Brisk Wind Advisory 56, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
451 AM AKDT Tue Mar 17 2020

.ANALYSIS AND UPPER LEVELS...

The upper levels currently are set up with a ridge over the
central part of the state, with a trough currently over the
eastern Bering moving into Southwest Alaska. Active weather ahead
of the trough is causing areas of precipitation across Southwest
Alaska, particularly over the Kuskokwim Delta. Meanwhile radar
returns over Bristol Bay only show some weak scattered rain and
snow shower activity. Precipitation is trying to begin across
Kodiak, but had difficulty doing so thus far. Meanwhile much of
Southcentral is dry for now.

&&

.MODEL DISCUSSION...

The models are in good agreement through the next couple days,
showing a progressive/fast-moving upper level pattern that will
bring chances of precipitation to the mainland every 2.5 days or
so, with breaks of sunshine expected in between. Regarding today's
expected snowfall, while the model consensus on the timing and
snowfall amounts is good, it is somewhat different than previous
runs solutions have been. All models now show the trough
developing an upper level low this evening that rapidly dives
southeastward into the Gulf by Wednesday morning. Previous model
runs had today's and now Thursday's front following after one
another in quick succession, with very little to no break in
between. Now there is a distinct strong ridge that moves over
mainland Alaska on Wednesday, clearing and drying everything out
before the next front moves in Thursday afternoon and evening. The
impact on the forecast is for today's snow to end more quickly
tonight, instead of it lingering on into Wednesday morning.
Forecast confidence is high.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist through this
morning. Snow moves in this afternoon and intensifies, dropping
visibility and ceilings to IFR category through sunset. Conditions
will gradually improve overnight to MVFR, but occasional IFR
conditions remain possible until the early morning, when all
precipitation should end. VFR conditions should persist through
the entire day on Wednesday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Today
through Wednesday night)...

The overall forecast for today remains on track, with widespread
snow expected across Southcentral Alaska. Two separate upper
level short-waves, closely spaced, will approach Southcentral
today with precipitation spreading northward from Kodiak to the
Kenai Peninsula this morning, then across the remainder of
Southcentral this afternoon and evening. The heaviest snow will be
right ahead of the first short-wave, which will track northward
up Cook Inlet and the Susitna Valley. The formation of a weak
surface low over the western Gulf will lead to just a little bit
of low level southeast flow across the Kenai and Chugach
Mountains. While this certainly won't be enough to dry things out
completely in the lee of the mountains, it does mean that the
heavier snow will be west of the mountains - up the Sterling
Highway corridor near Cook Inlet to west Anchorage, the western
Matanuska Valley and southern Susitna Valley. Temperatures will
warm enough along the Gulf Coast and Kachemak Bay to lead to a
change to rain (or at least a mix of rain and snow) this afternoon
and evening.

The first short-wave will exit to the north by late this evening,
while the trailing short-wave digs southeastward into the Gulf,
ultimately forming an upper level low. This is a change from
previous forecasts which were based on a more easterly track into
Southcentral. As a result, have backed off on snow amounts overnight
tonight into Wednesday morning and ended the snow earlier than the
previous forecast.

As the trailing trough digs into the western Gulf we will see a
quick burst of cold air advection. This will kick up winds across
the western Gulf and Kodiak Island, with the strongest winds through
bays and passes of the Alaska Peninsula. On Wednesday, the upper low
and surface low will both track quickly east across the Gulf as an
upper ridge builds in from the west. Deep northerly flow combined
with tightening pressure gradients will lead to gap winds along the
north Gulf coast and Prince William Sound. Winds will diminish and
dry and cooler conditions will settle in across the region Wednesday
night as the upper level ridge moves overhead.

&&

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

Low pressure over the eastern Bering is lifting a warm front toward
southwest Alaska this morning. Given the increased cloud cover and
initial push of precipitation overnight, low temperatures remained
in the lower to mid 30s for all but the far interior locations. This
has meant a changeover from mixed precipitation to all rain for
Dillingham to Bethel. As the front continues to push over the
southwest Mainland today, warmer air will move in, extending the
areal coverage of rain across the region. The exception to this will
be across the middle and upper Kuskokwim Valley where a couple of
inches of snow could fall before changing to a wintry mix. The
surface low then moves onshore along with the trailing upper-level
trough digging south from the northern Bering. As these features
work east, colder air will begin to filter back in and allow for a
changeover back to snow as the precipitation becomes more scattered
and showery in nature.

The shower activity will be mainly confined to the higher terrain
by Wednesday morning, tapering off by the afternoon hours as ridge
of high pressure settles in. Fog is also possible overnight
Tuesday into Wednesday morning, especially along the coast, as
the ridge builds overhead. The next weather front will move toward
the coast by late Wednesday, bringing another round of precipitation
for southwest Alaska Wednesday night through Thursday. Temperatures,
both aloft and at the surface, look to be colder than the previous
system. Thus, snow is expected, especially north and west of
Dillingham and King Salmon. For areas along Bristol Bay, enough
warmer air should work in to yield a mix of rain and snow.

&&

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

An area of low pressure over the eastern Bering this morning will
move east over the southwest mainland today. Upstream, a ridge of
high pressure continues to trap low stratus and areas of fog across
the central Bering and Aleutians, especially around the Pribilofs
where fog will continue to linger through mid morning.

Farther west, a developing storm-force low over the North Pacific
will lift north, tracking west of Attu later today and to the Anadyr
Gulf by Wednesday. This system will sweep a front over the western
Aleutians today, advancing to the Pribilofs and eastern Aleutians
Wednesday morning. Widespread rain and gales (with embedded storm-
force winds) will accompany the front as it tracks east. This front
then begins to slow and weaken as it moves to the west coast late
Wednesday night.

Strong cold air advection will trail the front, wrapping
underneath the low. This will act to increase instability,
allowing for numerous rain and snow showers to persist across the
western Bering. The instability will also produce very strong
westerly wind gusts across the western Bering and Aleutians
through Thursday.

&&

.MARINE (Days 3 through 5 Thu through Sat)...
On Thursday morning a front associated with a low in the eastern
Bering will enter the western Gulf bringing with it small craft
winds. There is low confidence that another triple point low will
form in the northern Gulf Thursday night. If it does, expect gale
force winds to occur through channels including Shelikof Strait,
the Barren Islands and through gaps and passes on the Pacific side
of the Alaska Peninsula through Saturday morning.

A gale force front over the Bering will continue to push
northeastward through Thursday before it exits the Bering
Friday. There is moderate confidence that another front will enter
the western Bering Friday night. Depending on the strength and
track of this front, gale force winds will be possible in the
western Bering through Saturday.

&&

.LONG TERM FORECAST (Days 3 through 7 Thu through Tue)...
Models continue to show good agreement that an upper level ridge
that extends from the Gulf north across the state, will continue
eastward into Canada on Thursday. Upstream, ahead of a large
Bering low, a strong front moves eastward across western Alaska
and the Peninsula. This front will impact the southwest coast
Thursday into Friday, bringing precipitation and strong southerly
flow. The track of this low still remains uncertain, though models
are showing the center departing the Bering to the north by
Friday. This will be followed by another upper level ridge moving
across the Bering and into southern Alaska through Sunday. Similar
to last week, warm temperatures and moist conditions will advect
into the Aleutians and Bering following the backside of the ridge.
This pattern will continue to alternate through Tuesday as upper
level troughs and ridges continue to follow one another across the
Bering.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning: 177 178.
Gale Warning: 130-132 170 173-176 179 181 185.

&&

SYNOPSIS AND MODEL DISCUSSION...JPW
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TM
MARINE/LONG TERM...CK