Small Craft Advisory 992, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
547 AM AKST Tue Dec 13 2022

.ANALYSIS AND UPPER LEVELS...

There is a surface low near Nunivak with the frontal boundary
extending into Bristol Bay and Kodiak. Overnight freezing rain was
observed at King Salmon and Pilot Point. The King Salmon radar
continues to be active this morning and the leading edge of the
precipitation shield is discernible on the Kenai radar. There is
another low in the northern Bering and a massive low south of
Adak. There is a jet core of 120 knots moving into Bristol Bay.
Suffice to say the pattern is dynamic and progressive.

&&

.MODEL DISCUSSION...

The models are in good agreement for 24 hrs. Then some differences
emerge with the timing and the placement of the next precipitation
event for Southcentral. And the phasing of an upper level low
moving east out of the Bering with a fetch of modified tropical
moisture from the Pacific. Make no mistake, this will have major
consequences for the Anchorage Bowl and portions of the Kenai
Peninsula on Thursday. As each new run of the models becomes
available, the situation is being re-evaluated.

&&

.AVIATION...

PANC...Cigs and Vis should remain VFR this morning. However, there
is another frontal boundary moving up Cook Inlet and the cigs will
decrease later today. MVFR cigs will be possible. Light winds
expected.

&&

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

Things are moving quickly in Southcentral with an upper level
wave moving through the region today. The clear skies overnight
have already given way to more clouds across Southcentral and snow
in the Cook Inlet region early this morning. Snow accumulations
today look to be less than were expected from the forecast
packages yesterday across Cook Inlet to the Susitna Valley with
forecast amounts about half of yesterday's forecast values. This
is due to more of the precipitation heading eastward which will
bring a little more snowfall to the eastern Prince William Sound
than yesterday's forecast.

The big weather challenge is the front moving into the area from
the south Wednesday into Thursday. This front will have a
southwest-to-northeast orientation and bring in a lot of moisture
from the Pacific. There are similarities with it to the storm that
just pummeled Southcentral with snowfall just two days ago. What
is different is that the warm air will be able to move farther
north than the previous system creating a question as to where the
snow will transition to rain. The dividing line looks to
generally run from Kachemak Bay, along the Kenai and Chugach
Mountains but not make it farther north than northwest Prince
William Sound. It will also not be a long period of rain should it
occur, but that can greatly change the nature and impacts of a
winter storm. Freezing rain looks unlikely due to the atmospheric
temperature profile. There will not be colder air trapped in lower
elevations as wind will be southerly in places that could turn
rain and therefore temperatures will be above freezing in the
rain. Part of the challenge is that many areas that get a little
above freezing at the surface will be cold enough aloft to remain
snow. Anchorage is right on the cusp of the rain/snow line for
Wednesday night though this set-up does lean toward the majority
of precipitation falling as snow. For both Kenai and Anchorage, it
is likely this will be another significant snowfall starting
Wednesday afternoon and going into Thursday morning.

&&

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

Busy weather continues through Thursday. For today, expect snow
showers as an occluding low tracks east across Southwest Alaska.
Parts of the region could see up to 4 inches of snow before
precipitation tapers off late this evening. Then, temperatures
drop tonight as northerly flow sets up between high pressure over
the Alaska interior and a North Pacific low approaching the
Alaska Peninsula.

Wednesday continues to be the day to watch. Moisture from the
North Pacific low will run up against a ridge in the Eastern
Pacific and stream up into Southern Alaska. The first forecast
challenge will be determining the likelihood of freezing rain as
warm air aloft overruns sub-freezing surface temperatures. While
possible, freezing rain is looking less likely at the moment as
models have trended the low and, consequently, warm air aloft
further south. The second forecast challenge will be determining
snow amounts from Wednesday into Thursday morning. There remain
some timing differences with the northward push of this moisture
plume, as well as some differences in its westward extent. As
such, the areas where snowfall amounts are least certain are the
Lower Kuskokwim Valley and from about Koliganek/Dillingham and
west in Bristol Bay. All told, current guidance suggests that snow
amounts could reach the double digits over a 24-hour period. Stay
tuned as we continue to refine the snowfall forecast and assess
the likelihood of freezing rain.

Precipitation should taper off from west to east through Thursday
morning. By Thursday afternoon, northeast winds look to pick up as
the pressure gradient tightens ahead of a strengthening front in
the Bering Sea.

&&

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

Multiple systems moving through the Bering Sea and Aleutian
Islands will keep weather active through Thursday. Today, winds
should diminish along the Eastern Bering Sea and Aleutians as a
low moves into Southwest Alaska. Meanwhile, a robust shortwave
will bring snow showers, freezing spray, and northwesterly winds
up to gale force as it tracks east across the Bering Sea, reaching
the Pribilof Islands late tonight.

As we head into Wednesday, two lows moving towards the Bering Sea
will bring widespread gale force winds and warmer temperatures.
First, a North Pacific low will approach the Alaska Peninsula and
Eastern Aleutians early Wednesday morning. The main story with
this system will be northerly gales across the Eastern Aleutians
and Southern Bering Sea. Further west, a low near Kamchatka
Peninsula will push its storm force front into the Western
Aleutians and Bering late Wednesday afternoon. As these features
interact, a broad swath of southeasterly winds will develop across
much of the Bering Sea. Offshore winds from Southwest Alaska could
contribute to heavy freezing spray in the Eastern Bering.

.LONG TERM FORECAST (Days 4 through 7: Friday through Monday)...

After a very active week of weather, a high-amplitude blocking
pattern looks to set up this weekend over southern mainland
Alaska, with calmer weather in store across the forecast area. On
Friday, the late-week impactful storm clears the area as a
digging shortwave trough shunts precipitation quickly eastward. A
weak low in the northern Gulf of Alaska exits east in tandem, with
cold air filtering across Southcentral on the back side of the
low. This will force gap winds out of bays and passes along the
north Gulf coast on Friday. These winds diminish Friday night as
high pressure begins to build across the Gulf of Alaska. An
upper-level ridge also begins to set up over the eastern Bering
Sea/Southwest Alaska, allowing for a period of dry weather across
the bulk of the southern mainland. The ridge draws up warmer air
into the Bering Sea and Southwest Alaska on low-level
southeasterly flow, leading to warmer temperatures there over the
weekend. Some warmer air has the potential to work into
Southcentral, but odds currently lean toward temperatures
remaining on the colder side, especially further east in the
Copper River Basin.

Out west, longwave troughing over the far western Bering will
focus active weather within the Bering Sea. South to southeasterly
flow with a series of embedded weak shortwaves will track from
the North Pacific into the Bering Sea, meaning rainy conditions
are expected for the Aleutian Islands and parts of the Alaska
Peninsula through the weekend. A swath of persistent near-gale
force easterly winds will be present across the Bering through at
least Sunday. Uncertainty grows by Monday when a shortwave diving
south from the Chukchi Sea reaches Southcentral Alaska. This may
induce low pressure in the Gulf, increasing the likelihood of
coastal precipitation developing going into the next week. For the
Bering Sea/Aleutian Islands, rainy conditions and easterly winds
are likely to continue on Monday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Storm Watch 101 111 121 145.
MARINE...Gale 130 132 150 155 170 172-176 178 180 412-414.
		 Heavy Freezing Spray 180 181
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...PS
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KC
LONG TERM...CQ

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