Brisk Wind Advisory 35, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
544 AM AKST Mon Feb 7 2022

.ANALYSIS AND UPPER LEVELS...

A low that was over Prince William Sound moved inland and
produced some last minute snow for Valdez and rain for Cordova
early this morning. Remnants of this system have moved inland with
light showers continuing in the Interior.

Out west, a stationary front is draped across the southern
Bering. Low stratus, light snow, and gusty northeasterly winds
have been observed across the Bering in association with this
front. Another front extending from a well-developed North Pacific
low is pushing into the western Gulf and lifting northward
towards the central and eastern Aleutians. The surface low is
conveniently located downstream of a positively-tilted upper level
trough and in tandem with the left exit region of 300 mb
southwesterly jet.

An Arctic airmass continues to spread southward across the Bering
and Southwest as the aforementioned upper level trough continues
to deepen. Northerly offshore flow combined with very cold surface
temperatures are producing dangerously cold wind chills for the
region, especially across the Kuskokwim Delta where wind chill
values are dipping into the upper -30s as of early this morning. A
Wind Chill Advisory remains in effect today for the Kuskokwim
Delta.

&&

.MODEL DISCUSSION...

A train of North Pacific lows are forecasted to quickly track to
Southcentral this week. Overall, models are in good synoptic
agreement with the speed and track of the upcoming low as it is
expected to continue north over Kodiak Island tonight and then
inland over the Kenai Peninsula on Tuesday. After this low,
model agreement decreases. The models continue to struggle with
the exact speed and track of the next low that approaches the
western Gulf Tuesday evening. The GFS is the slowest solution
while the NAM and EC are the fastest. Despite the current model
discrepancy with this low track, there is moderate confidence that
this storm will be the strongest of the lows to occur this week. The
last low in the series is expected to move near or over Kodiak
Island Friday morning and then near or over the Kenai Peninsula
Friday afternoon. With the current model runs, the GFS keeps the
low center on a more westward track, whereas the EC has the low
center tracking similarly to the previous low tracks with a more
eastward trajectory.

In summary, there is high confidence for active weather across
Southcentral through Friday, and periods of heavy precipitation
are expected as each storm passes.

&&

.AVIATION...

PANC...VFR conditions with scattered ceilings around 2000 feet
will persist through the TAF period. Southerly winds are expected
to shift to northerly this afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Wednesday night)...

A deep and cold upper level low will dig southward across into the
Bering Sea for the first half of the week, leading to increasingly
amplified upper level flow. The subtropical jet stream will cross
the North Pacific near 30N latitude, then take a sharp curve
northward to the Gulf of Alaska and Southcentral. As a result,
expect a series of fast moving storm systems to affect the region.
This will be especially impactful across Gulf of Alaska coastal
waters, where winds and seas will undergo rapid changes as each
storm moves through.

First off, a series of weak upper level short-waves crossing
Southcentral early this morning will exit to the north by
afternoon. Light snow will linger over the northern Susitna Valley
and along the Alaska Range, largely due to low level southerly
upslope flow. Meanwhile, an occluding low over the north-central
Pacific will track northeastward toward Kodiak Island, with
precipitation spreading well out ahead of the low center to Kodiak
Island this morning and to Southcentral this afternoon and
evening. In the upper levels, multiple out of phase short-waves
will cross the Gulf, many of them headed for the eastern Gulf and
Southeast Alaska. As a result, precipitation will initially be
fairly light, with the usual upslope enhancement along coastal
mountains. However, a short-wave along the Aleutians will phase
with a short-wave rounding the upper low over western Alaska and
head right for Southcentral late tonight through Tuesday morning,
bringing a quick shot of heavier precipitation, especially to
Prince William Sound and the Susitna Valley. Marginally warmer
air will advect northward across the Gulf, leading to a transition
from snow to rain for Kodiak and the Gulf coast. However, this
will be short-lived as arctic air quickly moves in with the upper
level short-wave tonight through Tuesday.

For the population centers from the western Kenai to Anchorage
and the Mat Valley, there may be a quick shot of light snow late
this afternoon through early evening, but downslope flow will
quickly strengthen and end any precipitation. The surface low
will head inland right over these areas Tuesday morning, ending
downslope flow. However, the upper level short-wave will be
rapidly tracking northward, so it will be a quick shot of snow
with little accumulation. Gusty southerly winds behind the surface
low will head inland. Oftentimes this results in a jump in
surface temperatures. In this case, there is enough cold air
advection that temperatures should max out around freezing.

As precipitation is tapering off over much of Southcentral Tuesday
afternoon, the next storm in line will move out of the Pacific.
With a higher amplitude flow and a stronger jet streak, this storm
will be more impactful, with much heavier precipitation across
the region. The key question and primary forecast challenge is the
low track. Models all depict a track a bit further east than the
first low. However, there is an east-west spread in solutions as
the low reaches Southcentral, with western-most solutions over
the Kenai Peninsula and eastern-most solutions in Prince William
Sound. This will be key to determining precipitation type and
identifying areas of heaviest precipitation. At the moment, it
does look like the Prince William Sound and eastern Kenai
Peninsula Mountains along with the Susitna Valley will see a
period of heavy snow Tuesday night through Wednesday, with
primarily rain at sea level. The population centers could see snow
accumulation Wednesday on the back side of this system with
strong cold air advection and persistent cyclonic flow aloft on
the eastern periphery of the western Alaska upper low. Impacts
from this storm should start winding down Wednesday night as the
best of the forcing exits northward.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3:
Today through Wednesday)...

Arctic air will continue to linger over the Northern Bering and
the Kuskokwim Delta through midweek. Dangerous cold wind chills
will remain possible through Tuesday morning.

A couple of quick moving North Pacific lows will transit northeast towards
Kodiak Island and impacting the AKPen and Bristol Bay along the
way. The first low will bring a few inches of snow through the
AKPen to interior Bristol Bay through Tuesday morning. Snow is not
expected to accumulate more than roughly 6 inches, but brief
heavy snow will be possible as this wave passes. The highest
accumulation looks to be near Iliamna and stretching north towards
the Kuskokwim Valley.

A second wave associated with the next low, moving in a similar
trajectory, will arrive overnight Tuesday into Wednesday morning.
There is lower confidence in the snowfall totals with this low,
but at this time appears to have less snowfall totals compared to
the first wave. Some clarity in the forecast will increase as
this initial wave passes.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3:
Today through Wednesday)...

Cold, arctic air will spread south and west across the entire
Bering Sea through midweek as a polar low lingers over the
Northern Bering. Northerly winds between 20 and 30 kts will aid
in advecting this cold air, causing widespread unseasonably cold
temperatures and increasing widespread heavy freezing spray
southward towards the Aleutian Islands through midweek. Freezing
spray may even reach south of the Aleutians by Tuesday morning.
Snow showers will spread away from the Bering sea ice over and
will impact the Pribilofs and the Aleutians over the next few
days. Lowered visibility is possible, especially with any isolated
heavier snowfall under some of these convective bands. Blowing
snow will also be possible at times.

&&

.Marine (Days 3 through 5: Wednesday through Friday)...

Gulf of Alaska:

A mature North Pacific low with an associated front will move
towards the Gulf late Tuesday night and Wednesday. This low will
produce a widespread area of gale force winds through Thursday.
The actual surface low will move inland early Thursday but the
tight pressure gradient will remain over the northern Gulf. The
models bring another Pacific low into the Gulf on Friday expect
another round of gale force winds.

Bering Sea/Aleutians:

The ice shield will continue to grow as arctic air gets advected
into the Northern and Central Bering. Just south of this ice
shield expect freezing spray and possibly heavy freezing spray.
With the thermal gradients in place, expect northwesterly gale
force winds funneling through gaps and passes along the Central
and Eastern Aleutians/Alaska Peninsula through Thursday
afternoon. Another North Pacific low will approach near the
Central Aleutians Thursday morning. There is a potential for
another round of gales and enhanced in associated with this low.
There is model uncertainty with the overall track, position, and
strength associated with this storm system. Therefore, forecast
will be updated and fine tuned as better model data becomes
available.

&&

.Long Term Discussion (Days 4 through 7: Thursday through
Sunday)...

The models all show an active and wet period for the North
Pacific and the persistence of the longwave trough over the Bering
and the Northwest Pacific. The models show 3 distinct lows for
this forecast period. However, there is substantial divergence
with the tracks and the intensities of these lows. Expect an
active weather pattern for the entire southern state as a series
of North Pacific lows dominate the region. Additionally, the
models show persistence with the brutally cold air over the
northern Bering and over most of the state. However, these Pacific
lows will bring relatively warmer air with them and places like
Prince William Sound could see rain and snow.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Wind Chill Advisory: 155.
MARINE...Gale Warning: 119, 120, 125, 130, 131, 138-140, 150,
                       155, 172, 351, 352.
         Heavy Freezing Spray: 138, 150, 155, 160, 165, 170,
                               171, 179, 185, 411-414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...AF
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...VR
MARINE/LONG TERM...PJS