Small Craft Advisory 934, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
439 PM AKST Fri Dec 4 2020

.ANALYSIS AND UPPER LEVELS...
A broad weak upper trough extends south across Southcentral
Alaska. A weak short wave moving through the trough brought a
little light snow to the Copper River Basin. Shallow surface
inversions over the Anchorage Bowl and Matanuska Valley trapped
low low level moisture allowing for low stratus and patchy fog
which lingered into the afternoon hours today. A weak short wave
moving east over Southwest Alaska is squeezing a little moisture
in the form of flurries out of the stratus over the region. That
said, there are a few areas in the eastern portion of the area
which have little cloud cover. A surface low over the West-
Central Aleutians is heading east and weakening producing some
snow and rain along the Western and Central Aleutians. A second
low center, located south of Unalaska, is moving northeast and
bring strong easterly winds and rain from the Western AKPEN to
the Eastern Aleutians.

&&

.MODEL DISCUSSION(through Monday afternoon)...
The main feature will be a storm force low heading north toward
the Gulf this weekend. The NAM/GFS slower solutions are preferred
over the faster ECMWF. Tweaks to winds and Pops/QPF will likely
be required.

&&

.AVIATION...
PANC...MVFR to LIFR conditions are possible with stratus and fog
during the overnight and morning hours. Light winds will prevail.

&&

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

Low clouds have persisted across areas surrounding the Knik Arm
and Turnagain Arm this afternoon, and a further expansion of
these low clouds is likely again tonight with very weak synoptic
flow in place. Some patchy fog is also likely. Any areas that
manage to sneak in some clearing tonight are likely to see low
temperatures head below zero.

The next significant weather maker will lift out of the North
Pacific Saturday night, with briefly accumulating snow expected
in Kodiak early on Sunday morning before a quick changeover to
rain. Late Sunday the front associated with this storm system
pushes towards the Gulf Coast with precipitation starting as snow.
The precipitation will gradually mix with rain near the coast and
lower elevations from the eastern Kenai through Prince William
Sound Sunday night into early Monday.

The Anchorage Bowl looks to stay mostly dry through much of
Sunday due to cross-barrier flow. Chances for some light
precipitation making it over the Chugach look to increase a bit on
Monday. The precipitation type around the Anchorage Bowl could be
an issue at that time, with models indicating a stout temperature
inversion that could result in a snow/freezing rain mix.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Saturday
through Monday)...

The main weekend weather story for Southwest is windy, clear, and
cold. High pressure over the northern Bering and a low approaching
the AKPEN will bring some brisk northeasterly winds to the
Southwest mainland. Clear skies and little diurnal heating should
allow temperatures to drop a few degrees from Saturday into
Sunday. Overnight on Saturday wind chill may reach as low as
30 below. A slight pattern change will begin to take place for
the interior late Sunday as high clouds to the north of a warm
front move over Southwest. This should help to bring temperatures
back above the 0F mark.

The AKPEN will be much more active than the mainland as the north
Pacific low approaches on Saturday. Precipitation is expected to
remain as snow but southern coastal areas like Cold Bay and Sand
Point may see some rain mix in. By late Sunday night the front is
expected to be just east of the western Alaska Range. Downsloping
should help to warm coastal areas from Nelson Lagoon north to King
Salmon. Some precipitation may make it over the mountains in the
form of snow. By Monday, light snow may push into the Middle
Kuskokwim but accumulations are expected to remain below 3 inches.

&&

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

Active weather is expected across the southeastern Bering through
Monday. This is due to a broad and deep North Pacific low. This
low is expected to move south of the AKPEN on Saturday and then
drift slowly northeast towards the Gulf by Sunday night. A storm
force front is expected on the Pacific side of the Chain from
Dutch Harbor to Chirikof Island on Saturday. As the storm moves
east, the winds will pivot to the north resulting in strong gap
winds over the eastern Aleutians. For the eastern Bering, strong
offshore flow and heavy freezing spray is expected near the coast.
By Monday winds and waves will subside as the low moves into the
Gulf.

&&

.MARINE (Days 3 through 5)...

Gulf of Alaska: Fresh to strong winds will persist until Monday
night. Over Kamishak Bay and southern Cook Inlet (adjacent from
Anchor Point), strong northerly winds and hazardous wave heights
peaking near 11 to 13 ft will diminish by Monday night into
Tuesday morning. As the low races through the northern Gulf, it
loses its energy. Its strong winds begin to weaken too by Monday
evening. Winds will gradually drop near 20 kt between Tuesday and
Tuesday night. Solutions become a bit sketchy by Day 5, so will
focus mainly on Days 3 and 4 which will be a bit quieter for the
forecast area.

Bering: Minimum gales possible over the eastern Bering Sea Mon
into Tuesday. The focus shifts to a strong low approaching the
Central Aleutians on Wed. There will be a wide swath of gales with
the system and front as it sweeps north and east across the
Bering/Aleutians Wednesday through Friday. There is also a strong
possibility that embedded storm force winds will accompany this
system. Stay tuned for updates as confidence increases.

&&

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

The model performance between Monday and Tuesday (Day 3 and 4)
shows the placement of synoptic features in close proximity,
except the GFS is a bit distant from most solutions. After day 4,
there's a lot of uncertainty among the models, yet they pick up
the next potential storm from the Northwest Pacific around the
middle of the week. Shortwaves from the upper low will make their
way across Southcentral and beyond the AKPEN through at least
Tuesday morning. Southerly winds bringing warm air advection to
Southcentral will yield to a rainy pattern for the first two days.
Over Southwest Alaska, a nearly stationary long wave from interior
Alaska extends southwestward to Nunivak Island on Monday morning.
Models pick up this upper level feature and a nicely uniform area
of northerly flow, including a broad area of cold air advection
over the Kuskokwim Delta. Meanwhile, warm air advection continues
to extend over the AKPEN and fills parts of Bristol Bay through
Tuesday. An upper ridge over the Bering will either shift eastward
or settles over the central Bering until the longwave over
Southwest AK phases out. All in all, the forecast confidence is
moderate to high for Days 3 and 4.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning: 150 155 172.
Gale Warning: 130 132 138 139 160-171 173-176 179-185.
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...CB