Small Craft Advisory 80, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
425 PM AKST Wed Jan 22 2020

.ANALYSIS AND UPPER LEVELS...
A deep vertically stacked low is passing south of the western
Aleutian islands, with an occluded front lifting northward toward
the Aleutians. This is leading to widespread gales west of Adak.
Light precipitation is also overspreading the area. Temperatures
are cold enough to support snow initially, but warmer air will
advect northward with the front.

A digging short-wave trough south of this low center is causing
amplification of a downstream ridge. This ridge is building
northward across the heart of the Bering Sea. Over the next couple
days this high amplitude pattern will lead to a change in weather
over the southern mainland, with cold air advection and resultant
winds and heavy freezing spray (over marine areas).

Currently over the southern mainland, a broad and weak upper
level short-wave trough is tracking eastward across Southwest
Alaska producing a small area of light snow ahead of it.
Otherwise, the region is cloud free in the mid to upper levels.
With steep surface-based temperature inversions in place, this
has led to formation of areas of fog, particularly in Bristol Bay
where a few stations have been reporting dense fog (1/4 mile or
less visibility) this morning.

Weak cyclonic flow out ahead of the trough is also producing a
few snow showers from eastern Prince William Sound up into the
Copper River Basin. A large area of low clouds extends from the
Susitna Valley southward to the Mat Valley, Anchorage, Cook Inlet
and the western Kenai Peninsula. With very weak low level flow
don't expect these clouds to go anywhere quickly. Lastly, there
are some localized gap winds along the Gulf coast due to tight
pressure gradients between a low approaching southeast Alaska and
high pressure northern to western Alaska.

&&

.MODEL DISCUSSION...
Models are in good agreement with large scale features over the
next few days and overall forecast confidence is above average.
Some of the challenges will be in the details of temperatures,
wind chills, and wind speeds as cold air advects across the
southern mainland the next 24 to 36 hours.

&&

.AVIATION...
PANC...MVFR ceilings, which are firmly entrenched across the Cook
Inlet region, will hold until we get enough wind to erode the
clouds. There could also be some period of flurries or light snow.
If visibility is reduced, it should be of a short duration. Low
level northerly winds will strengthen overnight tonight through
the day Thursday, which will advect in drier air and erode the
low level clouds. Best estimate to go VFR at the terminal is
sometime Thursday morning.

&&

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

Tonight through Thursday morning a few snow showers will linger
across the Copper River Basin with flurries elsewhere as an upper
level trough closes off a new low center over the western Gulf.
This upper level trough also pulls a cold air mass across the
southern mainland bringing colder trends through the end of the
week. Gusty offshore flow increases tonight across channeled
terrain and along the western Gulf, including Kodiak Island.
Gusty winds are expected to persist for Seward, Portage Valley
and Whittier over the next several days as the upper level low
becomes stationary over the northern Gulf. Northerly flow will
also increase for Broad Pass and Matanuska Valley as the pressure
gradient becomes favorable Thursday afternoon. Wind across
Thompson Pass and the Copper River Delta slowly increases Thursday
into Friday, with peak wind flow anticipated on Saturday. Winds
for these locations are highly dependent on the location of the
upper level low; so only minor adjustments were made as there is
still some uncertainty on the low position.

&&

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

Significant amplification of the long wave pattern over Alaska
over the next 24 to 36 hours is expected to cause another Arctic
outbreak of winter cold to move out of Siberia and into Southwest
Alaska. This cold air outbreak is already about to reach the
northern portions of the region, and will take hold across the
rest of Southwest Alaska tonight. Due to lingering cloud cover,
temperatures will only be marginally colder tonight than they were
last night, and it will only really be felt in most areas on
Thursday when skies gradually clear through the day. This will
keep temperatures from rising much during the day Thursday.
Thursday night is when the bottom will drop out, with widespread
lows between 20 below and 30 below across northern areas, and lows
between 10 below and 15 below into Bristol Bay. In addition to
the cold temperatures, strengthening high pressure over the
eastern Bering along with a low in the Gulf will result in
strengthening northerly winds across the Kuskokwim Delta and
Bristol Bay. Cold temperatures and winds combined will cause
another round of wind chills dropping to between 40 below and 50
below, especially along the coast of the Kuskokwim Delta and in a
corridor between Dillingham and Iliamna. Wind Chill Advisories
have been issued for these cold wind chills.

This pattern is expected to persist right through the weekend, as
the highly amplified and otherwise stagnant and unchanging upper
level pattern continues to support the strong high pressure off
the coast with a low in the Gulf. Thus, the very cold temperatures
and winds will also continue through the weekend.

&&

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

The Bering Sea will generally be split by a very persistent and
strong area of high pressure over the eastern part of the basin,
and a strong low south of the western Aleutians causing strong
east to southeasterly winds, occasionally reaching storm force,
and steady rain and snow showers. Under the high over the eastern
Bering, the primary weather story will be strong northeasterly
winds from off the mainland moving over the relatively cold
waters, causing widespread areas of heavy freezing spray,
especially by Thursday night over nearly the entire eastern
Bering. The strong winds will also be stronger as they are
funneled through bays and passes as well.

Across the central Aleutians and western Bering, strong easterly
winds on the north side of a strong North Pacific low are expected
to persist into the day Saturday, but they should gradually shift
northward with time. Thus, the strongest winds should be north of
the chain by Friday evening. The influx of colder air from further
east along with the mixing of warmer Pacific air around the low
will cause temperatures to remain around or slightly above
freezing, and as such, precipitation type issues are likely.

&&

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

Gulf of Alaska:

Weak low pressure will persist over the Gulf of Alaska Friday
through Monday and then a second weak low enters the GOA Tuesday
and Wednesday. A strong thermal gradient has set up where brutally
cold temperatures reside over land and mild temperature are over
the Gulf Of Alaska. The expectation is that enhanced offshore
winds and gap winds will develop resulting in areas of heavy to
light freezing spray and enhanced sea states. The areas of concern
at this time are the Copper River Delta along with Kamishak Bay
through the Barren Islands.

Bering Sea and Aleutians:

A strong low centered south of the western Aleutians will keep
widespread gales with occasional embedded storms ongoing across
the central and western Bering Friday. The Gales will likely be
over the western bering Sea Saturday and Sunday. Seas of 20 to 30
ft are expected over a large area of the central and western
Bering and Aleutians Friday and Saturday. Heavy freezing spray is
expected Saturday north and east of the Pribilof Islands.

&&

.LONG TERM FORECAST (Days 4 through 7: Friday through Tuesday)...

The big picture will be low pressure persisting over the Gulf of
Alaska and south of the Aleutians with strong high pressure over
the eastern Bering Sea and fierce cold air advection over the
mainland. For the mainland this translates to below normal
temperatures this forecast period. The Climate Prediction Center
which is part of the National Weather Service anticipates that
these below normal temperatures will persist into the 8-14 Day
Outlook. This pattern will result in enhanced offshore flow and
gap winds. Models are in good agreement with the longwave pattern
over the mainland, Gulf Of Alaska and Bering through the forecast
period.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Wind Chill Advisory 155 161.
MARINE...Gale 121 127 129 138 150 170 172-175 178 185
		 Storm 175 176 177.
Heavy Freezing Spray 121 127 129 130 132 138 139 150 160 165 179
180 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...KH
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JW
MARINE/LONG TERM...PJS