Gale Warning 748, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
456 PM AKST Tue Dec 1 2020

.ANALYSIS AND UPPER LEVELS...
A high amplitude trough along the Southwest Alaska coast is tracking
very slowly eastward. Deep southerly flow dominates ahead of the
trough, from the northeast Pacific to southern mainland, with
multiple weak waves embedded within the flow. At the surface, a
north-south oriented cold front extends from the western Alaska
Range southward to the south end of Kodiak Island. Weak surface
waves are tracking northward along the front, locally enhancing
precipitation and winds. The cold front is progressing slowly
eastward into the western Gulf. The airmass over Southcentral
has warmed significantly over the past 12-18 hours, as the deep
southerly flow spread northward. As a result, precipitation has
changed to rain across much of Southcentral, with snow holding on
over the interior and at higher elevations (snow levels roughly 2
to 3 thousand feet near the coast). Areas of snow continue over
Southwest Alaska ahead of the upper trough, but it is generally
light snow with minor snow accumulation. Persistently strong
east- west pressure gradients are also leading to strong gap winds
across Southcentral, most notably through Turnagain Arm and the
Upper Hillside of Anchorage as well as through the Knik River
Valley into Palmer. Pretty uniform gusty southerly winds extend
across much of the Gulf and Gulf coastal region.

Out over the Bering Sea and Aleutians, the weather is much
quieter. Gusty northerly winds and snow showers dominate the
far eastern Bering Sea, eastern Aleutians, and southern Alaska
Peninsula on the back side of the upper trough. A weak low and
broad trough along the western Aleutians are producing a few
showers out that way as well.

&&

.MODEL DISCUSSION...
Models are generally in good agreement with the current storm
system over the southern mainland of Alaska. The primary remaining
challenge will be the transition back to snow (and associated snow
accumulation) as colder air advects across Southcentral Wednesday
afternoon through Wednesday night. As this storm winds down,
attention shifts to the North Pacific, where we expect multiple deep
storms to form. Models have been struggling with what looks like a
complex pattern. They have come into better agreement with a gale
to storm force low approaching the central to eastern Aleutians
Wednesday night and Thursday, so will trend toward a stronger wind
field despite continued uncertainty in the exact low track.

&&

.AVIATION...
PANC...Persistent low level southeasterly flow will help maintain
VFR conditions until about midday Wednesday. Some marginal low
level speed shear is also possible. By midday Wednesday, a cold
front will move up Cook Inlet and toward the airport. Expect a low
level wind shift (to south to southwest) and commencement of
precipitation. While conditions will be warm enough for rain or a
mix of rain/snow at the start, expect a rapid transition to all
snow. As a result, ceiling/vis will drop to at least MVFR, if not
IFR. The snowfall does look short-lived, with maybe about 3 hours
of the worst conditions.

&&

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

An elongated area of low pressure moving northward over Kodiak
Island has a moisture connection from the tropics near Hawaii. As
a result, this has been a very warm and moisture laden system.
Most areas across Southcentral have warmed into the 40s with even
near 50 degree readings being observed at Fort Richardson this
afternoon. Many areas across Southcentral are also reporting rain
especially in the populated areas. Highest elevations above
approximately 2,500 to 3,000 feet were the only areas still
reporting snow. The warm air has even made it as far north as
Talkeetna where they changed over to rain this afternoon. Now that
the warm air has made it in, it will be here to stay for the next
18-24 hours. Winds off Turnagain Arm have also come into Anchorage
where they have been gusting as high as 33 kts earlier today.
These winds will be here to stay across west Anchorage, Turnagain
Arm, and the Anchorage Hillside through Wednesday morning. The
Matanuska Valley and Knik River Valley winds are also blowing this
afternoon which are expected to continue through Wednesday
morning before diminishing.

Next, where the winds stay up overnight, temperatures will not be
able to fall all that much especially with such a warm air mass in
place. Temperatures across the entire Southcentral region were
bumped up a couple degrees for tonight due to this. Where the
winds become light enough some regions may be able to fall to
freezing but most areas are likely to remain above.

Later tonight, a potent upper level trough moving through
Southwest Alaska will begin to push over the Alaska Range. This
will be the beginning of the cold air coming back for
Southcentral. First, the cold air will move back into Kodiak
overnight with temperatures falling throughout the day Wednesday
back down to and below freezing. The cold air will move back into
Southcentral from the southwest direction with Homer being next in
line seeing the cold air start to move back in Wednesday morning.
With this, any rain is expected to change back to snow by
Wednesday afternoon. With the cold air moving back in across
Homer, this will setup a southwest flow across the Inlet. Cold air
flowing over a relatively warm body of water will allow for some
moisture to be picked up to cause some "Cook Inlet effect" snow
showers for the Homer Bluff area and the higher terrain to the
north. However, there is a caveat to this as 700 mb relative
humidity is fairly dry, therefore, this will strictly be cold air
moving over warmer water, moistening and condensing into clouds to
squeeze out some snow. In other words, this will be a very
shallow moist layer about a mile above the ground.

Cold air continues progressing northward along the Kenai Peninsula
reaching Kenai Wednesday afternoon and then Anchorage and the
Matanuska/Susitna Valleys later Wednesday afternoon into the
evening hours. However, as this is doing so an upper level trough
is expected to swing through squeezing out whatever moisture is
left in the column out ahead of it. A relatively narrow band of
snow is expected to develop as a result of this. This band is
expected to impact Kenai first with around an inch of snow. This
band will continue moving northeastward with the upper level
trough with the potential to intensify some. As a result, general
snowfall amounts of 1-3 inches are anticipated across the
Anchorage Bowl, Matanuska and Susitna Valleys with the highest
amounts expected across the Matanuska Valley. This band is
expected to be relatively quick hitting not lasting more than a
few to couple hours in any one location, but it could be heavy for
a brief period.

Another weak shortwave may pass through Southcentral Thursday
morning with the potential for some snow showers with little to no
accumulation. Other than that, the cold air returns to
Southcentral with highs in the upper teens to low 20s Thursday and
single digits and low teens Thursday night.

&&

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

An upper level trough that has been anchored near the west coast
of Alaska is supporting a frontal boundary just west of the Alaska
Range and southward near Dillingham and east of King Salmon.
Light snow will continue across the Kuskokwim Delta tonight ending
by tomorrow morning with up to 2 inches of snow accumulation.
Light snow will continue from Iliamna and northward near
Sleetmute where 1 to 4 more inches of snow are likely. Gusty winds
and light snow flurries are expected along coastal areas of the
YK Delta and Nunivak Island. This could lead to some pockets of
blowing snow but it is not expected to be widespread. As this
upper trough moves east tonight and Wednesday the snow will come
to an end for the area with cold temperatures expected through the
end of the week. Areas of fog are possible in the YK Delta late
Wednesday night and Thursday morning as weak onshore flow develops
and high pressure moves over the area. Late Thursday through
Friday a weak low pressure system will bring light snow to the
Kuskokwim Delta with only minor snow accumulations.

&&

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

Cold air advection with brisk northerly flow will continue through
Wednesday with snow showers across the AKPEN and eastern Aleutians
with snow accumulations less than an inch for most populated
areas. High pressure will move across the western Bering tonight
and into the central Bering on Wednesday. This high pressure will
drift in the northern and western Bering on Thursday and Friday. A
series of lows will affect the western and central Aleutians
through Friday with light rain or a rain/snow mix with increasing
winds. Winds up to gale force start to show up Wednesday night
through Friday for the central Aleutians and southern Bering with
near storm force winds Thursday and Friday as a stronger low
develops just south of the central Aleutians. This low will need
to be watched with storm force winds possible.

&&

.MARINE (Days 3 through 5...Fri through Sun)...

Gulf of Alaska: Off-shore winds through gaps and passes will be
present in the Western Gulf on Friday as 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 since 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 it's 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...Fri through Tue)...

The long term starts with short wave ridging over the AKPEN, a
weak shortwave ridge over SW AK pushing into the Bering and
Southwest flow over Southcentral AK. This pattern then quickly
becomes almost indiscernible after Friday. Our primary
forecasting models have major disagreement in synoptic features,
which leads to low forecast confidence. Models are at least
hinting at longwave ridging over the Bering and longwave troughing
dropping down the West Coast of AK. One constant they do at least
have from previous runs is longwave ridging pushing into the
eastern AK from the West Coast. Models vary placement and strength
of this ridge each day, but the fact that it's still there leads
to a higher confidence level that we will see this ridge set up
over the weekend. Overall, the long term looks much the same as
the past couple of days...we know the weather will be active but
we will have to wait until the short term to identify specifics.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 145
		 Flood Advisory 125.
MARINE...Heavy Freezing Spray Warning 160 180 181
		 Gale Warning 174 130
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...MV
SOUTHWEST ALASKA/BERING
SEA/ALEUTIANS...RC
MARINE/LONG TERM...BB