Small Craft Advisory 918, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
701 PM AKST Sun Nov 29 2020

.ANALYSIS AND UPPER LEVELS...
The strongest weather feature on the map this afternoon is the low
that is south of the Aleutians. This strong low is located in the
forward-left exit region of a strong jet streak (around 185 KT at
250 mb) which is feeding the low. A weaker low east of Kodiak is
bringing some rain and snow showers to the Gulf region and
scattered snow showers inland in Southcentral Alaska. A very weak
upper level low near the Kuskokwim Valley is bringing enhanced
lift which is bringing some light snowfall to parts of southwest
mainland Alaska this afternoon as well.

&&

.MODEL DISCUSSION...

The models are in surprisingly good agreement to day, especially
considering the pattern and complexity of the lows. The main low
that bears watching is the one currently south of the Aleutian
Islands. The models all show the low elongating with two low
centers. The first low is the one that, while very consistent
with strength and track on the GFS and NAM, have different wind
directions over northern Cook Inlet on Tuesday. The expectation is
that the NAM direction, which turns winds to the southeast over
Anchorage and the Matanuska Valley is the correct one (the GFS
keeps northerly winds). The timing of the southeast winds in these
areas, and associated temperature and precipitation-type is the
main issue for the forecast.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist through this
evening in spite of a passing snow shower or two. Wind shear will
develop Monday afternoon and increase into Monday evening.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Tonight
through Tuesday evening)...

An upper level low and its associated surface low are currently
located just southeast of the Kenai Peninsula this afternoon. This
low will continue moving slowly to the northeast towards Prince
William Sound by tonight where it will open up into a trough. Once
it opens up into a trough it will gain some momentum and continue
moving northeastward over the Copper River Basin and then across
the border into Canada. As the aforementioned low continues
moving northeast, expect widespread on and off rain and snow
showers across Prince William Sound including the Valdez and
Cordova areas through the overnight hours. Mountain areas around
the Sound will continue to experience southerly flow that will
upslope and squeeze out more moisture from the atmosphere
continuing through the remainder of the overnight hours. A piece
of energy that extends west from the center of the upper level low
has pushed a band of snow through the Anchorage Bowl and the
Kenai Peninsula this afternoon. This band was currently located
over the western part of the Kenai Peninsula and is expected to
dissipate as we head into the evening and overnight hours. Only
very minor accumulations of snow are expected from this.

Attention then turns to the next system currently located to the
south of the Central Aleutians. This system will continue moving
eastward heading into tonight. Kodiak will be one of the first
places to feel the impacts from this system as the front
associated with it pushes through beginning Monday morning.
Precipitation is expected to start as snow in Kodiak before
transitioning over to all rain as warmer air is brought in. Around
an inch of snow is possible before the change to all rain. Over
the Gulf waters the front will bring storm force winds which are
expected to begin across the southern waters Monday morning. The
front continues moving rapidly to the north to the Gulf Coast with
the storm force winds ending by Monday night. Main concerns with
this storm will be the warm air coming northward with it. This
will cause increasing snow levels as milder is drawn northward.
Heavy wet snow accompanied with warming temperatures and even some
rain will create hazardous conditions for the mountains
especially in Turnagain Pass and Moose Pass raising the concern
for avalanches. Snow levels are expected to rise to upwards of
2500 feet on Tuesday with the warm air.

A Winter Storm Watch has been issued this forecast package for the
Susitna Valley, especially for areas north and west of Talkeetna,
for the possibility of a 12-24 inch snowfall from Mon eve through
Tue eve. A deep southerly and moist flow is expected to push up
against the Alaska Range and create for a possible very snowy
situation. The Parks Highway especially through the Chulitna area
has the potential to become very hazardous due to the possible
heavy snow amounts anticipated. Stay tuned to see if this gets
upgraded to a Warning in subsequent updates. The Anchorage Bowl
and the western Kenai Peninsula are all expected to start as snow,
though, only light accumulations are possible. Milder air is
expected to work in from south to north late Monday night and
through the day on Tuesday to create a rain/snow mix across the
area. Some regions are anticipated to even change to all rain.
Downsloping is a concern for the Anchorage Bowl which may have the
warm temperatures occur but with relatively minimal
precipitation. As the front lifts northward, expect the Turnagain
Arm wind to develop with gusty winds for the Hillside and the Arm
beginning Monday morning and continuing through the day Tuesday.
The Matanuska Valley wind and the Knik River Valley wind are also
expected to ramp up as this system moves through resulting in
temperatures spiking into the 40s for Palmer over to Wasilla.
Temperatures across Anchorage will also warm into the upper 30s to
near 40 when the Turnagain Arm makes it into town. Warming
temperatures even without any rain will make travel hazardous as
warm temperatures combined with frozen ground and melting snow
could make road conditions slick for the Tuesday morning commute.
Stay tuned for the latest forecast updates.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 2: Sunday
afternoon through Tuesday)...

Cold through tonight with mostly cloudy conditions as weak high
pressure briefly moves across the area. The next low pressure
system moves along the AKPEN on Monday this will bring
precipitation mainly in the form of snow to Southwest Alaska, a
rain/snow mix is likely along coastal areas along the AKPEN Monday
afternoon. As the low tracks north Monday afternoon a deformation
snow band is expected to set up across parts of Bristol Bay Monday
night and early Tuesday morning mainly from Dillingham and
northward with 6 to 11 inches of snow expected. Additionally,
winds will be gusty from the north at 25 to 35 mph. A winter
weather advisory has been issued for snow and blowing snow
for tomorrow afternoon through Tuesday for portions of Bristol
Bay and the lower Kuskokwim Delta. There is still uncertainty on
the exact track of the low with the GFS farthest west while NAM
and Canadian are somewhere in the middle. I blended GFS/NAM and
Canadian for QPF and used GFS Ptype however I did a few manual
edits to get mix around King Salmon and along coast of AKPEN.
However, it looks unlikely that the Kuskokwim Delta will get
enough snow for an advisory as most models keep bulk of
precipitation east of Bethel. As the main upper level shortwave
moves north by Tuesday morning most of the heavy snow will end,
however lingering light snow will continue along a slow moving
frontal boundary moving east across Bristol Bay towards the
Alaskan Range on Tuesday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 2: Sunday
afternoon through Tuesday)...

A low pressure system south of the central Aleutians is producing
near storm force winds of 45 to 50 knots as this system tracks
east tonight snow and strong winds are expected along some of the
Aleutians with light snow accumulations, but with temperatures
near or slightly above freezing only an inch or two is expected
in most populated areas. Do not expect to meet criteria for
blowing snow advisory for eastern Aleutians tonight. Late Monday
into Tuesday cold northerly flow will set up with snow showers
likely with only minor accumulations and winds in the 20 to 30
knot range. Tuesday a weak low will move near Attu Island bringing
some light rain to the western Aleutians but no impactful weather
from this low.

&&

.MARINE (Days 3 through 5 - Wednesday through Friday)...The period
commences with the last vestiges of a decaying low over the
Northern Gulf of AK while a new low moves from the North Pacific
towards the Aleutians. First in the Gulf, winds will finally
subside below gale-force as we head through the day Wed into Thu.
The rest of the week will feature primarily cold air advection
westerly flow with a strong low east of our area over SE Alaska.

Bering...The pattern will continue to feature rapid and dynamic
cyclogenesis. On Wednesday, broad northerly flow over the Eastern
Bering will yield to a new system developing south of Dutch Harbor
Wed night into Thu. There is high confidence in at least gale-
force winds with this low, however, storm-force winds cannot be
ruled out. This low will lift north into the Central Bering by
late Thu. A new low will then take its place on Fri. There is
solid model agreement and moderate confidence in the pattern until
the Fri system. At that point, it looks like at least gales will
once again be possible, but the exact strength and location by Fri
night is quite uncertain.

&&

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

Going into the second half of this week, guidance indicates a
pattern shift is ahead. Aloft, strong southerly flow pushes off
to the east and exits Southcentral AK. In Southwest AK and the
Bering/Aleutians, there are indications that shortwave ridging
will move in behind the exit of the longwave trough. Moving into
the weekend, model guidance shows this ridging moves over
Southcentral and phases with a longwave high pressure ridge
extending up the West Coast from high pressure in the Western US.
Low pressure then returns the the Western Bering.

On the surface, high pressure exists over Mainland AK while a
series of lows moves north through the Eastern Gulf of Alaska and
into the Southcentral Coast, keeping trough along the coast. Low
pressure also exists in the Bering. It should be noted that there
is considerable spread in the models both aloft in the general
pattern and at the surface with specific features. Especially with
specific low pressure systems, there is too much disagreement to
be able to discern what will happen in the long term. When it
comes to the pattern, there is disagreement in what will happen,
but guidance is at least indicating the pattern setup mentioned in
the previous paragraph. Therefore, we can have some confidence in
a pattern shift ahead, but there is too much uncertainty to
identify specific features.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Storm Watch 145. Winter Weather Advisory 161 152.
MARINE...Heavy Freezing Spray Warning 180 181.
		 Storm Warning 119 120 131 132 351 352 174 176 177
 		 Gale Warning 125 128 130 136-141 413 414
                      150-172 173 175 178 179 180 181.
FIRE WEATHER...NONE.

&&

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

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
957 PM AKST Sun Nov 29 2020

AKZ125-010200-
Western Prince William Sound-
Including the cities of Whittier, Seward, Girdwood,
and Moose Pass
957 PM AKST Sun Nov 29 2020

...A Wet, Windy, and Warm Storm to Start the Week...

After several cold, snowy storm systems, the next one in line will
bring much warmer temperatures. This will allow for a transition
from snow to rain with snow levels rising to near 2,500 feet by
Tue evening. The weather pattern will also favor heavy amounts of
precipitation. Here are the key take-aways:

Snow and Rain: The system will bring a bit of both. It will track
quickly from south to north. The initial front will allow for
Seward to see a couple inches of snow accumulation through the day
Monday. However, warm air will push in quickly behind the front
and transition precipitation over to all rain for all locations
near sea level. Precipitation will be nearly continuous for
places like Seward and Whittier where over 4" are expected by
Wednesday morning. For higher elevations, such as Turnagain Pass,
colder air will try to hold on. However, temperatures will rise to
near freezing, so expect very heavy, wet snow over the mountains.
Snow accumulations will be on the order of 1-2 feet.

Winds: East to southeast winds will steadily increase through
Passage Canal and into the Portage Valley through Monday. Winds
should peak late Monday evening into early Tuesday morning. Gusts
to 60 mph in Portage Valley and the Eastern Turnagain Arm are
expected.

Travel Hazards: This combination of rain, snow, and rain on snow
could result in challenging conditions across the area. Any rain
that falls on snow covered roads could make them especially
slick. Warmer temperatures near the surface can also result in a
thin layer of water on top of snow on the roadways. Please check
in with the DOT for the latest road conditions calling 5 1 1 or
visiting 511.alaska.gov. Avalanche concerns will also increase
with this pattern. Strong south to east winds will make for
uneven snow distribution. A moderate amount of rain will fall on
the new snowpack over the midslopes with heavy wet snow falling
along the highest terrain. Please visit cnfaic.org for the latest
on avalanche danger.