Small Craft Advisory 125, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
403 PM AKST Thu Feb 6 2020

.ANALYSIS AND UPPER LEVELS...
There is a weak upper level short-wave ridge over southcentral
Alaska. This is leading to relatively benign weather, with light
winds and just some showers over the Copper River and northern
Susitna River valleys. There is nearly vertically stacked low over
the Alaska Peninsula, moving slowly northeast and weakening. This
is producing precipitation over the Alaska Peninsula and the
coastal areas around Bristol Bay. The arctic front stretches west
to east across the Bering, located just south of the Pribilofs.
It then swings northeast through about Cape Newenham into the
lower Kuskokwim Valley. There is northerly flow to the north of
this front, with temperatures in the Kuskokwim Delta area
generally below zero F.

&&

.MODEL DISCUSSION...
The numerical models are in good agreement on the synoptic pattern
through Saturday afternoon. There are some differences in the
timing of a surface low crossing the eastern Aleutians late
Friday and then into southwest Alaska on Friday. The Canadian GEM
is the slowest with this system, lagging almost 6 hours behind the
other models. The NAM and ECMWF were the fastest, with the GFS
somewhere in between. Due to these differences and the impact this
has on significant weather, the forecast confidences is near normal.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist. There is a
risk of MVFR conditions in low clouds and/or light fog tonight,
but for now have left out of the TAF.

&&

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

Fairly benign weather will be in place for most of southcentral
through tomorrow. The majority of weather will be with a weak
front moving in from the southwest this evening. Rain and snow
will move through Kodiak, though it'll transition to all liquid as
temperatures continue to rise and will remain above freezing
through the weekend. The same front will bring rain to coastal
locations along the peninsula and Prince William Sound through
Friday morning. Besides clouds, Anchorage and interior areas may
see the occasional flurry, but should remain dry through Friday
afternoon. Atmospheric winds will run perpendicular to the
mountains, helping to keep most areas dry. There will be a 3(ish)
hour window Friday evening where flow at all levels turns out of
the southwest, and may give us a window for light snow.

The weekend brings on the biggest changes across the region as a
second, and stronger front, approaches from the southwest. This
front will pack much more of a punch with more rainfall and warmer
temperatures. As tends to happen in the winter, this front will
align with such an orientation that will funnel winds down
Turnagain Arm late Saturday morning and into the afternoon hours.
Unlike some of the events that Anchorage has seen this winter,
this is expected to be brief. The window for strong winds along
the arm and higher elevations looks to last no more than 6 hours.
Winds may peak close to 60-65 mph, but winds in town are not
expected to be stronger than 20-25 mph over a few hours. It's
during this time that we'll see a quick spike to our temperatures
(chinook wind). Daytime highs on Saturday will jump into the upper
30s to lower 40s. Mid 40s aren't entirely out of the question,
but as the evening approaches, winds will return to a
southwesterly orientation, bringing temperatures back into the mid
30s. Depending on the time of day and strength of the warming,
rain to wet snow will fall. Most areas may see up to a half of an
inch of snow, while upslope mountain areas will see considerably
more.

&&

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

Showers continue to diminish in coverage over the Kuskokwim
Delta, though areas of new snow on the ground continue to blow
around and diminish visibility. Meanwhile, a vertically stacked
low over the AKPEN is pushing a front towards Bristol Bay. South
flow ahead of the low will make for a good moisture source, and
expect stratiform snow/snow showers to spread over the Greater
Bristol Bay area north into the middle/lower Kuskokwim Valley
tonight into tomorrow. An Arctic front continues to sag south into
the Kuskokwim Delta. Temperatures along the Kuskokwim Delta coast
have remained below zero throughout the day, with wind chills
already to -40 in Chevak. Since the Arctic air mass is spreading
from the Bering Strait, expect the worst conditions along the
coast, particularly north of Kipnuk. However, wind chills should
still periodically get down to 40 below farther inland e.g. in
Bethel. A Wind Chill Advisory remains in effect this evening
through Friday night.

A strong low will approach from the eastern Aleutians Saturday.
Expect southwest winds along the Bristol Bay coast to gust to 50
mph Saturday afternoon and evening, and coastal flooding/ice
shoves from high surf is also possible. Since this is nearly 48
hour out, stay tuned for further updates. As for precipitation,
expect 1 to 3 inches of snow in the Bristol Bay area Friday night
into Saturday and in the Kuskokwim Valley. With some southerly
flow along the front, there is potential for snow to briefly
change over to rain or rain/snow mix south of Dillingham early
Saturday. However, precipitation should change back over to snow
in colder westerly flow behind the front. In the Kuskokwim Delta,
expect around 2 to 5 inches of snow Friday night into Saturday.
More importantly, northeast winds gusting as high as 40 mph
combined with snow falling will significantly impact visibility
Friday night. A Winter Weather Advisory has been issued for this.
On the bright side, a warming trend with this system will
eliminate the hazardous wind chills early Saturday.

&&

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

A compact low over the western AKPEN (Cold Bay/King Cove area)
will bring a small swath of gales south of the peninsula, while
widespread small craft advisory level easterly winds will persist
over much of the Bering this evening. Over the Pribilof Islands,
blowing snow continues to limit visibility. Easterly winds over
much of the Bering should hit gale force tonight into Friday with
heightened freezing spray concerns as Arctic troughing strengthens.
Friday afternoon, a low from the North Pacific will approach the
central/eastern Aleutians and bottom out around 960 mb. Expect a
wide swath of storm force winds with this, mostly on the Pacific
side of the central/eastern Aleutian coastal waters...though
there is some uncertainty with the low's position/track. Expect a
small swath of hurricane force winds. Along the AKPEN, expect
gusts to 60 mph Friday night into Saturday. Saturday
afternoon/evening, storms may make it into Bristol Bay. Besides
winds, expect a good shot of precipitation from the central
Aleutians to the AKPEN. Meanwhile, with storm force winds over the
Pribilof Islands and a steady round of snow Friday night, expect
significant impacts to visibility. A Blizzard Warning is in effect
Friday evening into early Saturday morning. Expect whiteout conditions.

&&

.MARINE (Days 3 through 5: Sunday through Tuesday)...
A gale force storm will move through the central and western
Aleutians into the western Bering from late Sunday into Tuesday.
Storm force winds are possible along the frontal boundaries. There
is still considerable uncertainty for the timing and strength of
this system.

For the Gulf, gale force winds over the northwest Gulf and
Shelikof strait Sunday morning will quickly diminish during the
day.

&&

.LONG TERM FORECAST (Days 3 through 7: Sunday through Thursday)...
We start the long term period Sunday with a weakening low near
the Seward Peninsula, high pressure south of the Gulf of Alaska
and another storm on tap moving toward the Aleutians and Bering
Sea. Models are in generally good agreement with the location of
the low moving into the Aleutians Sunday. However, they differ
significantly with the intensity of this storm. The models have
similar solutions with the timing of the progression of the front
from this storm across the Aleutians and Bering Sea through Monday
but continue to diverge significantly with the storm center
through Wednesday, thus leading to little confidence in the
current model solutions. The approach is to use an ensemble and
blended approach beyond Monday.

This pattern and scenario support the Arctic air pushing back
across the southern mainland Monday and persisting through
Thursday, thus returning the mainland to the colder air, similar
to the past few weeks. However, here the models differ significantly,
and temperature forecasts Tuesday through Thursday differ by
nearly 20 degrees, again leading to very low confidence in any one
solution and low confidence in the extended forecast period. The
Bering Sea and Aleutians will be in the highly active weather
pattern, with frequent periods of strong winds and precipitation
through the extended period.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Wind Chill Advisory 155.
         Winter Weather Advisory 155.
         Winter Weather Advisory 195.
         Blizzard Warning 195.
MARINE...Hurricane Force Wind Warning 172 174.
         Storm Warning 150 155 165 170 171 176 179 180 181.
         Gale Warning 130 131 132 138 139 141 160 173 175
                      177 178 185.
         Heavy Freezing Spray Warning 160 179 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...SS
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MM
MARINE/LONG TERM...BL/SA