Small Craft Advisory 923, NWS Anchorage, AK

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

.ANALYSIS AND UPPER LEVELS...

A high amplitude trough is in place along an axis that stretches
from the Seward Peninsula south to Bristol Bay while an upper
level ridge extends over the Yukon and British Columbia. On the
west side of this trough, cold air advection with northerly flow
is in place across the Bering. Meanwhile over the Gulf, an
impressive fetch of moisture is being ushered inland across the
northern Gulf coast. While most of this moist southerly flow is
steering over the eastern Gulf and Alaska Panhandle, the
southerly flow also extends across the western Gulf and
Southcentral is seeing ample moisture this morning as well.
Precipitation across northern areas of Southcentral is falling as
snow this morning while coastal locations are seeing rain.

&&

.MODEL DISCUSSION...

Overall model solutions remain in great synoptic agreement
through Thursday, however there is less agreement among the
localized effects. The biggest forecast challenge continues to be
precipitation type across Southcentral (ultimately driven by
localized winds and temperatures). Also, timing of the return of
cold air across Southcentral on Wednesday (including the Mat- Su
Valley and Anchorage Bowl) remains tricky and model solutions
continue to struggle with this, leading to overall lower
confidence on the transition from rain and/or mixed precipitation
back to snow.

&&

.AVIATION...
PANC...The forecast for the next 12 hours is still quite
challenging. Expect Turnagain Arm winds to continue, leading to
low level wind shear at the terminal through late morning as
surface winds are expected to remain out of the north. There is
still potential for a period of freezing rain at the terminal
during the morning hours, though ceilings and vis are expected to
remain VFR. Gusty southeasterly winds are expected to return to
the terminal by the evening hours.

&&

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

The very active stormy pattern continues across Southcentral
Alaska this morning. A developing north-south elongated area of
low pressure is the driver for all of the active weather. As
well-advertised, an unseasonably warm air mass has moved across
all of Southcentral. The warm air is overriding and gradually
eroding the cold air mass still in place over portions of the
area. The warm air has completely eroded the cold air along the
coast, in places like Seward, Homer, Valdez, Portage, and
Whittier. The strong gap flows have warmed temperatures above
freezing in Palmer and Wasilla, and have gotten Gulkana very close
at times. It's a pretty remarkable thing to watch Gulkana's
temperatures fluctuate this morning, depending on whether the
southeast winds are blowing or not.

The aforementioned area of low pressure that is forming near
Shelikof Strait is enhancing the precipitation pattern over the
area, keeping occasionally heavy snow ongoing in Skwentna and the
western Susitna Valley. The Winter Storm Warning for heavy snow
remains in effect there. For the southern Valley, Anchorage, and
the western Kenai Peninsula, the abundant moisture is fighting the
warm south to southeasterly cross-mountain flow, working to both
warm and dry the atmosphere. These two opposing forces are close
to canceling each other out, meaning these areas are seeing some
dry time, and some time with very light precipitation. Cold air in
place in portions of the southern Susitna Valley, such as Willow,
and a cooling northerly wind at the surface in Anchorage and the
western Kenai Peninsula are working to make a vertical atmospheric
profile that is favorable for freezing rain. The warmth and
moisture overriding a reinforced below-freezing air mass below it
are the conditions needed for freezing rain.

For the southern Susitna Valley in areas like Willow, freezing
rain that could accumulate to up to two tenths of an inch of ice
is expected through the morning. For Anchorage, a Winter Weather
Advisory for freezing rain is in effect until noon. The
temperatures in Anchorage are hovering just below freezing, with a
very light to calm wind, but very light precipitation rates.
Thus, the freezing rain, if it can become heavy enough should
quickly warm temperatures above freezing, so any accumulations of
ice this morning will remain very light. If the precipitation
rates stay as they have been, either none or light sprinkles, then
there simply won't be enough freezing rain to warm temperatures
above freezing. Both of these scenarios have the same result: a
light glaze of icing at most.

Persistent strong, onshore flow across the coastal mountains
continues this morning, with storm total precipitation of 5 inches
in Seward expected. This rain combined with melting snow up to
2,500 feet above sea level today and tonight could lead to some
localized flooding, so the Flood Advisory remains in effect
until Wednesday morning.

This afternoon through tonight, temperatures will warm above
freezing everywhere with limited daytime heating as the December
sun is at its weakest point in the year, but moreso attributed to
the continued influx of warm air. There remains some uncertainty
as to whether or not the Turnagain Arm wind will get into town
before tonight. If it does, then the temperatures will warm much
more quickly in the affected areas.

On Wednesday, the cold air will finally begin to make more
progress eastward into Southcentral, as the entire upper level
trough associated with the cold air begins to shift into the area.
There is some disagreement with the timing of when this will
happen, but for most areas, there will be a very anti-diurnal
temperature trend on Wednesday, with the warmest temperatures
early in the morning, then falling throughout the day. The cold
air returning will also allow Anchorage and the Mat Valley to get
a quick inch or two of snow, most likely Wednesday afternoon, but
depending on how fast the cold air returns. It should be through
all of Southcentral Wednesday night. This will work to dry out the
Cook Inlet region, but mountain upslope snow will persist for the
eastern Prince William Sound, including Valdez, Cordova, Thompson
Pass, and the surrounding areas into Thursday morning. This is
because the flow of much colder air, still southwesterly off the
Gulf, will still be taking moisture from the Gulf and slinging it
into the coastal mountains.

&&

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

An upper-level trough over Southwest Alaska continues bringing
unsettled weather to the region. Widespread snowfall persists
across the mainland through Wednesday afternoon, with the
heaviest snow falling today for Sleetmute and areas east. Snow
accumulations are expected to be from 4-8 inches for these areas
with locally higher amounts. Blowing snow and light snow
accumulations are expected for Nunivak Island and the Kuskokwim
Delta coast tonight into tomorrow morning. Significant cold air
advection behind the trough and lowered sea surface temperatures
allow for heavy freezing spray concerns along the Southwest coast.
Additionally, the Southwest mainland will see lowering
temperatures over the next couple days as the trough moves across
the region. The upper- level wave exits to the northeast by
Wednesday night while a ridge of high pressure sets up over the
Central Bering. This will allow for a drier and clearer pattern
for the Southwest Thursday into Friday.

&&

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

An upper-level trough over the Eastern Bering pushes onto the
Southwest coast this afternoon. The main concerns will be
significant cold air advection and a band of heavy precipitation
along the AKPEN behind the trough. Heavy freezing spray will be
confined to the Southwest coast, with light freezing spray
nearing the Pribilof Islands and along the Northern Bering.
Tonight into tomorrow morning, the AKPEN will see periods of heavy
snowfall with accumulations of 5-10 inches along the mountains.
Lowering temperatures along the Aleutian Chain is expected as the
trough continues east and cold air is advected in. Ridging sets up
over the Central Bering by late Tuesday night and will persist
through the forecast period. A North Pacific low enters the
Western Bering late Thursday night. Models are not in great
agreement with this low and will continue to be monitored over the
next couple days.

&&

.MARINE (Days 3 through 5)...

Gulf of Alaska: The remnants of low pressure linger to start
Thursday, but it is on its way out. The Gulf of Alaska then
quiets for the most part. Cooler air filters back into Gulf, which
will cause gusty winds through gaps and passes along the Western
Gulf, including through Kamishak Bay and the Kennedy Entrance.
Wind direction then becomes generally northerly through the
weekend. The Gulf continues to remain quiet with no major weather
systems in our forecast area during this time.

Bering Sea: The Bering starts out quiet with high pressure in
place. Flow then becomes easterly for the weekend and winds will
increase to small craft advisory level. This is due to a couple
weak lows moving through the North Pacific, just south of the
Aleutians. Confidence is not high with the placement of these lows
due poor model guidance, so wind speeds may need some adjusting.
However, we can anticipate no impactful low pressure systems
moving through the Bering.

&&

.LONG TERM FORECAST (Days 3 through 7)...

We are still expecting a pattern shift to occur late this week,
with the weather noticeably changing starting Thursday. Southerly
flow aloft supporting above average temperatures in Southcentral
AK will push off to the east and allow cooler, more seasonable
temperatures to return. Shortwave ridging looks to develop over
Southwest AK. An upper level low should exist in just south of the
Central Aleutians. As we progress into the weekend, a key
difference emerges from yesterday in that model guidance has back
off the shortwave ridge phasing with high pressure in the Western
U.S. There are still hints of it there, but it's more fuzzy now.
Longwave troughing still looks to return to the Bering Sea.

Since models are less agreement than they are yesterday, not much
can be said for starting to focus in on a forecast solution for
the longterm. This means we can only look out a few days at most
to discern specific weather features. Analyzing the long range
today we do know the weather pattern stays busy. So, we can
expect a pattern shift with continued active weather for our
forecast area. Due to significant model discrepancies, that's
about all we can say for now.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Advisory: 125.
         Winter Weather Advisory: 101, 152, 161.
         Winter Storm Warning: 145.
MARINE...Gales: 131. Heavy Freezing Spray:

160, 180, 181. FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...KO
SOUTHCENTRAL ALASKA...JPW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KM
MARINE/LONG TERM...BJB

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
325 AM AKST Tue Dec 1 2020

AKZ155-020200-
Kuskokwim Delta-
Including the cities of Bethel, Hooper Bay, and Nunivak Island
325 AM AKST Tue Dec 1 2020

...BLOWING SNOW ALONG THE KUSKOKWIM DELTA COAST EXPECTED TUESDAY
NIGHT INTO WEDNESDAY MORNING...

A trough of low pressure moving inland over Southwest Alaska on
Tuesday night will bring gusty northerly winds and periods of snow
and blowing snow to the Kuskokwim Delta coast. Visibility may be
reduced to one half mile at times in localized areas. Snow
accumulations of 1-3 inches are expected.