Gale Warning 659, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
528 PM AKDT Mon Oct 31 2022

.ANALYSIS AND UPPER LEVELS...

The key features of note are two upper level troughs on either
side of a weak shortwave ridge. The first trough axis is to the
east, aligned over the Alaska Panhandle while the second trough
is in place over the western Bering. At 300 mb, a 180 knot jet
streak is positioned over the Aleutian Chain. At the surface, a
persistent low over the northern Gulf has kept lingering snow
showers over the northern Gulf coast and has helped to accentuate
gusty northerly winds through channeled gaps and passes.
Additionally, a surface low centered over the eastern Bering is
approaching the Alaska Peninsula and southwestern mainland.
Satellite imagery and surface observations both confirm gusty
northwesterly winds across the Alaska Peninsula, Kodiak Island,
and Shelikof Strait. Ample cold air advection has produced visible
cloud streets over much of the Gulf today. Lastly, a much broader
surface low associated with the upper level trough over the
western Bering continues to spin in place, keeping widespread
cloud cover in place over the majority of the Bering.

&&

.MODEL DISCUSSION...

Model agreement continues to be a challenge as discrepancies still
exist, especially for the short term forecast. On Tuesday afternoon,
the GFS and Canadian both place the surface low farther north
over the eastern Bering. Meanwhile the NAM is the farthest south
solution, placing the low center over Bristol Bay Tuesday night.
Most recently, the 18Z NAM solution placed the low in better
alignment with the GFS/Canadian and offered a more northerly
solution. An additional complicating factor with this setup is
the development of a triple point low over Kodiak Island early
Wednesday morning. For the time being, there is generally good
agreement with this feature as it tracks into Prince William Sound
Wednesday afternoon, though a few timing differences will likely
influence the timing of precipitation.

&&

.AVIATION...

PANC...VFR conditions with northerly winds will persist. A brief
gust is possible this evening, but generally expect winds to
remain less than 12 kts.

&&

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

Strong northwesterly gap winds are continuing this evening along
the Gulf Coast and around Kodiak Island between a small low
spinning south of Cordova and an upper ridge over Southwest.
These winds should come down steadily tonight as the surface
pressure gradient relaxes and as upper level support also
diminishes. An upper trough axis will shift east during the day on
Tuesday, pulling the low stalled out in the northern Gulf north
into the coast near Cordova as it fills in. Light snow will spread
into the eastern Prince William Sound (PWS) near and north of
this feature, but amounts will remain light and generally 1 to 2
inches or less for spots including Valdez and Cordova.

Attention then quickly shifts to the next system pushing into the
Southwestern Gulf by Tuesday evening, initially in the form of a
strong front and associated gale force winds. A shield of
precipitation will spread northeast as the front approaches Kodiak
Island, but expect seasonably warm sea surface temperatures
combined with onshore winds to quickly change any snow over to
rain near sea level shortly after the onset during the late
afternoon and evening on Tuesday. As the front continues to lift
northeast through Wednesday morning, strong southeasterlies and
the band of precipitation will continue to push into much of the
rest of Southcentral. Temperatures should remain cold enough for
all snow to fall over inland areas, but a mix with or changeover
to rain will again be likely for coastal areas at times on
Wednesday, including Homer, Seward and Whittier. However, expect
the onset of any accumulating snowfall across the Mat Valley and
Anchorage area to be significantly delayed due to downslope drying
associated with the strong cross-barrier flow on the front end of
this system. In addition, there remains some uncertainty for the
track of a triple-point low expected to form along the front and
move somewhere near the southern Kenai Peninsula towards the PWS
area through Wednesday evening, and this is in turn casting
uncertainty for timing and duration, and amounts for snowfall
along the front and near the new low center. Anchorage should
still be on track to see a period of snow on the back end of the
system as the front pushes through and southeasterly winds aloft
shut off during the late morning to early afternoon on Wednesday.
This round will push through rather quickly, limiting snowfall
totals to a few inches at most across the Mat Valley vicinity
through Wednesday evening.

By Thursday, heavier coastal rain/snow and inland snow will end
from west to east as the low shifts into the eastern Gulf and as
another shot of Arctic air blasts into Southcentral from the
northwest. This strong push of northwest flow and cold advection
should once again ramp up north to northwest gap flow, especially
near the Barren Islands by Thursday afternoon. Light snow could
actually continue along and to the west of the Kenai and Chugach
Mountains for much of Thursday due to a combination of upslope
flow and additional upper waves rotating in from the north.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Tonight
through Thursday)...

Cold, calm conditions prevail across Southwest Alaska tonight as a
surface ridge transits the region. A deck of stratus lingering
across eastern portions of Bristol Bay and the Lower Kuskokwim
Valley will continue to hang in place, while clearer skies closer
to the coast begin to be replaced by high clouds moving in tonight
ahead of the next impactful system for the area. Models are coming
into better agreement with a strengthening North Pacific low
moving across the eastern Aleutians tonight poised to bring more
active weather over the next several days. There is now increased
confidence that the low will approach Southwest Alaska and track
inland near the Kuskokwim Delta, bringing a round of snow and
blowing snow to the region into Wednesday.

Snowfall will begin Tuesday afternoon along the coast and quickly
push inland with the arrival of a strong front. Snow will combine
with intensifying easterly to northeasterly winds to reduce
visibilities across the Kuskokwim Delta and Bristol Bay through
the overnight hours. This has prompted the issuance of Winter
Weather Advisories for both areas. Portions of Bristol Bay,
particularly from Dillingham northward, can also expect heavy
snowfall rates of 6 to 8 inches of snowfall accumulation in
around 12 hours through Tuesday night. As the front pushes
further inland Wednesday morning, snowfall rates will decrease
significantly and a southwesterly push of warmer air may lead
to a changeover to rain for areas along the Bristol Bay coast,
though the bulk of precipitation is expected to fall before this
occurs.

Coastal flooding will be another potential threat with this system
as it pushes inland. Westerly to southwesterly winds on the back
side of the low may lead to minor coastal flooding for locations
along the Bristol Bay coast from Jacksmith Bay to Togiak as storm
surge combines with high tide early Wednesday morning.

The low and attendant front continue northeastward on Wednesday,
bringing snow to the Lower Kuskokwim Valley as it clears out of
areas closer to the coast. Precipitation tapers to snow showers
across Southwest Alaska on Wednesday afternoon as cold, dry
northwesterly flow works in on the back side of the departing low.
This northwesterly flow regime is then expected to continue
through Thursday, with increasingly colder air working its way
south.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 3: Tonight
through Thursday)...

An intensifying North Pacific low crosses the central/eastern
Aleutians tonight as it moves into Bristol Bay, bringing
southeasterly winds to gale force through the islands and light
to moderate rain along the front. Colder air streams in across
the Bering behind the low as it pushes eastward. A decaying low
pressure system in the northern Bering Sea will also rotate
southward bringing rain and snow showers across the central Bering
and Pribilofs Tuesday night into Wednesday. At the same time, a
second surge of colder air begins to move southward across the
Bering Sea, bringing gusty northwesterly winds and snow showers
as far south as the eastern Aleutians by Wednesday night.
Conditions look calmer further west with a large surface high
pressure pushing into the central Aleutians by Thursday.

&&

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

At the onset of the forecast period, the longwave pattern features
a positive tilt trough axis over the Gulf Of Alaska through to
the Yukon Territory, a ridge axis over the Western and Central
Aleutians and another trough axis over eastern Russia. Suffice
to say, it is a highly amplified pattern. On Saturday, cyclonic
flow continues over the Gulf Of Alaska, the ridge axis has shifted
to the eastern Bering and a frontal boundary is east of Kamchatka.
By Sunday there is an inverted trough axis over the eastern Gulf
of Alaska and wide area of high pressure at the surface which
spans from the Pribilof Islands to the interior of Alaska. On Monday
the global models begins to struggle with the North Pacific and
the southern Bering. Cold temperatures will dominate the forecast
period with the most brutal temps expected in the Copper River
Basin which could see temperatures into the negative 20s to
negative 30s, depending on whether there is any fog or low stratus
hanging around.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 155 161.
         Coastal Flood Advisory 161.
MARINE...Gale Warning 120 130 131 132 150 155 160 165
                      170 172 180 181 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...KO
SOUTHCENTRAL ALASKA...AS
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CQ
LONG TERM...PS

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