Small Craft Advisory 930, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
400 AM AKST Thu Dec 5 2019

.ANALYSIS AND UPPER LEVELS...

An upper level shortwave ridge is in place over Southcentral. This
has led to the development of patchy fog in places. Meanwhile, a
weak low spins over the Gulf allowing offshore flow to continue
along the northern Gulf coast. Out west, a large low centered
over the northern Bering is lifting a front over Southwest Alaska,
bringing continued snow and blowing snow to areas along the
Southwest coast. Upstream of this low, an upper level ridge is in
place over the western Aleutians.

&&

.MODEL DISCUSSION...

Models are in generally good agreement through Friday afternoon.
Solutions indicate that an upper level Bering low will move
eastward by Friday night, though there are vast differences in
the track the low takes and where it is centered over the eastern
Bering by Friday night. Models also indicate that the upper level
ridge currently over Southcentral becomes more amplified as it
moves eastward over the Alcan border by Saturday afternoon, which
would potentially yield a more active pattern over Southcentral
as any developing North Pacific lows would be steered over the
southern Mainland. Overall, confidence in the forecast beyond 48
hours is below average given model inconsistencies.

&&

.AVIATION...
PANC...Conditions are once again favorable for fog/stratus. Light
overrunning snow will develop late today. Expect MVFR conditions
by the evening.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Fog over the Anchorage area should dissipate around noon as higher
clouds begin to move over the area, but the timing on this is
difficult to determine and it could clear earlier or linger into
the afternoon. Westerly flow aloft will continue over the area
into Friday afternoon. This will bring a weather front that is
currently over southwest Alaska into Southcentral tonight. This
system will be weakening as it moves across tonight into Friday
morning. Some accumulating snow is possible from the Kenai
Peninsula northward through Anchorage and the Matanuska and
Susitna Valleys, but amounts are expected to be fairly light.
Southerly flow aloft will begin to develop over the area on Friday
night, signaling the onset of a warming trend.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Blowing snow conditions continue for southwest Alaska as a front
is pushing through. The worst conditions are along the coast where
visibilities are dropping below one mile at times. The front will
weaken through the day as it moves farther inland. A second front
will move into the area Friday and will be warm enough to bring
some rain to the coastline around Bristol Bay. Other areas around
southwest Alaska will stay primarily as snow. Similarly to the
first front, there looks to be enough wind with the second front
to bring back blowing snow to the area into Friday night.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The front over the eastern Bering will keep pushing east and move
onshore today. Behind the first front a new low has spun up near
the central Aleutians. This low is bringing another front into the
area from the south. It looks to make it to the Pribilof Islands
and Alaska Peninsula by this evening and is pulling up enough warm
air to bring mostly rain to the area. Cold air will move in
behind the front transitioning rain back to snow from west to east
across the Aleutians/Bering through Friday.

&&

.MARINE (Days 3 through 5: Saturday through Monday)...

The Gulf of Alaska remains quiet until early next week, when
southerly flow and low pressure is expected to bring in gale force
winds on Monday. Confidence is moderate in this occurring. With
southerly flow we will definitely have elevated winds and waves,
but to what degree is a bit of a question. Gale force looks to be
a reasonable solution for now.

In the Bering, low pressure will be present in multiple areas. Two
lows will be present over the weekend, one in the western Bering
and the other over the Central Aleutians. Both are forecast as
gale force for now. Models are quite erratic and consistency is
poor both over time and between different models. We are confident
in low pressure producing elevated winds and waves, but exactly
where it will be windy and how strong those winds (some models
think storm force) could be is a big question. We are not
confident in the specifics. So we will need to hone this in the
next couple of days.

&&

.LONG TERM FORECAST (Days 3 through 7: Saturday through Wednesday)...

Long term analysis still points to southerly flow and considerable
warming over the weekend for Southcentral AK. These conditions
look to persist into early next week. The high amplitude ridge
responsible for this looks to form rapidly Saturday along the
ALCAN border and push all the way into the Arctic. We can also
expect low pressure embedded in this flow to bring precipitation
to coastal areas and elevate winds. However, model guidance is
continuing to be inconsistent over Southcentral, so details will
have to come in time. However, we are confident in a big warm up
- threatening our snow pack - for Southcentral this weekend.

Out west, broad low pressure is expected to be present over the
entire area. An entrenched long wave trough will have several low
pressure systems of varying intensity spin through for the
duration of the forecast period. Right now, the forecast starts
with two low pressure systems (called a double barrel low) with
one low in the western Bering and the other over the Central
Aleutians. Models are quite erratic in the long term so the
solution for low pressure will evolve, but there is enough
agreement in models to go with this for now over the weekend. Low
placement will likely change as guidance does and it should be
noted that by early next week multiple lows will be rotating in
the long wave trough and right now it's not clear where they will
be. So stay tuned...

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory...155 161.
MARINE...Gales...160, 180, 181, 185, 411, 412, 413.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...KO
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DK
MARINE/LONG TERM...BB