Brisk Wind Advisory 29, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
453 PM AKST Sat Feb 15 2020

.ANALYSIS AND UPPER LEVELS...

A zonal polar jetstream which extends across the Aleutians and
through the southern Gulf of Alaska is in the process of
amplifying. The subtropical jetstream remains south of 50N across
the North Pacific. Surface high pressure extends southwestward
across mainland Alaska from northern Yukon Territory. A large area
of low pressure approximately 300 nm northwest of Saint Paul is
slowly moving northward. A shortwave trough is moving through the
Kuskokwim and Bristol Bay bringing snow to the area with warmer
temperatures along the coast. Another area of light snow is moving
eastward through the central Gulf and Prince William Sound.

&&

.MODEL DISCUSSION...

Models are consistent with the overall synoptic pattern for
Sunday and Sunday night. However, some discrepancies arise on
Monday with respect to the deep low approaching southwest Alaska.
Even so, solutions are at least grouped together and all models
depict a storm force low. As a result, will overhaul the forecast
for the eastern Bering and Southwest Alaska to better depict the
impacts of this storm.

Major model disagreement begins on Tuesday, particularly with
development and track of a surface low across the Gulf of Alaska.
Conditions are favorable for cyclogenesis, as a high amplitude
trough progresses eastward underneath the western Alaska Low and
swings into the Gulf. Arctic air accompanies it into the Gulf.
There are differences with individual short-waves moving through
the larger trough as well as with the tilt of the trough. For now,
will not completely buy off any particular solution and wait until
there is better agreement before making any significant changes
to the forecast.

Forecast confidence is high for Sunday, moderate for Monday and
low Tuesday onward. Despite the differences in model output, we
are confident that the overall weather pattern will be active with
impactful weather likely across much of the forecast area. As is
typical, higher resolution models were favored for small-scale
localized effects in this forecast package.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

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

An amplifying pattern will bring a warm front across Kodiak Island
tonight. This warm front will bring moisture and warmer
temperatures, so the southwestern part of the island will see rain
while Kodiak City will start off as snow before transitioning over
to rain. As this front moves northeast across Southcentral
through Sunday into Monday night, snow chances will increase for
coastal areas, higher elevations and the Susitna Valley.

As the front propagates into the central Gulf, a barrier jet will
set up along the northern Gulf. The southeasterly flow associated
with this barrier jet will make it's way into Prince William
Sound. This is an indicator of the beginning of a Turnagain Arm
wind that will start early Monday morning and carry into Tuesday.
As the wind ramps up, warmer temperatures will make their way into
the Anchorage Bowl with the highest temperatures expected on
Tuesday. This flow pattern also indicates downsloping as the
mountains on the windward side of this flow pattern receive the
bulk of the precipitation. Areas on the leeward side, including
Anchorage and the western Kenai Peninsula look to remain mostly
dry with only up to a 20 percent chance of precipitation for these
areas.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Tonight
through Tuesday)...

Major changes were made to the forecast this afternoon, in
response to improving agreement between the models on the track
and likely impacts resulting from a strong low moving north up the
west coast from the North Pacific. More on that below.

The light snow ongoing across much of the Kuskokwim Delta, Lower
Kuskokwim Valley, and portions of Bristol Bay this afternoon are
the result of a dying front moving across the area. Visibilities
across the area should gradually improve overnight. The break
between the systems will be brief during the day Sunday. Late
Sunday afternoon, a warm front will move south to north across the
area. This front will increase the winds, particularly across the
Kuskokwim Delta. Much of the rest of the area will pick up 1 to 3
inches of snow with this front, but it will be a precursor to the
more active weather alluded to above. The front itself will also
fall apart and weaken overnight, but snow showers are likely to
persist across the Kuskokwim Valley. With the windiest conditions
expected along the Kuskokwim Delta coast, it is those areas where
visibilities are most likely to drop to one half mile or less late
Sunday afternoon and evening.

Following right behind this front will be the strong low mentioned
above that will cause much of the hazardous weather that will
impact Southwest Alaska. A second warm front at the leading edge
of the low will begin the precipitation over Southwest Alaska. The
warm air with this feature will be enough to cause any snow that
begins at the onset to mix with and change over to rain across
much of Bristol Bay, mainly after midnight Sunday night. Strong
southeasterly winds through the gaps, particularly across Lake
Iliamna, will lead to downsloping which will shut off the rain or
make it light from Iliamna through King Salmon. The main
precipitation will ramp up across the Kuskokwim Delta a bit later,
after midnight. From Kipnuk and Kuskokwim Bay south, the snow may
mix with rain, especially Monday morning. Elsewhere from Bethel
north and west, snow will be the sole precipitation type. Winds
will increase markedly with this low, especially along the coast.
A Winter Storm Watch for blizzard conditions is in effect west of
Bethel. The watch begins Sunday afternoon, but that is with the
aforementioned warm front bringing some wind and snow, but not
enough where blizzard conditions are expected. Late Sunday night,
with the main low, is when the worst conditions west of Bethel
are possible. From Monday afternoon on, there are some significant
differences between the models on the storm track. However, the
most common one involves the low center stalling off the coast.
While snowfall rates will be much lesser Monday afternoon and
Monday night at the coast, the gusty winds and already fallen snow
will keep blowing snow advisory conditions ongoing throughout the
night and into Tuesday morning. Cold air will quickly wrap around
the low by Monday evening, which will transition all of Southwest
Alaska back to snow.

The leading front, now occluded Monday evening could possibly
result in a deformation band developing for Bristol Bay and the
Lower Kuskokwim Valley. This could bring a few inches of snow to
many of the areas that changed over to rain or didn't get much
precipitation at all. This band moves north and east across the
Alaska Range Monday night. As the weakening primary low very
slowly drifts eastward into Southwest Alaska on Tuesday,
widespread snow shower activity is likely to persist throughout
the day, resulting in additional minor accumulations for the
entire area.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3:
Tonight through Tuesday)...

A strong low over the northern Bering keeping active weather
ongoing for much of the basin will continue gradually weakening.
A pair of compact North Pacific lows follows behind it. Both are
capable of producing small areas of gale force winds. The first
will track just east of Shemya, causing some snow or a rain/snow
mix along with windy conditions through Sunday night. The low will
then accelerate quickly south and eastward back into the North
Pacific by Monday morning. The second low tracks east of Nikolski
late tonight then then will move to near the Pribilofs as it
weakens into a trough Sunday night. Yet another low will follow
behind this second one.

This next low will be the main hazardous weather maker for
portions of the area. As the low track near Dutch Harbor Monday
night, it will be rapidly strengthening. Cold air streaming in
south of the low will lead to rapid pressure rises, causing high
winds in the waters around the eastern Aleutians. High Wind
Watches have been issued for the Alaska Peninsula and Eastern
Aleutians, with the primary threat area expected from Dutch Harbor
through King Cove. South to southwest winds gusting to 80 mph are
possible for a brief period late Sunday night. As the parent low
quickly lifts northward away from the area, the high wind threat
will rapidly diminish Monday morning. The low will then sit and
spin somewhere in the area between the Pribilofs, Cape Newenham,
and Nunivak Island through the day Monday. In addition to
widespread gales and small areas of storm force winds, it will
also draw down much colder air across the central Bering,
including around the Pribilofs. This will likely cause blowing
snow conditions across the Pribilofs. How severely visibilities
are reduced will entirely depend on the exact track of the low,
which remains uncertain. Thus, while no hazardous winter weather
products have been issued for the islands, residents should remain
on alert for blowing snow conditions and cold temperatures
returning to the islands. The low slowly moves into the southwest
mainland during the day Tuesday, as another front moves into the
western and central Aleutians late in the day on Tuesday.

&&

.MARINE (Days 3 through 5/Tuesday through Thursday)...The extended
forecast begins on Tuesday with a deep system in the eastern
Bering Sea. Widespread gales can be expected over much of the
Bering with this system as well as freezing spray concerns.
Another system enters the area on Wed/Thur and looks to come west
to east, possibly tracking just south of the Aleutians. No matter
the storm track, the secondary system will bring a colder air mass
to the Bering Sea along with freezing spray concerns and prolonged
northerly wind.

&&

.LONG TERM FORECAST (Days 4 through 7/Wednesday through
Saturday)... An active storm pattern will be taking shape in the
extended period. Guidance is pointing at a slightly different
track than what we've been experiencing, just south of the
Aleutians into the Gulf of Alaska as opposed to the north Pacific
into the Bering Sea. The setup will bring more active weather to
Southcentral Alaska. The focus for this time period will be a
front stalling out from northwest to southeast over the
mainland/Gulf of Alaska on Tuesday. Guidance is pointing to the
development of strong low pressure system along this boundary in
the Gulf, tracking into Prince William Sound. The main concerns
with this track will be snowfall potential in the Anchorage and
Matanuska Valley, as well as Valdez area. Behind that system, a
system from the Aleutians will move into the Gulf of Alaska late
in the week. No matter the exact storm track, look for another
shot of Arctic air coming back into the area.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High wind watch 181 185. Winter Storm Watch 155.
MARINE...Gale 150 160 171 173 174 177 178 180 185.
		 Storm 155 165 170.
		 Hurricane Force 172.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...ED
SOUTHCENTRAL ALASKA...CK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JW
MARINE/LONG TERM...MTL

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
1002 PM AKST Sat Feb 15 2020

AKZ101-111-161500-
Anchorage-Matanuska Valley-
Including the cities of Anchorage, Eagle River, Indian, Eklutna,
Palmer, Wasilla, Sutton, and Chickaloon
1002 PM AKST Sat Feb 15 2020

...Gusty winds and warm temperatures will likely return to
Anchorage and the Matanuska Valley late Monday into Tuesday...

Strong and gusty southeast winds will develop along Turnagain Arm
and the Anchorage Hillside during the day Monday and peak Monday
night. Gusts as high as 70 mph are likely Monday night. Gusty east
to southeast winds will also move into the rest of Anchorage and
much of the Matanuska Valley Monday night. Winds could gust as
high as 20 to 35 mph. The arrival of winds will cause
temperatures across the area to jump above freezing, possibly
reaching the upper 30s to lower 40s. Winds are expected to then
die down during the day Tuesday, with temperatures cooling back
down to freezing or below.

There is still some uncertainty in the track and strength of the
storm system causing these winds. Stay tuned to the latest forecasts
for this impending storm system by visiting weather.gov/afc.