Small Craft Advisory 931, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
529 AM AKST Thu Dec 3 2020

.ANALYSIS AND UPPER LEVELS...

At 300 mb, a strong zonal 160 kt jet is positioned south of the
Aleutian Chain. Meanwhile at the 500 mb level, several troughs
are in place. One is over the northwest extending from the
Kamchatka Peninsula toward the western Brooks Range, and a second
is positioned south of the central Aleutians. An upper level ridge
separates these two troughs over the eastern Bering. At the
surface, a band of snow showers extends from the southeast to the
northwest from Portage Valley to the Susitna Valley. Southwest has
clear and cold conditions this morning thanks to the departing
trough and the building ridge.

&&

.MODEL DISCUSSION...

Overall, models are in good synoptic agreement for the short term.
There are some differences of note with a complex North Pacific
low positioned south of the Aleutian Chain. While models agree on
the strength of this low and advertise storm force winds, the
track and timing are less certain. By tonight, this feature
becomes a double-barrel low, and models are struggling to resolve
the timing of gale force winds to the eastern Aleutians Friday.By
Saturday, model solutions have been fairly consistent with the
development of a 950 mb storm force low south of the eastern
Aleutian and tracking it northeastward into the Gulf Sunday.

&&

.AVIATION...
PANC...VFR conditions expected. Gusty southerly winds should taper
this morning and shift to northerly by the afternoon.

&&

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

The cold front has swept through most of Southcentral Alaska and
is now in the eastern Prince William Sound to central Copper
River basin early this morning. Temperatures are dropping behind
it and usher in much colder temperatures throughout the region
for the next couple of days. The upper level trough is currently
in the Susitna Valley just east of Skwentna into northern Cook
Inlet near Fire Island. This can be seen on satellite imagery and
has kept a band of light snowfall from Eagle River northward
through Talkeetna and eastward to the Talkeetna Mountains most of
the night. This trough is moving slowly eastward and weakening
which will shut off remaining snowfall this morning as it passes.
As alluded to earlier, Friday will see colder and drier conditions
across Southcentral.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3/Thursday
through Saturday)...

A polar surface low reaches the Southwest coast today. Impacts
will be minimal with very light snow accumulations for the
northwestern portion of the Kuskokwim Delta. Otherwise, the
Southwest mainland will continue to see clearer and drier
conditions as the upper level trough moves out and high pressure
moves in. Radiational cooling and persistent northerly flow will
allow temperatures to continue dropping over the next couple
days.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through
3/Thursday through Saturday)...

A series of North Pacific lows track just south of the Aleutian
Chain throughout the forecast period. Models are in decent
agreement through Friday morning with widespread gales and small
craft winds, as well as a brief period of storm force winds this
afternoon. Precipitation will be light and in the form of rain or
a rain/snow mix. Models begin to diverge Friday afternoon before
coming into decent agreement again by Saturday, widespread gales
and small crafts are likely with light widespread precipitation.
On Saturday, another round of storm force winds are likely south
of the AKPEN. Periods of heavy freezing spray will be an issue
along the Southwest coast through Saturday.

&&

.MARINE (Days 3 through 5)...

Gulf of Alaska: Off-shore winds through gaps and passes will be
present in the Western Gulf on Friday and 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 as 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 its 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...NONE.
MARINE...Storms: 174, 176.
         Gales: 130, 155, 170, 172, 173, 175, 413.
         Heavy Freezing Spray: 160.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...KO
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KM
MARINE/LONG TERM...CB