Small Craft Advisory 998, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
424 AM AKST Sun Dec 16 2018

.ANALYSIS AND UPPER LEVELS...

An upper level low centered off the coast of Hooper Bay is
building southeastward down the Southwest Alaska coastline this
morning. This feature and shortwaves tracking around it will be
the primary weather-makers over southern Mainland Alaska for the
next several days. For now, being ahead of the greatest forcing,
this is allowing light snow across Bristol Bay around the King
Salmon area to continue. As upper level lows are associated with
cores of cold air, and the one moving into Southwest Alaska is
particularly strong, this has made for bitterly cold temperatures
across the region. A separate vertically stacked low over the Gulf
is tracking northwestward with time. They are expected to merge
later this morning. In the meantime, Middleton Island radar shows
the next round of precipitation tracking northwestward across the
northern Gulf into Prince William Sound. This moisture will feed
the development of a deformation band that will bring snowy
conditions to the Kenai Peninsula. More on that below.

Most locations in Southwest Alaska are below freezing except right
at the coast. Generally the further inland you go, the colder it
gets. The coldest air is over the Kuskokwim Delta, closest to the
center of the upper level low, with 10s below zero around Bethel
and readings below minus 20 in Kalskag. Fortunately winds in those
bitterly cold areas are very light, keeping wind chills above
advisory criteria.

&&

.MODEL DISCUSSION...

Model agreement remains decent over the next couple days with the
current low in the Gulf now well-agreed upon to make landfall near
Seward late Sunday night. There is significantly worse agreement
on the development of another small mesoscale low over the western
Aleutians on Monday. However, the larger features, including a
strong front that will make its way into the Western Aleutians
Tuesday morning are well-handled. Model trends lately have been
trending weaker and drier with a shortwave trough expected to
bring a period of snow to Southcentral Sunday night through Monday
morning.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist until late this
morning, when a few light snow showers will move through, though
accumulations should hold under an inch. Then by midnight tonight,
another more significant round of snow moves in, which will
persist into the morning rush hour Monday morning before tapering
to snow showers. Accumulations from the second round of snow will
be in the 1 to 3 inch range.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...

The upper level low over the Gulf is in the process of becoming an
"open wave" as it gets caught up in the flow of a low along the
west coast of Alaska. A bent back occlusion associated with this
low will push westward through the northern Gulf and Southcentral
this morning, bringing some snow and warmer temperatures to the
area. This front will stall over the Kenai Peninsula. Significant
snow is expected over the western Kenai Peninsula today into
tonight with the stalled front. The front will then move north
tonight and Monday as the upper low becomes more or less fully
absorbed in the west coast low, and the front transitions to more
of a traditional occlusion. This will bring snow to much of
Southcentral, including the Anchorage area. This front will weaken
and move north of the area by Monday afternoon. Southerly flow
aloft will then be over Southcentral, but with very cold air
wrapping around the west coast low, cold advection will bring a
colder air mass into the area through Tuesday.

&&

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

Cold air is entrenching itself over the Kuskokwim Delta today as
a deep upper level arctic low centers itself over the region. The
upper low is allowing frigid arctic air to spill south from
Siberia and into Southwest Alaska. Temperatures in many locations
away from the immediate coast will approach or fall below -20F in
the morning over the next couple of days. Steady northerly winds
will also lead to wind chills approaching -40F, especially over
the Kuskokwim River Delta. On possible saving grace preventing
temperatures from dropping even lower is cloud cover from a
disturbance moving west across the Alaska Range and the Lower
Kuskokwim River Valley, which will bring a few inches of snowfall
to locations like Sleetmute and Stony River tonight into tomorrow
morning. By Monday and into Tuesday, cold arctic air will spread
across all of Southwest Alaska and even down the AKPEN. Most
locations will not even make it above 0F Monday and Tuesday with
lows dropping into the negative teens and twenties.

&&

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

Cold northerly flow will dominate the eastern two-thirds of the
Bering and Aleutians as ridging sets-up to the west and an upper
low shifts to the east over the Southwest mainland. This will
allow for cold arctic air to spill south across the Bering Strait
and south to the Aleutians. Strong winds will also kick up as the
pressure gradient tightens, with persistent 25-35 mph northerly
winds developing over the Eastern Bering by Monday. Strong cold
air advection with relatively warm ocean temps will lead to the
formation of widespread snow showers over the Bering from the
Pribilofs east and to the south over the northern side of the
Aleutians. Snow is expected to impact Aleutian communities from
Dutch Harbor east to Port Heiden through Wednesday morning.
Amounts don't look to be too significant at this time but there
could be localized heavier accumulations if training of individual
snow bands sets up. Blowing snow will also be a problem with the
aforementioned strong winds over the northern Aleutian coastline.
By Tuesday, the next Pacific system approaches the from the west
sweeping a strong gale force front across the Western Bering and
Aleutians.

&&

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

The forecast period commences with the longwave trough extending
from western Alaska to the Gulf Of Alaska and all the way to the
Hawaiian Islands. Upstream, there is a high amplitude ridge that
spans from NE Russia to the Central Bering to the North Pacific.
Meanwhile at the surface, there are the remnants of a cyclone
over Prince William Sound and a mature low near Kamchatka with an
occlusion over the Western Aleutians. As the forecast period
elapses, the low near Southcentral will continue to decay and the
western storm will develop a triple point low and advance
eastward. Expecting another round of gusty winds and rough seas
with the Aleutian low. By 12z Friday, the models start to
significantly diverge at which time the confidence level in the
forecast package radically decreases.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory: 121 131.
MARINE...Gale Warning: 119 120 130-132 150 176.
Heavy Freezing Spray Warning: 160 180-185.

&&

SYNOPSIS AND MODEL DISCUSSION...JPW
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KVP
LONG TERM...PS

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
857 AM AKST Sun Dec 16 2018

AKZ181-187-170900-
Alaska Peninsula-Central Aleutians-
Including the cities of Cold Bay, Sand Point, and Atka and Adak
857 AM AKST Sun Dec 16 2018

...Veniaminof and Cleveland Volcanoes Active...

Veniaminof Volcano, located 22 miles northwest of Perryville is
currently active. Indications are that occasional low level
emissions of ash are continuing this morning. The current
northerly transport winds will carry any ash to the south which
may bring a trace to a dusting of ash at Perryville through the
evening.

Cleveland Volcano, located 39 miles west of Nikolski is currently
active. Indications are that occasional low level emissions of
ash are continuing this morning. The current easterly transport
winds will carry any ash to the west which should keep it away
from any population centers.

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
1156 AM AKST Sun Dec 16 2018

AKZ181-185-187-170900-
Alaska Peninsula-Eastern Aleutians-Central Aleutians-
Including the cities of Cold Bay, Sand Point, Unalaska, Nikolski,
and Atka and Adak
1156 AM AKST Sun Dec 16 2018

...Veniaminof and Cleveland Volcanoes Active...

Veniaminof Volcano, located 22 miles northwest of Perryville is
currently active. Indications are that occasional low level
emissions of ash are continuing this morning. The current
northerly transport winds will carry any ash to the south which
may bring a trace to a dusting of ash at Perryville through the
evening.

Cleveland Volcano, located 39 miles west of Nikolski is currently
active. Indications are that a low level eruption occurred earlier
this morning. The current transport is expected to carry any ash
just to the north of Nikolski and Unalaska. Any ash in this cloud
is rapidly dissipating and ashfall is expected to be minimal and
north of any population centers.