Small Craft Advisory 910, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...UPDATED
National Weather Service Anchorage AK
831 PM AKST Thu Nov 26 2020

.ANALYSIS AND UPPER LEVELS...
A deep trough, anchored by a closed low over far eastern Russia,
is firmly entrenched over the entire Alaska region. A strong
zonal jet over the Pacific combined with a train of short-wave
troughs and abundant Arctic air is leading to a plethora of strong
and impactful storm systems.

Looking at satellite imagery there are four storms of note. A
potent short-wave and deepening surface low over the western Gulf
are tracking northward toward the Kenai Peninsula, with snow
overspreading the Kenai Pen and Prince William Sound and small
craft to gale force winds near the fairly compact low center. A
second short-wave and surface low over the North Pacific is
tracking northeastward into the southern Gulf, with small craft to
gale force winds accompanying it. Out west, there are two lows
approaching the Aleutians, a small compact one just west of Attu
and another larger low south of the chain tracking toward Adak.
While there is enough upper level forcing to cause both lows to
deepen, the low approaching Adak has much stronger baroclinicity
and is in a more favorable position ahead of a digging short-wave.
Conditions along the central to western Aleutians are rapidly
deteriorating as these storms approach, with rain, snow, and gale
to storm force winds.

In between the two sets of lows, a broad trough with numerous
embedded short-waves is producing widespread showers across the
Bering Sea region. Southwest Alaska is mostly dry, with some
some showers headed inland off the Bering Sea.

&&

.MODEL DISCUSSION...
Models have struggled mightily with track of the Gulf low. It
shifted from Prince William Sound in Wednesday morning's solutions
all the way west to the western Kenai Peninsula in this morning's
solutions. This necessitated a significant update to the wind and
snow forecast this morning. However, now that the low is well
formed, the forecast for Southcentral for tonight into Friday has
stabilized. Consistency with the track of the low headed into the
southern Gulf has also been quite poor. Prefer the NAM/GFS
solutions with a more consistent southerly track over the ECMWF
which lifts the low to the northeastern Gulf. Lastly, a trailing
short-wave results in interaction between the southern Gulf low
and the low headed for the Kenai Peninsula, with moisture
advection northward toward Prince William Sound. As the trough
approaches, expect a fairly narrow band of heavy precipitation
extending from the central Gulf northward to eastern Prince
William Sound. The challenge is figuring out where that band will
set up, because it has potential to produce a couple feet of snow.
Right now, it looks like it will be centered over eastern Prince
William Sound.

Out west, models continue to be grouped together with the two lows
along the Aleutians. However, small shifts from run-to-run and
differences among the models make it challenging to determine
where the strongest core of Hurricane Force winds will set up.
A High Wind warning is in effect for the Central Aleutians,
including Adak and Atka, but there is some question about whether
the strongest winds will hit there are track just south of the
islands. Minor differences in model low tracks persist all the way
through Saturday, though looking simply at the large scale there
is general agreement. In this very dynamic pattern, expect smaller
scale features will develop and make the difference in the local
forecast. Thus, overall forecast confidence is average to below
average.

&&

.AVIATION...
PANC...6Z TAF Update...There have been significant near term
model changes this afternoon/evening that have resulted in
increased snow chances for Anchorage later tonight into early
tomorrow. It now appears a convergence zone will set up along or
just to the west of Anchorage starting around 12Z tonight that
will bring increasing chances for IFR snows. If this band sets up
directly overhead there is a risk of some LIFR periods between 12z
and 20Z. Confidence on this time period is low with models not
settled on exactly where this heaviest band sets up just yet.
Conditions should improve after 20Z as this convergence zone moves
east of the area.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3:
Tonight through Sunday)...

Two separate lows will at least bring mixed rain and snow through
the weekend. The first low lies to the northwest of Kodiak this
evening, which will trek northward along the Cook Inlet then
moving inland tonight and dissipating. The amount of snow and rain
from this system will vary across the region through Friday, with
the most snow accumulation up to 10 to 14 inches near higher
elevations over the western Prince William Sound (PWS) tonight.
The next developing storm from the Bering will bring another round
of snow on Saturday over Kenai Peninsula and much of Southcentral
Alaska between Saturday and Sunday. At Kodiak Island, scattered to
widespread rain is expected Friday night through Saturday night,
including snow showers.

Cold air advection from the north combined with more cold air
from behind the first low in conjunction with strong winds will
generate freezing spray over the Cook Inlet and Turnagain Arm
tonight through Friday morning. These same spots will see another
episode of freezing spray as the next major storm and its frontal
boundary enters the southwestern Gulf Friday and Friday night.
Overall, an active mixed snow/rain pattern and associated strong
winds along the frontal boundary and lows will continue for the
next couple days.

&&

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

Snow showers across Southwest Alaska will continue to taper off
this evening into Friday morning. Chances for snow increase Friday
afternoon for the Alaska Peninsula and Bristol Bay as a low
pressure system moves eastward staying below the Alaska Peninsula.
Cold temperatures over the next few days will keep snow as the
main precipitation type across the mainland, with a few inches of
snowfall possible across the Bristol Bay region. By Saturday
afternoon, snow showers across the area will start to diminish as
the aforementioned low pressure systems continues into the Gulf.

&&

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

A strengthening low just south of Adak will continue to move along
the Aleutian Chain, bringing a swatch of hurricane force winds. A
High Wind Warning remains in effect for the central Aleutians,
including Adak and Atka, which could see strong winds and gusts
associated with this system. However, today's model runs have
shifted the system farther south, keeping the strongest core of
hurricane force winds south of the Chain. High seas and
widespread precipitation will also accompany this system as it
works its way eastward moving south of the Alaska Peninsula
through Saturday morning. A secondary storm force low develops
behind this first system over the western Bering on Friday, which
will then track over the central Aleutians and weaken by Saturday
afternoon.

As one systems exits eastward into the Gulf on Sunday, another
strong low pressure system approaches the western Aleutians and
Bering by Sunday morning. Models are in good agreement with
placement and strength of this next system, which will likely
bring another storm force front to the forecast area, with the low
center moving south of Adak by Sunday afternoon.

&&

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

Gulf: The active pattern continues with high winds and waves
following back to back storms into the Gulf. A gale force low
will move into the Gulf Saturday afternoon, expected to dissipate
throughout the day on Sunday. Monday morning, a front associated
with a low south of the Chain will enter the western Gulf. This
system could bring storm force winds as it propagates towards the
northern Gulf Coast with a barrier jet looking to set up Monday
evening. After this, strong southerly flow will overtake the Gulf
Tuesday morning, expected to dissipate throughout the day.

Bering: A large and strong area of low pressure looks to skirt the
Chain beginning on Sunday morning bringing with it the potential
for storm force winds and waves. Expect high winds and waves into
the central Bering between Sunday morning and Tuesday afternoon.
Tuesday evening, most of the high winds and waves look to remain
confined to the eastern Aleutians and Bering which should begin
to dissipate through Wednesday. After this, there looks to be a
short break in the active pattern through Thursday with calm winds
and seas.

&&

.LONG TERM FORECAST (Days 4 through 7/Monday through
Thursday)...

The active weather pattern continues throughout the long term.
Precipitation will continue for the northern Gulf Coast with snow
for the higher elevations on Monday, as an area of low pressure
continues to weaken as it moves further inland. Temperatures look
to remain warm with this feature, with rain expected for the lower
elevations and snow for the highest elevations. Right behind this
is a front associated with a low in the North Pacific, bringing
another round of precipitation to Kodiak Island and then to the
coastal areas Monday afternoon, along with an increase in winds.
Expect precipitation to make its way inland Monday evening with
this front. Temperatures aloft look to remain relatively warm,
with rainfall looking to be the dominant precipitation type with
this system. We look to dry out a bit after this, with
precipitation mostly remaining across the coastal locations and
higher elevations Wednesday. This looks to continue through
Thursday, with an increase in precipitation expected, as another
low is anticipated to move from the southern Gulf into Prince
William Sound. This could switch rain, from the previous low, to
snow as warm air is replaced by cold air.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...
Hurricane Force Wind Warning: 172,174,176
Storm Warning: 155,173,175,177,178,411
Gale Warning:
119,120,130,131,132,136,138,139,140,150,160,165,170,180,351,352,413,414
Heavy Freezing Spray: 180,181
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...SB
SOUTHCENTRAL ALASKA...CB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AH
MARINE/LONG TERM...KM/CK