Small Craft Advisory 902, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
443 PM AKST Tue Nov 24 2020

.ANALYSIS AND UPPER LEVELS...

The large front we have watched move through the Bering is
currently pushing into Southwest Alaska. Surface observations show
warm is pushing inland as expected with this front, switching
snow to rain. The well defined trough and trailing edge of the
front is currently located over the Pribilofs stretching south to
the Eastern Aleutians. Behind this front surface analysis shows a
prominent post frontal shortwave moving through the Central
Bering, with satellite confirming this with a band of cloud cover.
Northwesterly flow and colder temperatures are behind this shortwave.

Southcentral is having a brief break in the weather, as the low
that was inducing offshore flow and gaps winds has moved southeast
out of our forecast area. This has allowed winds to subside. A
surface high is also in place over Southcentral AK, helping to
keep weather quiet.

All of this is supported in the upper levels by a 500 MB closed
low just off the coast of Siberia with a 500MB trough extending
south through the central Bering. This is system is part of
broader longwave troughing encompassing all of the Bering and
Eastern Siberia. Moving east to Southcentral, a 500 MB trough is
in place with the ridge axis about over the Susitna Valley. This
synoptic pattern is in the process of progressing east, which is
allowing upper level troughing to push into mainland AK.

&&

.MODEL DISCUSSION...

One the synoptic scale models are picking up on the general
features and look to be in agreement. It should be noted weather
features are moving about three hours faster than models show, so
this change is accounted for in the forecast. Otherwise they are
handling the pattern well. They do develop inconsistencies when
focusing on the mesoscale, especially for Southcentral AK and the
Gulf of Alaska. In Southwest AK and the Bering models seem to be
performing well, save for depicting precipitation type which is
typical. In Southcentral AK, models have considerable disagreement
after about 36 hours, including the frontal system on late
Thanksgiving Day. Therefore, higher forecast confidence out west,
with lower forecast confidence after Wednesday for Southcentral.

&&

.AVIATION...

PANC...VFR conditions will persist. Light northerly winds will be
present at the surface. However, southeasterly winds are expected
to increase with Turnagain Arm winds tonight. These winds will
remain elevated just above the surface and will create a low-level
wind shear environment over the airfield.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Tonight
through Friday)...
The next storm system, which will bring widespread wind and
precipitation to Southcentral late tonight through Wednesday
night, is largely on track. Have made only minor adjustments
with this afternoon's forecast package. The biggest question going
into today was how much wind lower elevations of Anchorage and the
Matanuska Valley would see on Wednesday. The latest forecast
soundings are in good agreement with the overall evolution of the
profile through the day tomorrow. While we'll start the day out
with a weak low level inversion, moistening of the airmass near
and above mountain top, will result in some cooling, resulting in
a fairly deep near isothermal profile. As the front passes through
around 3pm tomorrow afternoon, low level cold air advection will
quickly ensue, resulting in a weak unstable layer. At no point is
there an obvious critical level, which is key to helping force
high winds down to the surface. However, high winds (gusting to 80
mph) along west Turnagain Arm and on into the upper Hillside of
Anchorage look on track, so have upgraded to a High Wind Warning.
Still expect some gusty winds at lower elevations, but these winds
should remain well below key thresholds to produce damage and are
expected to be fairly brief (on the order of a few hours). For
areas near the mountains as well as the Matanuska Valley, expect
peak wind gusts of around 40 to 50 mph Wednesday afternoon.

Strong winds across the region ahead of the triple point low and
frontal system will lead to a fairly rapid warm-up at sea level
Wednesday morning, with rain expected across most of the Kenai
Peninsula. However, inland areas will likely see some wet snow
accumulation before changing to rain. The Seward Highway corridor
will be particularly messy, with strong winds and areas of rain
and snow. The snow will be heavy at times through Turnagain Pass.
As the front and low head inland Wednesday night, colder air aloft
will quickly move in behind it. However, surface temperatures
across Anchorage and the Mat Valley will likely be sitting in the
mid to upper 30s, largely due to the strong winds earlier in the
day. Thus, expect a mix of rain and snow transitioning to snow as
we head into the evening hours. The upper trough will move
through fairly quickly, so accumulation will generally be light
across the region Wednesday night.

Another much weaker storm will track across the western Gulf
Thursday and head into Southcentral Thursday night. This looks
to be primarily a snow producer Thursday night through Friday,
with little concern for strong winds. However, confidence in the
track of this storm is low, so will have to wait for future
forecasts to lock in on snow amounts. The active weather will
continue Friday, as a large and deep Aleutian low approaches
Kodiak and the western Gulf. This will bring another round of
strong winds and heavy precipitation to the region as we head
into the weekend.

&&

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

A front associated with a strengthening low in the northern Bering
continues to impact the southwest this afternoon. Gusty winds
continue across the area with a mix of precipitation throughout.
Almost all areas have turned over to rain, as temperatures have
increased due to warm air advecting into the region. Confidence
has increased regarding the Kuskokwim Valley and if colder
temperatures would prevail. Sleetmute has stayed below 32 degrees
throughout today, increasing confidence for the area that
precipitation will continue to fall as snow. For the areas
experiencing rainfall, expect precipitation to change over to snow
during the overnight hours, with continued rainfall expected
tomorrow during the day. Gusty winds will continue for much of the
southwest until tomorrow afternoon, as the front moves to the
east and out of the area.

As the warm air moves east on Wednesday, it will be replaced by
cold air beginning, first, with the coastal regions before making
its way across the rest of the southwest inland areas. This will
change all precipitation back over to snowfall as the warm air
moves out of the area. Expect continued snow showers for Thursday
before it dries out a bit Friday.

&&

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

Rainfall and gusty winds continue to accompany a front associated
with a strong low in the northern Bering. This will continue until
tomorrow, when cold air advection takes over and transitions rain
to snow for the central and eastern Aleutians. After this, widespread
snow showers are expected across the Bering and Aleutians from
Wednesday morning until Thursday morning, when a front associated
with a low in the North Pacific makes its way across the western
Aleutians. This will bring another shot of storm force winds,
warm temperatures and increased chances of rain to the area before
moving into the central Aleutians Thursday and the eastern
Aleutians on Friday. Challenges are expected with this feature, as
with the last, in precipitation type and timing of the transition
from snow to rain and from rain back to snow.

&&

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

Gulf of Alaska:

Fair confidence with overall pattern but much uncertainty remains
in the details during the 3-5 day timeframe. Friday, a Gale force
low will move across the southern Gulf. The low will weaken and
move into the eastern Gulf Friday afternoon. however low
confidence on track and strength. Friday afternoon a gale force
front moves into the Gulf there is a chance of near storm force
winds southeast of Cordova with the barrier jet, but low
confidence on strength and higher confidence with track. This
front will move inland and weaken by Saturday afternoon. A gale
force low will move in behind the front by Saturday afternoon.
This low weakens as it moves into the northern Gulf Sunday
morning, fair confidence on track and intensity.

Bering Sea:

A storm force low moves just north of the Aleutians across the
southern Bering on Friday and Saturday. Most of the storm force
winds are expected on Pacific side of Aleutians. This low will
move east into the Gulf Saturday evening. Good confidence on
track, but lower confidence on strength of low. Saturday night
through Sunday another storm force low is expected track south of
the Aleutians, low confidence with track and strength of this low.

&&

.LONG TERM FORECAST (Days 4 through 7...Saturday through
Tuesday)...

Progressive flow will continue to dominate the long term with a
series of fronts and lows moving across the Aleutians and Bering
into Southwest and Southcentral Alaska. These systems may bring
chances of accumulating snow to Southwest and Southcentral. Models
are hinting at a slight pattern shift as the storm track and
upper level jet may move south keeping the storms south of much of
the Aleutians and Bering, with the exception of the Gulf which
may be influenced by a strong North American ridge which will
direct storms into the Gulf and possibly into Southcentral by the
middle of next week. Good confidence on the overall pattern, but
lower confidence on details of track and strength of systems.

Saturday morning an occluded front will move into Southwest
Alaska with rain near coast and snow farther inland. This front
will stretch across the AKPEN to near Kodiak Island and across the
Gulf bringing heavier precipitation to Kodiak Island and
northward across the Kenai and Prince William Sound. As the front
moves north models are starting to show a triple point forming
near Kodiak Island and tracking north, however the track and
strength of this low is very uncertain. Rain or snow is possible
across Southcentral late Saturday into Sunday as the front moves
inland, but details uncertain especially with precipitation type.
This front will also produce Gale force winds across Gulf and
strong winds, especially near the coast and the usual local effect
winds. The parent low to this front will move south of the
eastern Aleutians and AKPEN Saturday. This low will produce storm
force winds mainly on the Pacific side of the Aleutians and weaken
to Gales as it moves into the Gulf by Saturday evening. This low
then moves into northern Gulf late Sunday and weakens. Fair
confidence with this system, however lower confidence with the
details of local winds and precipitation type.

Yet another storm force low tracks just south of the Aleutians
Sunday and Monday still a bit of spread among the models, but
relatively good agreement considering its 5 days out. Depending on
the track of low and front Southwest Alaska may get some snow out
of this system as the EC and GFS show the possibility of snow in
the deformation zone, but too much uncertainty to say this will
happen at this point. The front associated with this low will
track into the Gulf on Monday bringing another round of winds,
rain, and snow. Tuesday the front will move inland and weaken,
while much of the Bering and Aleutians will be under quieter
weather as northerly flow sets up as models hint at the upper jet
dipping farther south keeping the storm track south of the area.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning 101.
		 Coastal Flood Advisory 155.
MARINE...Gale 120 128-139 141 150-178 180.
		 Storm 119 125.
FIRE WEATHER...NONE.

&&

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