craft Advisory 928, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
418 PM AKST Wed Dec 2 2020

.ANALYSIS AND UPPER LEVELS...

A long wave upper trough extends from the Chukchi Sea to over
Southwest Alaska. A short wave embedded in the trough is moving
slowly northeast into the Cook Inlet Region. In the lower
elevations, cold air advection spreading north and east across
Cook Inlet, in conjunction with the upper short wave, is
producing snow across the Kenai Peninsula, Anchorage Bowl, to the
MAT-SU Valleys. Rain showers continue across the northern Gulf
Coast. Further west. A broad area of open cell cumulus snow
showers, associated with cold advection, extends across much of
the eastern Bering Sea, East Aleutians, and Ak Peninsula.
Generally mostly cloudy skies continued over the Southwest
Mainland. A couple of lows near the Western Aleutians continued
snow showers and locally gusty winds over the region.

&&

.MODEL DISCUSSION...
Out west...models are in relatively good synoptic agreement with
broad offshore flow developing over the Southwest Mainland
Thursday night. Models struggle however on low placement and
accompanying winds over the Aleutians beyond Thursday.

Southcentral...The overall synoptic pattern is similar between
models through Friday. Models struggle over the eastern Gulf on
Saturday. The NAM and GFS show a developing low moving north over
the eastern Gulf Saturday, with the NAM much further north and
stronger. The ECMWF and CAN NH have little there at that time.
The thought for now is that the NAM is the outlier and will
likely limit grid adjustments in the Saturday timeframe.

&&

.AVIATION...
PANC...MVFR to IFR conditions will continue through tonight.
Light southerly winds will shift north on Thursday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (through Saturday morning)...
Tonight's forecast will focus on a band of snow traversing the
Anchorage Bowl. The band will continue to work north and east
through the evening hours. While there will be lingering snow
showers behind the band, most of the accumulating snow will fall
with the band itself. Colder air will filter up the Inlet over the
evening hours, ending most of the snow changes. Snow will linger
the longest, through the mid-morning hours in the Matanuska Valley
as cold air drives up against the mountains. Areas of snow will
remain for the Valdez area through the Copper River Basin on
Thursday. Dry conditions follow for the Cook Inlet region along
with a colder air mass returning low temperatures in the teens
Thursday morning, with single digits on Friday morning.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...

A polar surface low tracks across the Northern Bering towards
Nunivak Island through tonight. Models are not in great agreement
on the location or track of this path, however, most of the
models stall the low near the Kuskokwim Delta coast. Although this
is not a strong system, it does have significant cold air
associated with it, meaning precipitation will be a dry, powdery
snow. Strong winds are not needed to produce blowing snow with
this set-up. Thus, snow and blowing snow is likely along the
Kuskokwim Delta coast on Thursday afternoon with snow
accumulations of around an inch. Elsewhere should see relatively
drier conditions through Friday night. Winds will remain light and
become primarily northerly by Friday. Clearing skies, radiational
cooling, and an inflow of cold arctic air will continue dropping
temperatures across the Southwest mainland over the next few days.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

Relatively quieter pattern across the Bering through tomorrow
morning when a complex North Pacific low approaches the Central
Aleutians. Main concern with this will be gale force winds along
the coasts and a brief period of storm force winds along the
southern edge of the Central Aleutians. Precipitation with this
should remain light and work its way west up the Chain tomorrow
morning through Friday. This should be a rain/snow mix or all rain
as slightly warmer air enters with this system.

A polar surface low tracks across the Northern Bering towards
Nunivak Island through tonight, approaching the Kuskokwim Delta
coast by tomorrow morning. Elsewhere, high pressure will dominate,
with southeasterly flow in the Western Bering, and northerly flow
in the Eastern Bering and along the Southwest Coast. Heavy
freezing spray will be a concern today along the Southwest Coast.

&&

.MARINE (Days 3 through 5)...

Gulf of Alaska: Off-shore winds through gaps and passes will be
present in the Western Gulf on Friday as cold air filters into
the Gulf. Outside of these gap winds, the Gulf stays quiet. There
is a weak low moving north into our forecast area Saturday, but
there is low confidence in this since long range models have been
very inconsistent. We are more confident in northeast flow that
develops over the Gulf on Saturday, which will enhance winds in
the Lower Cook Inlet and Shelikof Strait to small craft advisory.
However, there are still no major weather systems forecasted and
no winds above small craft advisory.

Bering: A low pressure system that has been becoming more
pronounced in models the past couple of days will be present just
south of the Central Aleutians Friday. It is expected to produce
weak gale force winds in the Central Bering Friday. We have at
least moderate confidence in this since it's been in models for a
couple days now. This low should weaken and stay south of the
Aleutians over the weekend. Winds then shift to the northeast for
the Bering and could become as strong as weak gale force winds in
the Eastern Bering, but this part of the forecast is still being
honed. Otherwise the Bering remains quiet.

&&

.LONG TERM FORECAST (Days 3 through 7)...

The next storm from the North Pacific will drive a couple shortwaves
over the region. The first shortwave will enter the Aleutians and
AKPEN on Saturday, followed by the next wave on Saturday night
aiming for Southcentral Alaska. Meanwhile, a long wave over the
area will wrapped in a developing upper low from Kuskokwim Delta
on Saturday night. The potential for shortwaves and longwave
passing through the forecast area could be continuous in the
first couple days of the long term. An upper level high over the
Bering is recognized by the models between Saturday and Saturday
night. Warm air advection from the Gulf and North Pacific will
invade Bristol Bay and Southcentral between Saturday and Monday,
which will keep the bulk of cold air advection well west and north
of the region. As the upper ridge over the Bering marches
eastward, the bulk of cold air advection across the Kuskokwim Bay
and Kuskokwim Delta will push eastward. Models share similar large
scale synoptic features up to day 4 (Sunday), then gradually
differs on the onset of complex lows from the Northwest Pacific.
Overall, the trend of shortwaves moving across the region is consistent
with the models, and the forecast confidence for day 3 and 4 are
moderate.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 145.
MARINE...Gales: 130, 172, 173, 175. Storms: 174, 176
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...BC
SOUTHCENTRAL ALASKA...ML
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CJ
MARINE/LONG TERM...CB