Small Craft Advisory 938, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
417 PM AKST Sat Dec 5 2020

.ANALYSIS AND UPPER LEVELS...
Low level temperature inversions allowed the stratus and fog to
persist across Turnagain Arm, Anchorage Bowl, Matanuska Valley,
and Copper River Basin. Satellite imagery and observations showed
some lifting of low conditions across the Anchorage Bowl. With the
exception of the aforementioned areas, generally fair skies
prevailed across Southcentral. Southwest Alaska for the most part
experienced fair skies with broad offshore(north) flow. Gusty
northerly winds combined with temperatures below zero in spots
produced wind chills in the minus 20s. Satellite imagery showed
an intense fully occluded surface low just south of 50n and near
160w. The low with its associated front were moving north. Winds
in advance of the front were increasing with the approach of the
front and should reach storm strength in the coastal waters south
of the Alaska Peninsula. Gale force easterly winds were evident
over the eastern Bering Sea becoming northerly along the
Aleutians.

&&

.MODEL DISCUSSION...
Models synoptically are in good agreement through Tuesday
afternoon. Hand edits and higher resolution models will be used to
capture the finer details.

&&

.AVIATION...
PANC...light northerly winds are expected through tonight. Low
stratus and fog continues to lurk across the Anchorage Bowl and
Knik Arm. The hope is that the northerly winds will remain steady
and minimize the potential for low conditions to redevelop at the
terminal. That said, still can't rule of the potential for
occasional lower ceilings/visibility overnight tonight.

&&

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

Low clouds, patchy freezing fog and flurries have continued
around the Anchorage area into this afternoon up against the
Chugach Mountains. Northerly flow will increase tonight which
should result in a gradual erosion of these low clouds.

Attention now turns to a powerful North Pacific low that will be
lifting into the region over the next couple of days. This low is
currently analyzed at a stout 944 mb and is likely at its peak
intensity, with gradual weakening expected as it lifts into the
western Gulf of Alaska late this weekend into Monday.

Ahead of the low, a front will lift towards Kodiak Island tonight
into early tomorrow morning, bringing a couple of inches of snow
before a rapid transition to rain later tomorrow morning.

The front then lifts north and begins to impact the eastern Kenai
Peninsula through the north Gulf Coast starting later tomorrow
morning, with precipitation near the coast starting as a rain/snow
mix, but changing to mostly rain in the lower elevations by
tomorrow evening. The Anchorage Bowl looks to see little in the
way of precipitation on Sunday thanks to strong cross-barrier
flow.

As the front pushes further into the interior, snow levels will
steadily rise, climbing to 1,500 to 2,500 feet from the Eastern
Kenai Peninsula through Prince William Sound on Monday. There
remains the potential for some mixed precipitation around
Anchorage on Monday, but any mixed precipitation that does occur
looks to be quite light given continued moderate cross-barrier
flow.

On Monday night/early Tuesday the low heads towards Prince
William Sound while continuing to weaken. Cross-barrier flow off
the Chugach will weaken allowing for increasing precipitation
chances across the Anchorage Bowl. While a wintry mix is possible
during the early morning hours, colder air should work in by late
morning changing things over to all snow. Some light accumulations
are not out of the question for Tuesday afternoon/evening.

&&

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

Increasingly active weather is expected across Southwest to start
the week. This is due to a strong and broad North Pacific low
moving northeast from 45N 160W Sunday into the north Gulf Coast
late Monday. As the storm slowly moves northeast, its broad
influence will help strengthen an elongated upper level low as it
moves south and over the region from the northern interior. Lee
side troughing is also expected to occur which will aid in the
development of a surface low over Bristol Bay late Monday. This
storm should bring widespread light snow to Southwest with the
heaviest snow over the Y-K Delta under the occluded front.
Moderate northeasterly winds will also bring the potential for
blowing snow late Monday into Tuesday.

&&

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

A storm force low in the North Pacific will move slowly northeast
into the Gulf of Alaska by Monday night while high pressure
exists over the northern Bering. This will result in persistent,
strong northeasterly winds over the eastern Bering through the
forecast period. Development of a low over Bristol Bay on Monday
will to reinforce these winds on Tuesday with snow likely as well.
Heavy freezing spray is also possible along the coast of
Southwest Alaska.

&&

.MARINE (Days 3 through 5)...

Bering: Tuesday starts with strong north-northeasterly flow in the
Eastern Bering and quiet elsewhere. Tuesday night into Wednesday
models are showing a strong low pressure system moving into the
Western and Central Aleutians. Models have a good amount of
disagreement on this low, but there is enough information to know
that this system will have a front with at least gale force winds
with it. These winds will impact the Western and Central
Aleutians/Bering. This low should also induce easterly flow for
the eastern Bering for most of the week.

Gulf of Alaska: On Tuesday a strong low will just be exiting the
Gulf of Alaska and weather should be calming down. Southwest winds
should be in place behind it. Another low is anticipated to enter
the Gulf of Alaska on Wednesday, but will be much weaker with
little to no marine impacts. Models remain in disagreement on the
track of this next low, but all do agree it won't be as strong, so
we at least have moderate confidence in the forecast of low
pressure but little weather from it next week.

&&

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

Significant uncertainty persists in the Long term guidance. This
poor performance leads to little confidence in what will happen
mid to late next week. Right now, we know there will be a strong
low just south of the Western and Central Aleutians Wednesday and
low pressure in the Gulf of Alaska. However, placement of these
systems vary substantially between models. To further compound
the issue, moving forward in time from mid week to the weekend,
models progress these systems in very different directions.
Therefore, it's not possible to tell what will transpire, which
results in low confidence in overall in for the long term picture.
So, we know we will have generally unsettled weather for maritime
and coastal environments, while interior areas look to be quiet.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning: 138 150 155 172.
Gale Warning: 119 120 130-132 139 160-171 173 174 179-185 352 412 414.
Heavy Freezing Spray Warning: 139 140 160 180 181.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...RMC
SOUTHCENTRAL ALASKA...BG
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CJ
MARINE/LONG TERM...BB