National Weather Service United States Department of Commerce


000
FXAK68 PAFC 290220
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
520 PM AKST Sat Nov 28 2020

.ANALYSIS AND UPPER LEVELS...

The upper level pattern continues to present a complex longwave
trough over our forecast area. This trough has shifted east over
the past day and is now centered over Southwest AK and encompasses
almost all of our forecast area, save for a shortwave ridge over
the Western Aleutians. A couple of closed upper low centers exist
within this longwave, with one of the lows centered over the
AKPEN. Several shortwave troughs are also embedded in this flow.
There are two prominent shortwaves in our forecast area. One
weakening trough in the Bering supporting a decaying surface low
over the Aleutians. The other is over the Gulf of Alaska
supporting a surface low moving into the Southcentral Gulf Coast.

On the surface, radar is capturing precipitation associated with
the low along the Gulf of Alaska moving into Southcentral AK.
Surface observations are also showing the trough moving in, with
continued snowfall in areas like Portage and Whittier. Slightly
warmer areas farther south on the coast like Homer are seeing a
mix of snow and rain. The latest low center has pushed ashore near
Yakutat, with the next low right on its heels, currently located
south of Kodiak. In southwest, radar also shows wrap around
moisture in Bristol Bay from this system. A weak decaying low is
currently moving south through the Bering and not producing much
weather.

&&

.MODEL DISCUSSION...

As with the past few days, models are handling synoptic features
well in the short term and depicting the longwave pattern well.
They are continuing to struggle on the mesoscale and with local
effects, especially for Southcentral AK. There has been almost
continuous adjustment to the forecast the past couple of days as
a result. Today is a little better, in part because the pattern
has briefly quieted. However, models still struggle after about
48 hours. In our western domain, models are performing better. So,
we have high confidence in models out west, with lower confidence
in Southcentral.

&&

.AVIATION...

PANC...VFR conditions and light winds will persist.

&&

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

A gigantic area of low pressure east of Kodiak Island remains to
be the primary weather producer for Southcentral this afternoon.
A weakening frontal zone over Prince William Sound (PWS) continues
lifting inland and dissipating. Still, more snowfall from this
feature will spread across PWS and Copper River Basin, including
Thompson Pass tonight. A shortwave behind this frontal axis will
arrive across PWS and eastern Kenai Peninsula later tonight while
the upper level low make its way toward the northern Gulf and
inland (east of Cordova) overnight tonight. Once this upper low
moves inland and well east of PWS later tonight, snowy conditions will
taper off over much of the area east of the Anchorage Bowl. This
will last only for a short while before the next upper level
system arrives Sunday morning.

Over Shelikof Strait, a developing upper low is parked there
before moving toward PWS Sunday morning. This will cause another
heavy round of heavy mixing rain and snow to return by late
Sunday afternoon if not sooner over PWS and the Copper River
Basin. Meanwhile, a shortwave north of the upper low will break
away and drift northward past Anchorage Bowl by Sunday night. In
response, more snow will spread over Anchorage, Matanuska Valley
and western Kenai Peninsula on Sunday and Sunday night.

Temperature-wise, most of the area will stay at or above normal
this weekend, though Sunday night into Monday could offer a brief
return to colder, more normal temperatures. This will be brief,
however, because the next low in this parade of storms looks to
bring much warmer conditions and rain chances to much of Southcentral
by early next week.

&&

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

An upper level low continues to spin over the region with its
associated surface low to the east of Kodiak Island. The surface
low will continue moving northward through the Gulf of Alaska
while weakening. However, energy associated with the upper level
low is keeping light snow and low visibilities across the
Kuskokwim Mountains and across northern and central Bristol Bay in
areas such as Koliganek and New Stuyahok. This energy will only
ever so slowly move southward out of the region heading into the
overnight hours. Therefore, light snow was kept in the forecast
with some accumulation until Sunday morning before finally
dissipating. Due to the light snow, only an additional inch of
snow accumulation is anticipated. Radar imagery was showing some
returns possibly making it into the Bethel area this afternoon
through the evening hours. With this in mind, we opted to just
put flurries in the forecast as some moisture may make it into the
area.

This system finally pulls away by late Sunday morning. Skies are
expected to clear the most out over the Yukon-Kuskokwim Delta
with partly cloudy to mostly cloudy skies elsewhere. With the
recent snow and residual moisture in the air, areas of patchy fog
are possible for Monday morning. Other than that, expect a dry but
cold Sunday afternoon through Monday morning before the next
system approaches. Temperatures Sunday afternoon across the
interior areas are expected to be in the upper single digits and
teens with 20s along the coast. Temperatures will become very cold
for Monday morning with single digits below zero across the
interior areas and single numbers and teens along the coastal
areas. The one thing possibly going against it getting this cold
Monday morning is the potential cloud cover that may be around. If
skies clear, the potential is there for temperatures to fall even
further than already forecast. Also, areas of patchy fog will
also play a role in the temperatures for Monday morning.

Lastly, the next storm system moves into the area Monday morning
and continues to affect the area through Tuesday night. This event
should be all snow for areas west of a Lime Village to Koliganek
to New Stuyahok to Dillingham line. Areas east of the
aforementioned line have the potential for a snow changing to rain
situation as warmer air comes in and then back to snow when the
cold air comes back in. These areas may also just mix with rain
briefly especially areas around Iliamna. King Salmon is a tough
forecast for this system as the gradient between warm and cold is
expected to be sharp making any shift in the storm track
detrimental to the type of precipitation that will fall. A further
west track means more rain and warmer for King Salmon with
further east colder and more snow. Of course, the downsloping risk
is there as well with this system and depending on how strong the
downsloping is will factor into the amount of precipitation seen.
Interior Bristol Bay and the Middle Kuskokwim Valley have the
potential to see significant snowfall (greater than 6 inches) from
this event where a band of snow might set up. Where this band
sets up remains in question but where it does could see
significant snowfall and headlines will be possible with the
coming forecast packages.

&&

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

Cold air flowing over the Bering is creating an unstable
atmospheric setup. This is seen on visible satellite imagery in
the form of cumulus clouds. Some of these grow enough to cause
snow showers that may briefly reduce visibility and lower
ceilings for the Pribilofs and the eastern Aleutian Chain (Cold
Bay and Sand Point). This will continue for the duration of this
forecast package, especially for the Pribilofs north and west.

The next weather system is expected to affect the areas mainly
south and west of the Pribilofs with storm force winds especially
in the marine zones south of the Aleutian Chain. The timing on
these storm force winds are expected to be early Sunday morning
through Monday morning. Seas of 20-30 ft are expected south of the
Chain with 16-21 ft seas for the Bering side. The one other thing
to note is the potential for blowing snow Monday afternoon into
early Tuesday for the eastern Aleutians as the cold air is coming
back in and precipitation is still occurring. The better risk for
this is around Cold Bay and False Pass which will have a better
chance of keeping the cold air in place. Sand Point has the
better risk to have rain occurring and having the precipitation
shut off before the winds ramp up Tuesday morning for any blowing
snow to occur.

&&

.MARINE (Days 3 through 5 - Tuesday through Thursday)...

Gulf: A well-developed low moves over the Western Gulf through
Thu. Its model track and development are mixed with the wrapped
low as it weakens. However, widespread southerly high-end small
craft winds and waves with local areas of gale force winds move
over the Gulf through Thu. These winds will diminish to
southwesterly small craft over the Gulf and local southeasterly
small craft winds around the Barren Islands, Kamishak Bay and
lower Cook Inlet.

Bering: A well-developed low moves into the Gulf of Alaska weakens
by Thu. The model track and development of the wrapped low vary
through Thu. Northerly gale force winds and waves over the
Central and Eastern Bering and Alaska Peninsula diminish to small
craft winds over the Eastern Bering and West Coast waters. Cold
high pressure dips into the Bering for Thu, but an upper level
trough brings easterly small craft winds to the West of St Matthew
Island Wed and Thu.

&&

.LONG TERM FORECAST (Days 4 through 7/Wednesday through Saturday)...

A weakening low pushes through Cook Inlet on Wednesday, keeping
the Southcentral coastal areas under showery conditions.
850mb temperatures remain warm enough to keep precipitation as all
rain, followed by a brief period of rain/snow mix on Thursday.
Models differ slightly with this low, where the Canadian/ECMWF
dissipate the low as it makes its way onshore, while the GFS
lingers it over Bristol Bay through Friday. This will have an
impact on a second low that enters the Southern Gulf by Thursday
afternoon where the GFS path is pushed more towards the Southeast
Coast. Both the Canadian and ECMWF solutions track this low north
towards the Southcentral Coast with slight differences between
strength and center location. These differences in model solutions
can impact precipitation type and intensity for the Southcentral
Coast for Friday and will need to be monitored over the next few
days. A third system tracks south of the Aleutians late next week
and will bring gusty winds to the central/eastern Aleutian Chain.
Additionally, persistent upper-level troughing lingers over the
Southwest region through Friday. Flow with this will be
predominantly northerly, thus, below average temperatures can be
expected over the Southwest mainland and the AKPEN/eastern
Aleutians.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Heavy Freezing Spray Warnings 160 180 181.
Storm Warning 172 174 175 176 177 178.
Gale Warning 150 155 160 165 170 171 173 132.
FIRE WEATHER...NONE.

&&

$$

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


851
FXAK69 PAFG 282339
AFDAFG

Northern Alaska Forecast Discussion
National Weather Service Fairbanks AK
239 PM AKST Sat Nov 28 2020

.SYNOPSIS...
The storm track remains over the Gulf of Alaska which is keeping
weather over the Interior rather quiescent. Strong northerly flow
continues over the West Coast. Light snow will move into the
region as an arctic trough digs into the West Coast and a weak
upper level shortwave trough drifts into the Interior as the Gulf
low ejects into the Panhandle/Yukon. A minor chinook warming event
with gusty south winds is expected for the Eastern Interior on
Tuesday and Wednesday.

&&

.DISCUSSION...

Upper Levels and Analysis...
There is a deep and cold polar low (approx 479 dam) over the
Canadian Arctic, with the trough axis stretching into eastern
Arctic. There is a wrapped up occluded low moving into the Gulf of
Alaska, with the surface center around 970 mb, and slowly
weakening. Satellite imagery clearly depicts a mobile shortwave
trough along the eastern periphery of this low moving up toward
the ALCAN from the Gulf of Alaska. Over the Brooks Range, there is
a leftover frontal zone stretching along a deformation zone, thus
it is weakening. The overall gradient is directed from north to
south with a continued Tanana Valley Jet pushing east winds south
of Fairbanks, enhanced by the southerly flow aloft. Besides strong
north winds along the capes of the West Coast, weather is rather
quiescent.

Model Discussion...
All the models depict a similar synoptic pattern, and mostly
diverge Tuesday and Wednesday as a strong North Pacific warm front
moves up along the Gulf Coast and shifts inland, merging with a
West Coast arctic trough. This is a classic pattern in winter, and
there will likely be moderate snow accumulations under the
deformation band that forms in between these two features aloft.
However, all the models, while similar, have different placement
and timing, and also eject subtle shortwaves northward across the
AK Range at different times and with different amplitudes. As a
result, a blend of models is favored to average out these subtle
differences, with this forecast increasing chances for snow
Tuesday and beyond, and increasing winds across the Ak Range in
the gaps as well as the Tanana Valley Jet.

Central and Eastern Interior...
Quiet weather over the as northeast gradient and a storm track
over the Gulf of Alaska keeps the area dry. An easterly wave
coming in from the Gulf of Alaska will rotate into the Interior
tonight and tomorrow, then merge with an arctic trough out west,
bringing light snow to the area tonight and tomorrow. Snow amounts
look to be less than 1-2", with the best accumulations over the
Central Interior and Upper Koyukuk.

Southerly flow aloft begins to assert itself Monday with
increasing pressure gradient across the AK Range, and gradual
warm air advection aloft. The mid level warm front will move north
across the AK Range Monday evening through Tuesday, bringing light
snow threats back, as well as even stronger gap winds and a TVJ.
Tuesday the chinook pushes over the AK Range with temps shooting
up into the upper 20s to low 30s over the Tanana Valley. Light
snow and possible mixed precip lifts north as the front moves
toward the Brooks Range.

West Coast and Western Interior...
Quiet weather except for strong northeast winds along coastal
areas, accompanied by ocean effect snow showers along the Capes
and over Saint Lawrence Island where light accumulations with
blowing snow is expected. Light snow moves into the Western
Interior on Sunday as weak southerly flow and a subtle disturbance
interact with one another. Snow amounts will be light, with
widespread 1-2" amounts from Galena northward through Monday.
Another stronger snowband develops on Tuesday and will persist
into late week. this looks to bring much more snow to the Western
Interior with the possibility of several inches of snow. Stay
tuned...

North Slope and Brooks Range...
Stratus and light flurries are the name of the game along the
Arctic coast. Snow chances increase Sunday night and Monday as an
arctic trough digs into the West Coast and develops a southerly
flow aloft. Strong northeast winds will continue along the
Northwest Arctic coast with an increasing easterly gradient
developing on Tuesday along the coast, and increasing southerly
Brooks Range gap winds.

Extended Period Forecast Days 4-7...
Beginning Wednesday, an active pattern will be in place as general
arctic troughing dominates over the West Coast and southerly flow
over the eastern half of the Mainland. Snow initially over the
Western Interior will gradually shift east as a series of lows
tracks across the Alaska Range, and the Arctic trough moves east.
Confidence is very low on precip threats by midweek, however,
even through the pattern is conducive to periods of snow. Stay
tuned...

Coastal Hazard Potential Days 3 and 4...None.

&&

.AFG WATCHES/WARNINGS/ADVISORIES...

Heavy Freezing Spray Warning for PKZ200-PKZ210-PKZ220-PKZ225-
PKZ230.

Small Craft Advisory for PKZ225.

Gale Warning for PKZ210-PKZ220.

&&

$$

AHSENMACHER NOV 20



000
FXAK67 PAJK 282352
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
252 PM AKST Sat Nov 28 2020

.SHORT TERM...A storm force low is located south of Prince William
Sound. The associated front has been surging through the
panhandle. A variety of hazards have blanketed SEAK ranging from
High Wind Warning for Yakutat, Winter Storm Warning for Haines and
Skagway, as well as a multitude of gale warnings and strong wind
headlines. Damage reports have been filtering in this afternoon
with downed trees in Yakutat and Juneau. The associated front is
expected to continue to push through the panhandle this evening
with the overall trend of decreasing winds through the night.

The main band of precip with this system will shift south tonight
causing rain to become scattered from north to south. For the
northern and central panhandle rain chances decrease through the
night while a wave develops across the far southern panhandle
keeping rain chances elevated. For Monday, scattered showers will
persist before our next strong system impacts the panhandle monday
night into tuesday. This system has atmospheric river
characteristics with deep moisture support. Preliminary 24 hr
rainfall amounts of 3 to 6 inches is expected but more time is
needed before a more accurate estimate can be made.

Overall changes were minimal as main focus was on evolving
conditions with current weather system.

.LONG TERM.../Monday through Saturday night/In a continuation of
previous thinking, a strong atmospheric river to start a new
month is racing towards and through the northeast gulf coast and
northern portion of the panhandle. Models have fairly solid
agreement on the event occurring. Standardize Anomalies range
between 2.5 and 3.1 for precipitable Water, and IVT anomalies
are between 4 and 6. Based on the potential rain fall, have issued
a Hydrological Outlook (ESF) for the time as a start to share
some of the information. Rain totals in the current forecast for
that timeframe ranges up to 5 to 11 inches for the 2.5 days.

When the front makes its initial appearance, storm force winds
will be in evidence over coastal waters, and strong wind headlines
(gusts of 40 to 55 mph) will be likely for the coastal land
zones. To top all of this off the Haines and Klondike Highways may
be cold enough to start or stay snow for a while and heavy rates
may keep it snow for the highways as well. The snowfall and
amounts are the least confident part of the late Monday to
Wednesday forecast.

A wave or two on the N-S front will slow eastward progression
through Wednesday night the way it looks at this time. Longer
range models are also suggesting a second low moving northward
over the gulf late Thursday impacting somewhere between Kodiak
Island and Yakutat. A third feature may be trying to develop for
Saturday.

Forecast confidence is in the above average for Monday-Tuesday,
and below average from Thursday onward because of the model
consistency.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Strong Wind until 9 PM AKST this evening for AKZ026.
Strong Wind until midnight AKST tonight for AKZ027-028.
Strong Wind until 3 PM AKST this afternoon for AKZ021-022-024.
Strong Wind until 6 PM AKST this evening for AKZ020-023-025.
Winter Weather Advisory until 5 AM AKST Sunday for AKZ029.
Winter Storm Warning until 9 PM AKST this evening for AKZ018-019.
High Wind Warning until 4 PM AKST this afternoon for AKZ017.
Strong Wind until 6 PM AKST this evening for AKZ017.
MARINE...Gale Warning for PKZ012-013-021-022-031>035-041>043-051-052.
Small Craft Advisory for PKZ011-036-053.

&&

$$

CC/Bezenek

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