High Wind Watch 1, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
540 AM AKST Fri Dec 31 2021

.ANALYSIS AND UPPER LEVELS...

In the upper levels, a broad trough with an embedded Arctic low
centered over the Beaufort Sea extends across the the Brooks
Range. A high amplitude ridge extends from the North Pacific to
the southeastern Gulf coast through the Yukon Territory. To the
west of the upper ridge, a shortwave trough and associated strong
surface low situated underneath the left exit region of a 130+ jet
streak is crossing south of the eastern Bering Sea near the
Alaska Peninsula.

At the surface, high end gale force winds can be seen on the
ASCAT passes near Saint Matthew Island and extending south of the
Aleutian Chain in response to a strong thermal gradient as polar
air wraps around the low. Latest observations are reporting rain
across the central Aleutians, with snow showers north of the
Pribilofs. A warm front extending east of this low has occluded
and exited Southwest AK, leaving cooling air aloft in its wake.
This helping to transition precipitation type to all snow showers
across the Y-K Delta, Lower Kuskokwim Valley, and Bristol Bay
regions.

For Southcentral, this front is moving east of Kodiak Island that
provided some rain over the region. Additionally, a triple point
low is developing along the frontal boundary near the southern
coast of the Kenai Peninsula. Based on the CIMSS MIMIC imagery, a
deep fetch of moisture of 0.5-1.00 inch of precipitable water
extends northeastward to southern Alaska. A combination of strong
dynamic support, moist environment, and colder air over the
interior is bringing snow for much across Southcentral.

&&

.MODEL DISCUSSION...

The latest operational model runs are in good agreement with the
placement and intensity of synoptic scale features at both the
surface and in the upper levels. For Southcentral, there are some
model differences with the placement of the Gulf low on Saturday
as it nears Prince William Sound. Also, the exact precipitation
timing and storm total amounts are still being ironed out with the
current forecast package, but overall remains on track for a long
duration light snow event to continue through tonight. As we head
into the weekend, an Arctic outbreak of cold air will spill
across the Alaska mainland, leading to gusty winds for the bays
and passes, heavy freezing spray, as well as cold wind chills.

&&

.AVIATION...

PANC...Light steady snow will continue for much of today,
bringing MVFR to IFR reductions to ceiling and visibility at the
terminal. Conditions will improve to VFR as snow gradually
diminishes this evening with gusty winds out of the north
starting tonight.

&&

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

We are in the midst of a pattern change over Southcentral Alaska
today. A warm front aloft passed over the region Thursday evening
through the overnight hours and brought some snow. A second batch
of snow is moving up Cook Inlet early this morning. This snow is
not associated with that warm front but with an upper level low
moving in from the west that is phasing with the main upper level
low that is in the Interior of the state. This combination is also
ushering in colder air into Southcentral. So the weather story for
today into tonight will be the snow with a general 2 to 5 inches
over most of Southcentral for this event.

As the cold upper level low begins to really dig down over
Southcentral Alaska late tonight into Saturday, it will do a
number of things. It will push the snow eastward and bring in
colder and drier air into the area. There will be a tight pressure
gradient that develops over the area and this will really start
to kick up the winds Saturday into Sunday. Strongest winds will be
in Thompson Pass and perhaps making their way into the town of
Valdez as well. Kodiak will also see strong winds with gusts 60 to
65 mph likely through channeled terrain. Basically all the
northerly gaps will see strong winds develop by late tonight and
persist into Sunday.

While the typical areas that are susceptible to northerly winds
will be most affected, however, these winds will likely mix down
over much of the area and not just through the gaps. This presents
a challenge with the temperature and wind chill forecast for
Saturday night in particular. For the most part there should be
winds and good mixing in the low levels which will keep an
inversion from developing in most areas and keep temperatures
higher. However, the temperatures aloft will have dropped
significantly over the two day period and may just make it cold
and windy with very low wind chills. Coldest wind chills Saturday
night should be in Thompson Pass and Broad Pass, but more
significant wind chills could also develop across the Copper River
Basin as well as along Cook Inlet.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Day 1 and 2)...

Early this morning the Bethel radar was active with the remnants
of an occluded front. The surface low will dive southeastward
today towards Kodiak. The steering flow will change directions and
a brutally cold and dry air will filter into the region. By
Saturday, interior locations will struggle to reach 0F and Bethel
and Aniak will see high temperatures between -7F to -10. Wind
chills could get as bad as 40 degrees below zero Saturday night.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Day 1 and 2)...

The remnants of an occluded front from the central Bering and into
Southwest Alaska are visible this morning. Also discernible is
the tropical fetch associated with it which extends from the
central Pacific west of Hawaii. Meanwhile, there is another front
to the west of Attu Island (which also has a tropical fetch from
the West Pacific) and a ridge separating the 2 systems. The models
have this ridge amplifying this forecast period which will impede
the eastward movement of the Kamchatka low. The models show the
jet core pushing northward and remaining west of Attu. However, a
sliver of the precipitation will move over Attu and Shemya this
weekend but the bulk of the moisture will be steered over the
western Bering and NE Russia. Cold air advection over Southwest
Alaska combined with the position of the ridge will result in
strong gap winds the eastern Aleutians and Alaska Peninsula late
Friday/early Saturday morning. Heavy freezing spray will also
become increasingly widespread over the northern and eastern
Bering and south of the Alaska Peninsula.

&&

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

Bering...Enhanced northerly winds across the Eastern Bering
should be coming to an end on Sunday, and with it the threat for
heavy freezing spray, as a tightened pressure gradient shifts
inland of the coast and into the Gulf. Meanwhile, a highly
amplified ridge will be lifting northward over the Bering along
with strong southerly flow over the western Bering. This should
promote small craft conditions over the eastern Bering while
widespread gales develop over the western Bering Sunday into
Monday, including the Western and Central Aleutians. A very cold
air mass will remain in place over interior Alaska and the
advection of cold air and associated outflow winds is forecast to
persist across the Kuskokwim Delta and Bristol Bay coastal areas.
By Tuesday high pressure over the Bering will lift further north
allowing a trough over interior Alaska to retrograde and amplify
westward into the southern Bering. Guidance suggests a pocket of
intense cold air advection may accompany this trough,
overspreading Bristol Bay and Alaska Peninsula by midday Tuesday.
Intense northeasterly winds will become an increasing concern
along with a returning threat for heavy freezing spray for the
eastern Bering.

Gulf of Alaska...The main features over the Gulf of Alaska during
the 3 to 5 day period will concern a large trough of low pressure
centered over the southern Gulf and an arctic air mass inland of
the Gulf Coast. Weather put into effect by these features will
include snow showers scattered across the Gulf as well as the
continued presence of strong outflow winds spilling into the Gulf.
Gale to storm force northerly winds will likely exist across all
of the gaps, bays and passes on Sunday with a high threat for
freezing spray also existing. Very rough seas to potentially high
seas in some locations of 15 to 25 feet are forecast for much of
the Gulf Sunday into early Monday. Low pressure will begin
retreating southeastward and into the North Pacific by early next
week helping to slow down previously strong cold air advection
into the Gulf. This should allow for winds and seas to finally
begin diminishing Monday into Tuesday.

&&

.LONG TERM FORECAST (Days 4 through 8: Monday through
Thursday)...

A ridge of high pressure is forecast over the eastern Bering early
next week. This ridge will separate northerly winds and a cold
arctic air mass to the east of the ridge from southerly flow over
the Aleutians and strong warm air advection into the northern
Bering to the west of the ridge. Conditions across Southwest and
Southcentral are expected to remain cold and dry with little in
the way of any precipitation. Through the latter half of the week
high pressure over the Bering is forecast to slowly lift further
north, eventually breaking down as a shortwave trough moves from
interior Alaska into the Alaska Peninsula. Cold temperatures
previously kept inland by the ridge are forecast to sweep westward
across the Bering with previously warm temperatures over the
Aleutians to advect northward with the high. As a result, gusty
northeasterly winds are likely to occur with the cold air
advection along with snow showers and a threat of heavy freezing
spray.

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Hurricane Warning: 150
Storm Warning: 130-132 138 155 178
Gale Warning: 120 121 125 127 128 136 137 140 160 165 170-177 179-181
185 352 411-414.
Heavy Freezing Spray Warning: 130-132 136-139 141 150 160 165 185 414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...MF
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PJS
MARINE/LONG TERM...BL