Small Craft Advisory 56, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
358 PM AKST Thu Jan 18 2018

.ANALYSIS AND UPPER LEVELS...
There are 2 primary features across Alaska today. The first is a
cut-off, vertically stacked low parked just south of the Kenai
Peninsula. This low continue to wrap moisture across most of
southern AK. For the southwest, this is resulting in lingering
snow showers from Bethel down through King Salmon. These snow
showers are definitely over overachieving as they are working with
very little forcing and moisture. However, they are not much
producing much in terms of snow accumulation with King Salmon only
receiving 0.2" from 8am to noon. For areas from the Cook Inlet
through the Copper River Basin, this moisture is resulting in low
clouds and some extensive fog along the Copper River.

The other primary feature is a strengthening surface high under a
building ridge near the Gulf of Anadyr. As anti-cyclonic
(clockwise) flow continues around this feature, it will have
access to abundant cold air. It is already starting to pull some
of this cold air southward from the Chukchi Sea (and Siberia). It
could be aiding in keeping the snow showers going over SW AK. The
cold air will become the story for the weekend and into next week.

&&

.MODEL DISCUSSION...
Models in the short-term are doing quite well and forecast
confidence is rather high. The few specific areas of uncertainty
are the snow showers in SW, timing when fog dissipates, and how
much moisture can work back from the east to the west over South
Central in the upcoming cold pattern. Models are not doing great
with the current snow showers, so satellite and radar trends were
used to address those. They will continue to die off through the
day. Higher resolution models were used in trying to depict the
fog and lower stratus across Southcentral. There is not much
forcing to help "kick" that stuff out of the lower lying areas, so
have leaned towards keeping it in the forecast longer. Then
finally is how much moisture will make it up to the North Gulf
coast over the next several days. Models all show various pieces
of energy/moisture working from east to west in broad cyclonic
flow. As the cold air digs in from the north and west, we will
closely watch this moisture and see if it could lead to any snow
for inland areas.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist. Some patchy low
clouds from the Cook Inlet could float near the terminal, but
don't expect them to become widespread.

&&

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

The main forecast challenges heading into the weekend are gusty
offshore winds and colder temperature trends. A weak upper low
remains stationary through tonight then becomes and open wave by
Saturday. This center aids in keeping clouds about the area
through Friday which allows for temperatures to slowly moderate
downward. Areas of fog develops again tonight across the Copper
River as low level moisture remains abundant and steep temperature
inversions set up just off the surface. Improvement is expected
during the day on Friday, however this cycle continues through
Saturday as the pattern remains stagnant over this area.

Offshore flow, which developed today, gusts along typical gaps
locations through the weekend. The strongest gusts are expected to
increase late Saturday as the ridge forces the cold air mass from
the Southwest mainland to the western Gulf. The building ridge
over the Bering moves further east with subsidence drying along
its eastern periphery. This should briefly clear out skies across
the western Gulf, Cook Inlet to the western Susitna Valley. A low
pressure system tracking along to the eastern Gulf limits the
ridges eastward progression as it takes a northward turn late
Saturday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA...
The main story for the next couple days will be increasing
northerly winds and colder air moving into Southwest. The first
push of much colder air comes Friday. Winds will be strongest
across the Kuskokwim Delta and coastal Bristol Bay, and with cold
air moving down from the Arctic, some areas (especially the
Kuskokwim Delta) can expect wind chill values to drop to -25F
Friday night and as low as -35F Saturday and Saturday night.
Otherwise, this air mass will be bringing down much drier air, so
any flurries/patchy fog tonight should be replaced by clearing
skies.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS...

A weakening low over the western Bering Sea and the associated
stalled front will fall apart tonight as the Bering Sea comes
under increasing influence of offshore flow originating from the
Bering Strait and Southwest coast. Snow showers will be possible
through Friday, but mostly dry conditions and northerly flow will
begin to dominate. However, a gale force front will move into the
western Bering Friday night then stall in place as high pressure
builds over the central Bering Sea.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The long term begins Sunday night and Monday with a high
amplitude upper high over the Bering Sea and a trough to the east,
which includes a closed low in the Gulf of Alaska and another over
northern Alaska. There is weak ridging in between the two lows
over Southcentral. Strong northerly flow will be over the Alaska
west coast and Alaska Peninsula. During the first half of next
week the Gulf low will slide east, and the Bering high will
weaken. This will allow the north Alaska low to dive southward,
reaching southern Alaska around the middle of next week. The exact
path and timing of this southward traversing low is not agreed
upon exactly by the models, but agreement is good enough for a
fairly high confidence forecast for this time range. The end
result of all of this is a fairly cold regime with temperatures
below normal over mainland southern Alaska. Some precipitation is
possible through the period for the Gulf coastal areas. There is a
chance some precipitation could get further inland, depending on
how the Gulf low behaves, but confidence in this is low.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 177 178 180.
         Heavy Freezing Spray 160 180 181.

&&

SYNOPSIS AND MODEL DISCUSSION...MSO
SOUTHCENTRAL ALASKA...KH
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
LONG TERM...BL