Small Craft Advisory 4, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
636 PM AKST Sat Jan 1 2022

.ANALYSIS AND UPPER LEVELS...

A hybrid bora-gap wind event is well underway across southern
Alaska this morning, with widespread strong gusty winds and
falling temperatures and wind chills to start the new year. The
supporting pattern is a high amplitude ridge over the Bering Sea
and corresponding high amplitude trough from the eastern mainland
of Alaska southward across the Gulf of Alaska and into the
northeast Pacific. An upper level jet streak between the two is
oriented north to south along the west coast of mainland Alaska
with estimated speeds of 140 kts+. A short-wave trough is digging
southeastward across the western Gulf with axis of strongest low
level cold air advection from interior Southwest Alaska southward
across the Alaska Peninsula into the western Gulf. A surface low
over the northern Gulf is exiting eastward with a secondary of
cold air advection across Southcentral and into the northern Gulf.

Surface pressure gradients are extremely tight across the region,
in between a 975 mb low in the Gulf and a 1046 mb (or higher)
high over the northern Bering Sea and on up into the Arctic. The
very cold and dense air mass (-30 degrees Celsius or colder at 850
mb inland) is advecting across southern Alaska, accelerating
winds as it moves over mountain ranges and down the other side.
Winds are particularly strong through and near mountain passes
and bays along the coast as this cold dense air is pushed through
the gaps and aided by the very large pressure gradients. With
fresh snow in place from the past couple days, there are areas of
blowing snow and reduced visibilities. One of the worst areas is
the Glenn Highway near Paxson in the Copper River Basin, where
blizzard conditions are being observed. Wind chill readings have
dropped to -20 to -30 degrees across much of Southwest Alaska and
interior Southcentral, and continue to fall as the colder air
moves in and air temperatures drop. Heavy freezing spray is
developing across the Gulf of Alaska and Southwest Alaska coastal
waters in response to the strong winds and very cold air. Lastly,
ash from the 1912 Katmai eruption is being resuspended and
transported across Shelikof Strait to Kodiak Island. Webcams along
the west side of Kodiak Island show some very hazy conditions.
While much of the ash will remain elevated in the atmosphere the
strong gusty winds could also drive some of the ash down to the
surface.

Weather is quiet over the central Bering Sea and Aleutians
underneath the ridge. To the west of the ridge, a warm front is
draped along the far western Aleutians, producing gale to storm
force winds and rain.

&&

.MODEL DISCUSSION...
Models remain in very good agreement with the large scale pattern
over the next few days. The forecast challenge will be the many
details of wind and wind chills associated with the ongoing bora
event.

&&

.AVIATION...
PANC...Strong northerly winds will persist all the way through
Sunday evening. While there will likely be some fluctuations in
the strength of the wind, the strongest winds look to be during
the day Sunday as the core of winds coming out of the Matanuska
Valley tracks southward down Knik Arm and to the Anchorage
terminal.

&&

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

Low pressure over the Gulf and high pressure over the Bering is
pulling cold arctic air into the area with cold temperatures and
strong winds the story through the rest of the weekend.

A high wind warning is in effect for the MAT valley with Palmer
and the Glenn highway near the Knik river getting winds gusts up
to 60 mph. These winds will slowly make their way into Wasilla
tonight and then bending into northern and western parts of
Anchorage Sunday morning. This will push the high winds into the
northern and western parts of Anchorage where a high wind warning
will be in effect Sunday morning through Sunday evening.
Otherwise strong winds and cold temperatures will create wind
chills of 40 to 50 below for Broad Pass, Thompson Pass as well as
northern parts of the Copper River Basin through Monday morning.
Other inland valleys will experience wind chills of 20 to 30 below
zero through Monday morning.

Ocean effect snow due to the cold air blowing over the warmer
ocean waters will create bands of snow which are expected to move
over portions of Kodiak Island. These bands of snow along with
strong winds will create blizzard conditions across parts of
Kodiak Island tonight and tomorrow morning due to snow and blowing
snow.These bands will eventually shift west over the Shelikof
Strait and weaken by Monday.

On Tuesday the upper high will begin to move over the Alaskan
Interior creating a Rex Block over the region this will continue
to keep Southcentral cold, however winds will decrease late Monday
and Tuesday. The only other significant weather to speak of on
Monday and Tuesday is shortwaves rotating east to west across the
southern Gulf and towards Kodiak Island with chances of snow.

Widespread heavy freezing spray will continue in most of the
coastal marine areas. The worst region will be from lower Cook
Inlet through Shelikof Strait and to the Shumagin Islands. Parts
of this region will likely see not only heavy freezing spray, but
extreme rates of freezing spray with potential for 2 to 4 inches
per hour of ice accumulation in the strongest core of the winds.

&&

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

An upper low over the Northern Gulf is beginning to move
southeast and an elongated trough will stretch across the Alaska
Peninsula as it dips south from the Alaska interior. A cold,
arctic air mass over the entire southwest portion of the state
will continue to suppress temperatures over the through Tuesday
morning. Coupled with strong northerly flow and a tightened
pressure gradient, cold gusty winds will cause dangerously cold
wind chills around 50 below from the Kuskokwim Valley through the
interior Bristol Bay to King Salmon. Areas in the Kuskokwim Delta
will also experience cold wind chills in the 30s below zero
tonight and Sunday night. Iliamna, New Stuyahok, and King Salmon,
but not quite as far west as Dillingham, are especially gusty
today (to roughly 50 kts) coinciding with the upper level jet
support and the highest winds may stick around through at least
part of Sunday. Many locations are reporting blowing snow.

&&

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

A high amplitude ridge and an omega block across the Bering and
Aleutians has kept a deep upper trough anchored over the Alaska
Peninsula and Gulf and a North Pacific trough from progressing too
far east along the Aleutian chain. The most impactful weather
will be centered on the western and eastern side of the ridge for
the next couple days, with dramatically contrasting air masses
between the upstream and downstream portion of the ridging aloft.
The eastern Aleutians, on the cold side and near the upper level
trough, very strong, northerly flow coupled with Arctic air
advecting south will support extremely strong winds through much
of Sunday, with Hurricane force gusts as high as 80 kt possible
out of bays and passes through Sunday morning. Winds will weaken
as upper level flow veers from northerly to northeasterly, but
will remain gale force to storm force through Monday morning. The
winds coupled with cold temperatures dropping into the teens will
result in areas of very heavy freezing spray, with the highest
rates of accretion occurring with the core of highest winds
lasting through Sunday morning.

Meanwhile much warmer frontal system will stall across the far
western Bering as it runs up against the blocking ridge of high
pressure. Steady rain and southeast winds between gale and storm
force will follow along and ahead of the front across the western
Aleutians, with most of the rain staying west of Adak through late
Monday.

&&

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

Bering...Seas will begin to subside to near 20 ft by Tuesday
afternoon. A smaller area of northeasterly gales near the
Kuskokwim Delta and Bristol Bay will shift into the northern
Bering by Tuesday morning and then diminish. Tuesday afternoon a
broad trough with embedded lows over the North Pacific induces
another round of easterly gales from Bristol Bay into the central
Bering through Wednesday. By Wednesday morning, seas begin to
build to 20 ft in the central Bering. Southeasterly gale force
winds will impact the central Aleutians Thursday morning and will
stretch along the central Bering as an area of low pressure moves
eastward just south of the Aleutians.

Gulf of Alaska...Localized areas of gales continuing to linger
beyond Tuesday, particularly near Kamishak Bay and Shelikof
Strait as low pressure exits into the North Pacific. A front moves
north into the Gulf on Tuesday afternoon, allowing widespread
easterly gales to develop in the southern Gulf. Gale force winds
may also develop through the Barren Islands and southern Cook
Inlet and persist through Wednesday. Seas build to near 20 ft by
Wednesday morning in the far southern Gulf. A weak low develops in
the eastern Gulf Thursday which will bring localized small crafts
near the Barren Islands.

&&

.LONG TERM FORECAST (Days 4 through 8: Wednesday through
Saturday)...

There is general model agreement that a strong amplified upper
level ridge sitting over the Bering this weekend shifts northward
into the Chukchi Sea and over the mainland. A large closed upper
low continues to retrograde along the ridges southern periphery
into and across the Bering. Ahead of the low, a blast of cold,
Arctic air, previously sitting over the Alaska mainland, is pushed
across the Bering, creating widespread snow showers and the
potential for freezing spray, especially near the Southwest coast.
Various surface lows look to move northward from the North
Pacific and push a broad front across the southern Gulf, AKPEN,
and the Aleutians, leading to a return of warmer, wetter
conditions there by Wednesday. The bulk of the warm, moist air
looks to remain confined there as the strong ridge to the north
keeps the front stalled out, leaving Southwest and Southcentral
Alaska colder and drier. Weak warm advection on the back side of
the upper low nonetheless allows a bit of a warm-up and lighter
winds over the Alaska mainland through the week.

The GFS and EC both are agreeing that by Thursday, stronger
upper- level ridging extends further south over mainland Alaska.
This will likely keep the area clear and dry. The GEM, despite a
further west placement of the strong upper-level anticyclone,
also indicates the same general northwest to southeast ridge axis.
This is setup is more prone to draw surface lows northward into
the Bering. Therefore, the GFS and GEM are indicating the
potential for a surface low to draw warm, moist air deeper into
the Bering and possibly into Southwest Alaska by the end of the
week. This could cause temperatures to warm to near or above
freezing across parts of Southwest late Friday into Saturday.

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning: 101, 111.
         Blizzard Warning: 141, 171.
Wind Chill Advisory: 131, 141, 145, 152, 161.
MARINE...Hurricane Warning: 130, 150
         Storm Warning: 131-132, 138-139, 155, 176-178, 411
         Gale Warning: 119-121, 125-129, 136-137, 140-141, 160, 165,
         173-175, 180, 351-352, 412-414
         Heavy Freezing Spray Warning: 119-121, 125-132, 136-139, 141,
         150, 155, 160, 165, 180-181, 185, 414
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...SB
SOUTHCENTRAL ALASKA...RC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...VR
MARINE/LONG TERM...NS