Heavy Freezing Spray Warning 169, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
543 AM AKST Mon Nov 15 2021

.ANALYSIS AND UPPER LEVELS...

An upper level low continues to move in the southeastern Gulf
with a band of clouds and associated showers pushing across
Southeast, AK. This low is also responsible for gusty, offshore
winds through the gaps and passes, including Whittier, Seward, and
Thompson Pass. Additionally, an impressive polar low can be seen
on the water vapor imagery descending south from the Arctic Ocean
into the North Slope. Out west, a negatively tilted trough with an
elongated surface low is situated north of Adak. A series of
shortwaves rounding the base of this trough is providing the
necessary dynamics for light to moderate rain/snow showers and
gusty winds across the Bering and the Aleutians. As the trough
axis shifts south, a surface low will begin developing southeast
of Cold Bay, and be playing a key role for an active weather
pattern in Southcentral as it enters the Gulf on Tuesday.
Meanwhile, an amplified ridge is building ahead of the upper low
with benign conditions for the Southwest mainland. The Nighttime
Microphysics (RGB) this morning shows areas of low stratus and
patchy fog in parts of the Matanuska Valley, Knik Arm, the Kenai
Peninsula, and the Copper River Basin. Elsewhere, clear skies,
light winds, and strong radiational cooling is allowing overnight
temperatures to drop down in the single digits to below zero
across the lower and higher elevation regions.

&&

.MODEL DISCUSSION...

The latest operational model runs are in good agreement with the
placement and intensity of synoptic scale features at both the
surface and in the upper levels. Much like yesterday, the
challenge this morning is how long the fog layer near Knik Arm
will persist given the clear skies and deep snowpack on the
ground. Current thinking is that the offshore gradient will help
limit the extent and clear out a little sooner, but will
nonetheless be closely monitored. Otherwise, the only minor
discrepancy is the position and track of the aforementioned low as
it enters the Gulf over the next 24-48 hours and the
precipitation amount for Kodiak Island and the Copper River Basin.
Models are also handling quite well with the polar low as it
continues moving southward across the Alaska mainland and the
potential impacts for Southcentral for the rest of this week.
Therefore, forecast confidence remains above average.

&&

.AVIATION...

PANC... Areas of low stratus and fog near Knik Arm may impact the
terminal early this morning with a reduction to ceiling and
visibility from IFR to LIFR. VFR conditions with light winds out
of the N-NE will then prevail by this afternoon through the
remainder of the TAF package.

&&

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

This period of weather across Southcentral is what happens when
Arctic high pressure is too firmly in control and wandering lows
through the Gulf interact with it. Frigid is the word of the week.
For as cold as it is right now...and 15 degrees below normal this
time of year is remarkable...you ain't seen nothin' yet.

Today's weather is really a continuation of the frigid cold,
localized freezing fog, with wind through the gaps weather we have
been seeing all weekend, with a few more degrees shaved off the
thermometer from the past couple days.

This evening into tonight, a weak, relatively unimpressive low
will approach southern Kodiak Island. While Kodiak is the warm
spot in Southcentral, relatively speaking, and the low will bring
somewhat warmer air in with it, all the associated precipitation
will fall as snow. It looks to brush by the island, perhaps
dropping a quick 2 to 4 inches of snow before departing into the
Gulf. There is some uncertainty as to how far north the
precipitation shield will get, but few areas are expected to pick
up much more than the 4 inches on the high end of the forecast.

As the low moves into the north central Gulf Tuesday, it will
intensify a bit as it interacts with the increasingly bitterly
cold air mass over mainland Southcentral. There is considerable
uncertainty as to how far west the moisture shield associated with
the low gets, however the general consensus is that it will spread
over eastern areas of the Copper River Basin. The moisture will
make it over the Chugach mountains thanks in part to a strong
upper level low simultaneously diving south out of northern
Alaska. The air mass over the eastern Copper River Basin area is
already very cold. Thus, any moisture added to that air mass will
produce a very powdery high snow-to-liquid ratio snow. It will
accumulate very quickly. A Winter Storm Watch has been issued for
that area.

Meanwhile further west across the Susitna Valley, the core of
coldest air moving overhead Tuesday night and the winds squeezing
through the gaps in the mountains will combine to produce
dangerously cold wind chills through Broad Pass, as wind chills
drop as low as 50 below. A Wind Chill Advisory has also been
issued for that area. With the extra cloud cover further east
across the Copper River Basin, there is much more uncertainty as
to how cold the temperatures will get towards Paxson and the
Isabel and Mentasta Passes. Dangerously cold wind chills are also
possible across the northern Copper River Basin Wednesday night
and beyond.

Now onto the winds: The very strong pressure gradient between the
polar high over interior Alaska and the Gulf low will make all of
the gaps feature strong winds, though some are stronger than
others. For Seward and Whittier, there will be periods where the
winds gust to 60 mph, but fortunately the upper level low and
associated energy diving south will not allow the winds to align
to produce warning level winds. In fact at times the upper level
may be opposed to the surface flow. Nevertheless a consequence of
the unusually cold air mass in place, which will be reinforced
with the upper level low, is that the winds will still be very
strong. Interests in any of the passes through Southcentral should
expect strong winds. Further south towards Kodiak, the upper level
and surface flows will align during the day on Wednesday. Thus,
winds through Marmot and Chiniak Bays will also gust to 60 mph or
more. We will continue to refine that forecast.

The combination of the strong winds and cold temperatures will
cause widespread heavy freezing spray concerns from Wednesday on.
Mariners should expect to encounter potentially dangerous heavy
freezing spray in many of the coastal waters. Temperatures may
struggle to reach 10 above for highs by mid to late week in
Anchorage, with subzero lows. This reflects temperatures that
will be 15 to as much as 30 degrees below normal for this time of
year.

Finally, interests on the Kenai Peninsula along southern Cook
Inlet and Kachemak Bay may have to contend with lake-effect (or
more specifically, Inlet-effect) snow on Wednesday. As winds turn
more northerly and strengthen, they will pick up quite a bit of
moisture due to the extreme instability between sea surface
temperatures around 40 above, and air temperatures a mile up to 10
below. The big question is how organized the band(s) get, and
which unfortunate souls feel their greatest wrath. As of now, the
community of greatest concern is Seldovia. However, much of the
worst weather appears to stay just west of there, though the
town should see periods of snow if the lake-effect wanders that
far east. Interests in Seldovia and along the Sterling Highway
from the Homer Bluff to Ninilchik should monitor future forecasts
for potential updates to snowfall amounts. Any snow will be
accompanied by gusty winds which will easily blow the snow around
and locally reduce visibilities.

&&

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

Over the next few days, much colder below zero temperatures will
hit hard across Southwest Alaska in response to cold air advection
trailed by an Arctic low moving over the Alaskan Range by Wed
morning. Moderate to fresh cold northerlies along the west flank
of the low will push right through the lower Kuskokwim Valley and
parts of Bristol Bay on Wed, leading to high clouds dominating the
eastern Southwest. This cold dense air combined with strong
coastal winds and subzero temperatures will also invite freezing
spray offshore. All in all, dry but really cold temperatures for
Southwest will be closely monitored during short term.

&&

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

Multiple shortwaves will make their way across the Bering as a few
lows passes through the western Aleutians and well west-northwest
of Pribilof Islands during short term. Meanwhile, a string of
emerging lows along the frontal axis could elevate the potential
for high end gale gusts over the Bering and parts of the western
Aleutians Tue and Tue night. Steady rain could occur mostly west-
southwest of Pribilof Islands and west of eastern Aleutians, with
the rain/snow mixed anticipated along the eastern Aleutians and
eastern Bering as cold temperatures linger there the next few
days.

&&

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

Gulf of Alaska...On Wednesday, a digging trough will push cold,
Arctic air offshore, driving strong winds out of gaps along the
Gulf coast and contributing to the formation of a low in the
northern Gulf. These gap winds will reach storm force and produce
heavy freezing spray, especially in the vicinity of Kamishak Bay
and the Barren Islands. Mariners should take heed of this freezing
spray potential as it could be an extreme event. The only variable
that is not ideal for freezing spray is still warm SSTs across
parts of the Cook Inlet. Winds will diminish through Friday as
the low and upper- level trough move into the eastern Gulf. Seas
remain below 20 ft.

Aleutians/Bering...Localized areas of gales are possible near the
center of a low in the western Bering/Aleutians on Thursday. Its
front pushes northward and may develop a more widespread swath of
gales in the western and central Bering Thursday night. Seas
remain below 20 ft.

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

Confidence is increasing that an upper-level low digs over
Southcentral, dropping into the Gulf by Thursday, leading to a
blast of cold, Arctic air for southern Alaska. Temperatures look
to reach their coldest on Thursday night, when most of the
southern mainland could see temperatures 20 to 30 degrees below
normal in locations where skies remain clear. Cold air advection
will also enhance gap winds, keeping temperatures warmer in those
coastal areas with increased mixing. A low in the Gulf on Thursday
may squeeze out some lingering moisture along the north Gulf
coast in the form of snow. Elsewhere, conditions are dominated by
Arctic high pressure, so little precipitation and mostly clear
skies are expected through the end of the week.

Model guidance is in good agreement that both the Gulf low and the
upper- level low will eject eastward by Saturday, with offshore
flow weakening through the weekend. Guidance diverges beyond this
point with the next trough digging southward across the state. How
far west the trough is located, ranging from the eastern Bering
to over Southcentral in various model solutions, will have an
impact on temperatures this weekend. The GEM keeps temperatures
well below normal with a further east solution, while the GFS and
EC place the trough further west, allowing a weak ridge to pass
aloft and low-level southerly flow allowing a warmer air mass to
slowly work its way in over parts of the Southern mainland.

Out west, a high amplitude ridge persists over the eastern Bering,keeping
the storm track in the western and central Bering through at
least Friday night. Models tend to break down this ridge over the
weekend as a result of the second digging trough, with the GFS and
EC bringing colder air streaming over the Bering from the Alaska
mainland.

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Wind Chill Advisory 145
Winter Storm Watch 141
MARINE...Gales 178
Heavy Freezing Spray Warning 140 160 180-181
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...MF
SOUTHCENTRAL ALASKA...JPW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
MARINE/LONG TERM...CQ