Gale Warning 756, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
503 PM AKST Mon Dec 23 2019

.ANALYSIS AND UPPER LEVELS...
Several weak lows are affecting the area this afternoon. One
heading into Prince William Sound is producing widespread
convective showers from Cordova up through Valdez. A second low
is crossing the Alaska Peninsula. It will track up the coast and
support snow over Anchorage and the Mat Valley tomorrow afternoon
and evening. A new developing low over the North Pacific well
south of Shemya will impact the Aleutians tomorrow and move into
the Gulf for Christmas Day. More on that low below.

Temperature-wise, cold air persists across the Kuskokwim Valley
this afternoon. In addition to winds through the Kuskokwim Delta,
wind chills remain south of 20 below. The cold air will only be
reinforced over the coming days.

&&

.MODEL DISCUSSION...
While the models are in good agreement on the large scale overall
pattern, small scale differences in the models would result in
large differences in the forecast. For example, regarding the
snowfall in Anchorage tomorrow, the NAM is wetter and begins the
snow much earlier (tomorrow morning), than all of the other
models, which hold off on the snow until tomorrow afternoon.
Elsewhere, the track of the developing low over the Aleutians will
play a large role in the potential for heavy wind-blown snow
across much of the Chain, or if it falls as a cold rain. The
further north the track of the low, the better the chance it falls
as a rain. The center of the low quickly moves into the
southwestern Gulf by Christmas morning.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist through the
night tonight. Sometime during the day tomorrow, most likely after
noon, snow will build into the area. Several inches of
accumulation are expected as a result. Visibilities are expected
to drop to 1 mile or less during the afternoon. The snow will
persist into the evening, though intensities should slowly
diminish with time.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Snow showers are mainly impacting the North Gulf Coast this
evening with some inland flurries elsewhere. Generally quiet
weather will persist tonight before activity ramps up Tuesday as a
negative tilt shortwave trough tracks across Southcentral from
west to east. Snow will track from the Kenai Peninsula and move
north through the afternoon. With cold air in place and cold air
advection following behind, along with potential convective
forcing in the Gulf, thinking there is a good chance for
widespread 2-4" snow from the northern Kenai all the way to the
northern Susitna Valley including Anchorage. Some localized areas
will likely see slightly higher amounts along and near the
mountains. This trough will track through the Copper Basin
overnight, also bringing light snow accumulations. Another strong
low will move into the Gulf Wednesday, initially bringing snow to
Kodiak before changing to rain. Light snow will begin moving
inland as early as midday Wednesday, with heavier snow along the
Gulf Coast. This low moves inland Wednesday evening (see extended
discussion), but not before impacting the coastal waters with
strong gale force winds.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A negatively tilted upper trough remains situated over western
Alaska, with Arctic high pressure over northern Alaska. Areas of
snow persist over portions of the Southwest Mainland, with
temperatures mostly below zero in the Kuskokwim Valley/Delta. Any
precipitation will diminish Tuesday, and then bitter cold becomes
the main story. Temperatures will plummet as the upper trough digs
southward and north flow strengthens. A wind chill advisory has
been issued for the Kuskokwim Delta, including Bethel and is in
effect from Tuesday night through Wednesday evening. Wind chills
down to -50 are possible in the Kuskokwim Delta. Temperatures will
likely drop to 20 to 30 below zero in the middle Kuskokwim Valley
Tuesday night, but surface winds should stay below 10 mph.
Meanwhile in the Greater Bristol Bay area expect overnight lows
Tuesday into Wednesday 10 to 20 below zero.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A low currently centered roughly 600 miles south of Shemya will
deepen tonight into Tuesday, bringing gales to the southern Bering
and central/eastern Aleutians Tuesday, with an embedded swath of
storm force winds. With very cold air on the north side of the
low, there is concern about snow impacts in the Aleutians. The
projected low track has gradually shifted northward over the last
several model runs. This, combined with current sea surface
temperatures make blizzard conditions unlikely. Precipitation
types will be a challenge, and will likely vary from snow,
rain/snow mix, and rain. However, with strong winds likely and
periods of heavy snow possible, expect visibility reductions in
the central and eastern Aleutians Tuesday. A winter weather
advisory for blowing snow is in effect for Adak and Atka through
Tuesday evening. The driving low will proceed eastward Wednesday,
possibly driving gusty gap winds south of the chain on its
backside.

&&

.MARINE (Days 3 through 5)...
(Thursday through Saturday)

...Bering Sea and Aleutian Coastal Waters...

Winds will increase on Thursday across the western and central
Aleutians as a rapidly deepening area of low pressure approaches
Adak from the south. Sustained middle to hi-end gale force winds
will develop near the island chain, with some storm force winds
possible on the Pacific side, accompanied by seas of 25 to 30
feet. On the Bering side, look for sustained gales and seas of 20
to 25 feet.

The storm then rapidly transitions east as it for Friday and
Saturday, as it gets absorbed into a developing Gulf low, allowing
for improving conditions to commence.

...Gulf of Alaska...

An area of low pressure will move from near Kodiak Island to
Prince William Sound on Thursday, allowing for hi-end sustained
gales and seas of 15 to 25 feet to develop across the Gulf and
near the Barren Islands. These conditions will improve by Friday
afternoon as the system weakens and moves ashore. This will be
short-lived though, as another rapidly deepening area of low
pressure moves north into the Gulf late Friday night and into
Saturday, with sustained gales and an increasing sea state once
again possible.

&&

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

...Southcentral...

An active pattern with periods of snow is expected for Thursday.
Snowfall then winds down by Friday morning, but this will be
short-lived as another deepening area of low pressure moves into
the Gulf. Where this low goes is still up for debate. The European
model (ECMWF) takes the parent low towards Yakutat, with another
weaker low near Hinchinbrook Island, as an inverted trough
extends back. This solution keeps most of the snowfall in the
Copper Basin and Valdez area.

Meanwhile, the GFS Model tracks the parent low much further west
bringing the center into the Valdez area, placing more of the
western Kenai into the Anchorage bow. and Matanuska Valley
(perhaps southeastern Susitna as well) into the deformation
snowfall. The Canadian Global takes a middle ground approach but
is closer to the euro solution. Thus, it looks like most of the
steadier snowfall for Saturday will be confined to the Copper
Basin, with some very light snowfall perhaps extending back to the
eastern slopes of the Chugach Mountains.

Then on Sunday, a third storm system moves into the Alaska
Peninsula, with a warm front approaching the Southcentral Coast,
bringing more snowfall potential as an over-running setup
develops.

As for temperatures, some colder air does make it into the
Southcentral as each storm helps draw down colder air via
northeast winds as multiple lows pass mainly to our south and
east. However, the models have backed off the amount of cold
advection, as 850 mb temps around -15 deg. C are expected compared
to -30 deg. C yesterday. Thus, although it'll still be quite
chilly this weekend, it's not looking as cold as it was looking
like it could be with the models from yesterday.

...Southwest Alaska through the Bering Sea and Aleutians...

Very cold conditions are still anticipated across southwestern
Alaska through the Pribilof Islands, as cold air advects into the
region from central portions of the state. Here, 850 mb temps
dropping into the -20 to -30 deg. C. range are expected, with the
coldest temps locates across the southwest mainland. A dry
northeasterly offshore flow will keep this region mainly dry,
while snow squalls can be expected for much of the Bering and
Aleutians, as the colder air moves over the warmer sea surface
temperatures, creating sufficient instability for them to
develop.

For the Aleutians, a pair of lows will approach the chain while
staying south of there. This places the region in a favorable
over-running setup as elevated warm fronts attempt to approach the
coast. This warm advection helps moderate temperatures some from
Dutch Harbor east, while helping extend the colder air from the
Pribilof Islands across the western half of the region. In either
case though, precipitation will be fairly widespread as each low
approaches.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 187.
 Wind Chill Advisory 155.
MARINE...Storm Warning 170 173 174 176.
 Gale Warning 130 132 155 160 165 171 172 175 177 179.
 Heavy Freezing Spray 160 181.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JW
SOUTHCENTRAL ALASKA...JA
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MM
MARINE/LONG TERM...PD