Small Craft Advisory 857, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
453 PM AKST Tue Nov 10 2020

.ANALYSIS AND UPPER LEVELS...

An area of low pressure, centered over the southern Bering, has
an associated surface boundary stretched eastward across the
eastern Aleutians. As this system continues to move east, the
front will swing northeast across southern Alaska allowing the
cloud cover and precip to overspread southwest and southcentral
AK. In the upper levels, a 180kt jet, located over the North
Pacific, is rounding the base of the low pressure system over the
southern Aleutians. Meanwhile, the low level jet on the back side
of this storm system looks quite impressive, enhanced by a strong
storm system rounding the base of the trough as well as moderate
cold air advection. Dutch Harbor airport reported a peak wind at
Hurricane force (68 kt) while other observations across the
eastern Aleutians and southern Alaska Peninsula have reached
Storm force (50-55kt)

Temperatures in the Copper River Basin have reached the
teens, with readings mainly in the 20s and 30s in Southcentral.
Temperatures across southwest AK and the Aleutians have climbed
into the 30s and low 40s.

&&

.MODEL DISCUSSION...

Models are in general agreement with the synoptic features through
Friday. The low pressure system over the southern Bering will
continue to move east then pivot northeast until it, in effect,
stalls across western to southcentral AK. This leaves broad low
pressure from western AK to Southcentral from late Wed through the
end of the week.

While the broad synoptic scale features are in fairly good
agreement, there are differences in the details of what happens
under/behind the trough. In particular, models differ on the
transition of the upper center from Southwest Alaska to
Southcentral Alaska Wednesday night, along with location/track of
multiple short-waves rounding the main low center. This results in
low confidence in the precipitation forecast for Southcentral
Wednesday night through Thursday.

&&

.AVIATION...
PANC...Expect significant low level wind shear as an occluded
front over the Gulf lifts northward across the Kenai Peninsula
tonight. Low level flow will rapidly diminish as the front passes
the terminal around 18Z tomorrow morning. This is the point of
greatest uncertainty in the forecast. The weakening flow should
allow some precipitation to reach the ground. Warming above the
surface due to strong chinook winds tonight will likely be enough
for precipitation to initially fall as rain. However, marginally
cooler air will move in behind the front, so depending on
intensity there could be a rain/snow mix or even a brief period
of all snow. In addition, to complicate matter further, southerly
winds could move into the terminal right behind the front.
However, with a new low tracking from Bristol Bay into the
northwest Gulf gradients will switch to favoring a north wind.
Thus, if southerly winds do move in, it should only be for two or
three hours (roughly 18z-21z).

&&

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

Breezy to strong winds with gusts up to near 45 mph have whipped
across the Barren Islands and much of the western Gulf late this
afternoon. A surface trough over the Gulf will be replaced by an
incoming frontal boundary exiting the Bering Sea tonight. A mix
rain/snow is expected along with strong gusts will spread over
much of Kenai Peninsula, Prince William Sound, Anchorage and
Matanuska Valley tonight in response to the trough over the Gulf.
Warm advection south of the approaching front will reduce the
coverage of mixing rain/snow on Wednesday, while more rain will
push through the bays, Prince William Sound coastal areas, and
western Kenai Peninsula. As for Anchorage Bowl and Matanuska
Valley, rain will move in a bit late in the afternoon around the
Anchorage Bowl, while a good chance of mixing rain/snow will pass
through Matanuska Valley on Wednesday. Colder air advection from
the north of Talkeetna Mountains will extend to the east of Palmer
and to the north of Valdez and Thompson Pass Thursday night. At
850 mb, temperatures between 8 to 15 degrees Fahrenheit will
stretch over the north side of Chugach Mountains through Wrangell
Mountains between Friday and Friday night, and the surface
temperatures will trend downward thus reflecting this event.

Over Kodiak, Kamishak Bay and Barren Islands, breezy conditions
will linger through at least Friday afternoon. The aforementioned
frontal boundary will move inland and across Prince William
Sound by Wednesday afternoon, while winds up to 30 to 40 mph will
persist over the Gulf in the next couple days before it starts to
weaken Friday evening.

&&

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

A complex pattern continues today, with a series of lows and
associated fronts moving across the Bering. The first system
approaches the Southwest coast by Wednesday, bringing band of
widespread precipitation and a round of warmer air. Thus,
precipitation will be mostly rain, with snow or a rain/snow mix
likely for the higher elevations. Additionally, gusty winds are
expected across the region into tomorrow, diminishing by late
Thursday evening. Temperatures will increase briefly as the front
pushes through, before dropping again once the second front
approaches. A coastal flood advisory has been issued for tonight
through Wednesday for the northern Bristol Bay coast, as
persistent southerly flow causes surges to grow ~3-6 ft above
normal high tide.

A second front pushes into the Northwest Bering on Friday and
pushes towards the Southwest Coast. Models are still in
disagreement on the timing of the formation of the surface low,
with the GFS being the quickest. However, all models show a low
forming at some point as the trough approaches the Northern
Bering. Significant cold air advection is behind the front,
causing decrease in temperatures over the Southwest Mainland
enough to allow for snow or other freezing precipitation.

&&

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

A strong low continues to track east across the Bering through
tomorrow, pushing onshore by tomorrow evening. The biggest impacts
with this will be winds, with a period of high end storms expected
tonight in the Central Bering south of the Pribilof Islands.
Elsewhere, widespread gales will be prominent, especially along
the Aleutian Chain. Northwesterly flow will be dominant behind the
front and will persist over the next few days. Thus, expect an
inflow of colder air into the Northern Bering.

A second low approaches the Northwest Bering by Friday. Model
agreement is low on when the surface low forms, but all show the
formation of a low at some point on Friday. Significant cold air
advection follows this front, thus, freezing precipitation and
freezing spray may become impactful as sea surface temperatures
begin to decrease.

&&

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

Gulf of Alaska:

A Northern Gulf low drifts eastward through Sun. While track and
development of the low varies, easterly small craft winds linger
along coastal waters through Fri.  Confidence is good for
widespread westerly high- end gale force winds and waves from the
Alaska Peninsula across the Gulf to diminish on Sat. A new front
crosses the Alaska Peninsula Sat with northwesterly small craft
and local gale force winds near channeled areas entering the
Western Gulf Sun.

Bering Sea:

A polar front sweeps south over the Bering through Sat.
Confidence is good for widespread small craft winds and waves over
the Bering with local gale force winds with a St Matthew Island
low Fri that crosses the Eastern Aleutians Sat, and all parts
diminishing through Sun. A well-developed front approaches the
Western Aleutians and Bering Sat. Increasing confidence for
widespread southerly high-end gale force winds developing across
the Western Aleutians and Bering through Sun.

&&

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

On Saturday the primary feature will be a highly amplified upper ridge
developing across eastern Siberia into the far western Bering.
Ahead of this ridge, strong northerly flow and cold advection
looks to set up across the Bering, Aleutians and western coastal
areas. Expect this northerly flow to bring widespread snow showers
across the Bering/Aleutians and western coastal areas along with
gusty winds, especially through the gaps in the Aleutians. On
Sunday, this northerly flow will weaken as a storm system
approaches the far western Aleutians. This storm will bring a
return to southerly winds and warmer temperatures for the Western
Aleutians while another cold system starts to approach the Bering
Strait region.

Fairly quiet weather will be found across the mainland through
the weekend outside of a few snow showers along the Northern Gulf
Coast. Offshore winds could be a bit breezy in some of the local
gaps across Southcentral. Temperatures could fall slightly below
normal (Anchorage-High 27F, Low 15F) for mid- November with clear
skies.

For Monday and Tuesday, high pressure looks to park over mainland
Alaska, with a broad/weak low in the Gulf. This should result in
mostly dry weather, with the main risks being for occasional
strong winds along the Northern Gulf Coast due to a cold airmass
remaining in the Copper River Basin and a continued moderate
pressure gradient along the Northern Gulf Coast.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning: 150 155 165 170-174 176 177 179 412-414.
Gale Warning: 119 120 125 130-132 138 160 175 178 180 181 185 351
352.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...NR/SB
SOUTHCENTRAL ALASKA...CB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KM
MARINE/LONG TERM...MK/BG