Heavy Freezing Spray Warning 161, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
528 AM AKST Sat Nov 27 2021

.ANALYSIS AND UPPER LEVELS...

Teleconnections continue to support a very cold weather pattern
across the Alaskan mainland by supporting for deep upper level
troughing across the Alaskan mainland with upper level ridging
over the Bering/Aleutians. An upper level low inside of the
aforementioned upper trough is currently over the interior Alaskan
mainland and remaining nearly stationary. A second low is
currently developing near 50N 177E and is moving northward. A
second low is developing near 44N 157W and is moving
northeastward. The jetstream remains south of 50N until it reaches
southern British Columbia.

Closer to the surface, convective snow showers continue to move
from the northern Gulf into Prince William Sound along a meso-low
located just southeast of Seward. Observations show strong cold
air advection across Southcentral as northwesterly winds and cold
temperatures continue across the region.

&&

.MODEL DISCUSSION...

Guidance is showing significant differences with respect to the
placement and intensity of synoptic scale features at both the
surface and in the upper levels. Guidance also shows little to no
consistency between model runs. The ECMWF is the only model that
has some degree of run to run consistency and therefore it was
used in this forecast package. Models also continue to struggle at
resolving the placement of convective snow showers in Prince
William Sound. Overall forecast confidence remains low.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 to 3...Saturday
through Monday)...

As has been the case for the past several days, broad cyclonic
flow and cold air remain over much of Southcentral due to a wide
upper level trough stretching across the northern Gulf and a
surface low located near Montague Island. Northwesterly flow
persists through Kamishak Bay, southern Cook Inlet, the Barren
Islands, and Shelikof Strait, continuing the Gale and Heavy
Freezing Spray Warnings for those areas through tonight.
Intermittent snow showers persist for Prince William Sound while
quasi-linear snow bands remain persistent along Cook Inlet and
south of Homer. A tight temperature gradient remains along the
Alaska Range and continues to produce strong westerly outflow
through Whittier and Seward. Due to the gusty winds and light,
powdery snow, a Winter Weather Advisory for blowing snow remains
in effect for Whittier. Later today, a shortwave passes over the
northern Gulf and creates the potential for more snow for portions
of Prince William Sound and Kachemak Bay through the weekend. It
does appear that the winds will align from the west-southwest over
southern Cook Inlet later today. This could bring another round of
localized, but impressive snow accumulations to parts of Homer.

The long-lasting system we've seen for the past few days finally
starts to see its demise Sunday into Monday as a North Pacific low
tracks toward Kodiak Island and induces a shortwave ridge over the
northern Gulf. As a result, the energy from this trough flattens
out and disperses across Southcentral. The North Pacific low
approaches Kodiak Island Monday. Many details associated with this
low have yet to be ironed out as the models are still trying to
lock down the exact track of the low. However, this could be a
considerable snow and blowing snow event from Kodiak Island up
through Turnagain and Thompson Pass.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Today
through Mon night)...

Very cold polar air and mostly northerly winds farther inland
will essentially maintain bitter overnight cold temperatures
around 10 to 30 degrees Fahrenheit below zero through at least
Sun night. Meanwhile, overnight temperatures along Southwest
Coast could be in the teens below zero, then transitions to
improving temperatures but still below zero along Bristol Bay in
response to warm air lifting across the Alaska Peninsula Mon and
Mon night from the Gulf.

Isolated shortwaves from an upper low upstream could potentially
trigger steady light snow especially over the lower Kuskokwim
Valley into Sun afternoon. Meanwhile, an extensive wave south of
the same upper low will bring increased clouds farther north and
inland of Southwest with a chance of snow the next few days.
Trailing south of the latter system is a ridge aloft moving right
across the Alaska Peninsula on Sun night, which pushes off the
upper low farther west near Hooper Bay by Mon afternoon. This
scenario could trigger steady light snow over Kuskokwim Delta
between Mon and Mon night.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3:
Today through Mon night)...

Strong gales and storm-force winds are headlined in the forecast
for mostly western and central Aleutians in response to a
generally broad frontal system wrapped across the Bering and the
western Aleutian this morning. This same front will extend
eastward the next couple days leading to a wider coverage of gales
and storm force winds through the southern Bering and south of
Alaska Peninsula into at least Mon evening. Placement of complex
lows connected to the aforementioned front will make a difference
in the forecast should it pushes north of the Aleutian Chain. In
the meantime, ECMWF and NAM guides these lows directly south of
the Aleutians into the Gulf into Mon. Strong northerly winds
combined with colder temperatures will enhance snow and blowing
snow for Dutch Harbor and mainly over Eastern Aleutians starting
tonight into Mon morning. For this, a Winter Storm Watch has been
issued for the Eastern Aleutians effective tonight as the
potential for blizzard conditions is possible.

Large seas of more than 20 ft should appear near where stronger
gales and storm force winds will be through the short term. All
in all, active weather will stick around the weekend into early
next week.

&&

.MARINE (Days 3 through 5: Monday through Wednesday)...

Gulf of Alaska: Confidence is steadily increasing for a deep
North Pacific low to enter and cross over the Gulf as it tracks
mainly northeast towards the North Gulf Coast between Monday and
Wednesday. Winds around the low are expected to be at least gale
force, with storm force winds also possible along the low's
attendant front as it moves over the central Gulf on Monday and
approaches the Gulf coast at some point on Tuesday. Storm force
winds are also more likely where northeast flow is accelerated by
terrain near the southern Cook Inlet and Shelikof Strait, mainly
on Monday. Seas will also build to at least 20 ft over much of the
open Gulf waters, with highest seas coinciding with peak of
gale/storm force winds on Monday and Tuesday. Confidence is high
for at least gale force winds, with lower confidence remaining for
the low track.

Uncertainty increases further for Wednesday, with models diverging
on how quickly and where the low center reaches the Gulf
coastline. It currently appears winds will pick up once again up
to gales out of the west as the low potentially moves inland over
Southcentral on Wednesday. This will be especially likely where
cold air wrapping around the low from the northwest is
accelerated out of gaps along the AKPen and Aleutian Range.

Bering Sea/Southwest: A strong North Pacific low will be moving
south of the Aleutian Chain by the start of the period on Monday.
This will likely produce storm force winds along mainly the
central/eastern Aleutians and far southern Bering as it interacts
with modified Arctic air streaming southwest across the rest of
the Bering. Easterly gale force winds are expected farther north
across the rest of the southern/eastern Bering, becoming more
northerly as the low shifts towards the Gulf on Tuesday. Advection
of arctic air from the AK mainland over ice free waters will
continue to support heavy freezing spray from Monday to Tuesday,
with the most persistent and significant freezing spray closest to
the ice edge. As the initial system moves out of the region on
Tuesday, another low will enter the western Bering. Confidence for
this next system is lower at this time, but expect at least gales
near the low as it tracks across the western Bering/Aleutians
from Tuesday to Wednesday.

&&

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

The highly amplified longwave trough currently dominating much of
the state appears on track to re-assert itself for the first half
of the period, resulting as a deep North Pacific low moves into
Southcentral and merges with the existing upper trough over the
mainland on Tuesday. Upstream and to the west, a longwave ridge
will progress eastward into the Bering and amplify markedly
between Tuesday to Thursday. This will effectively further
reinforce the longwave troughing focused over mainland AK,
allowing more Arctic air to spill south into much of Southwest
and, slightly later in the week, Southcentral. This will
unsurprisingly lead to mostly below average temperatures
continuing through at least Thursday for inland locations,
especially across the Southwest. The bigger question will involve
the fate of the aforementioned deep low in the Gulf, with model
differences persisting for the progression of the system as it
approaches the North Gulf Coast sometime between Tuesday and
Wednesday. Even with these differences, there remains good
potential for areas near Prince William Sound, Kodiak Island and
much of the Copper River Basin to see significant snowfall on
Tuesday/Wednesday as the low increasingly interacts with the
entrenched Arctic air in place farther inland. The low will begin
to slow as it occludes on Wednesday and eventually drift north
over Southcentral, potentially spreading more snow north along
with it into parts of the Mat-Su Valleys and Cook Inlet region
Wednesday through AM Thursday.

A pattern change is looking more likely by the end of the
workweek, with the Bering ridge possibly accelerating east into
the western mainland by Friday morning, finally bringing a halt to
the endless stream of Arctic air from the north. A strong front
associated with a potent Kamchatka low could impact much of the
Bering/Aleutians by this point as well, with strong winds,
rain/snow and much warmer temperatures moving in tow as it races
swiftly east late on Thursday and into Friday.

&&
.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Watch 185.
Winter Weather Advisory 125.
MARINE...Heavy Freezing Spray Warning 121 127 129 130 131 137 138 139 140
141 150 160 165 180 181.
Storm Warning 170 173 175 413.
Gale Warning 130 131 138 155 165 171 172 174 176 177 178 179 352 411 412 414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...ED
SOUTHCENTRAL ALASKA...AF
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
MARINE/LONG TERM...AS