Storm Warning 31, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
504 PM AKST Sun Jan 30 2022

.ANALYSIS AND UPPER LEVELS...

A low is currently tracking northward over the western Aleutians.
Strong southerly flow out ahead of it is bringing temperatures
into the 40s and periods of moderate rain near Unalaska. This low
will quickly move into the central Bering through this evening.
Otherwise, things are extremely quiet elsewhere with weak
northerly flow keeping skies mostly clear for nearly all of the
mainland. Areas of low clouds are firmly entrenched across
portions of the western Kenai Peninsula, Anchorage, and the Mat-
Su. With a steep low level temperature inversion in place (as per
the 12Z Anchorage sounding), it will be hard to get rid of these
clouds. Locally gusty winds are being observed through the usual
coastal terrain gaps, however these are relatively weak.

&&

.MODEL DISCUSSION...

Model differences exist with the next main low/front approaching
the Aleutians on Monday morning. The NAM is significantly slower
with the frontal progression than other models, with QPF remaining
south of Bristol Bay late Monday while others show it already
overspreading the Kuskokwim Delta by then. Likewise, by midday
Tuesday through Wednesday, it is much more aggressive with
downsloping over Anchorage and shows very little snow accumulation
whereas other models currently show around 1 to 3 inches. Given
that the low is not overly strong, the weaker flow would tend to
favor the latter solutions. Confidence continues to increase.

&&

.AVIATION...

PANC...The ceiling forecast remains a big challenge. A hole has
opened up in the MVFR cloud deck sitting over the region,
resulting in clearing over the terminal this afternoon. With a
light northerly wind, the clouds could easily drift back in from
the north. Will monitor trends in satellite and adjust forecast
as necessary. Bottom line, these low clouds will likely remain in
the vicinity, if not overhead, through tomorrow.

&&

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

Dry northerly flow around a ridge of high pressure has managed to
finally allow for a sunny afternoon for most, but not all, of
Southcentral. Stubborn low stratus hanging on over low valleys
across parts of the MatSu, Anchorage, and the Kenai Peninsula is
slowly developing breaks, though much if it may try to hang on
into Monday with no major change in the overall pattern and the
related subsidence inversion until later in the week. Away from
where low clouds persist, temperatures will drop down to near or
below zero for the next couple mornings, especially over the
Copper River Basin, where morning lows between -20 and -35F are
possible through Tuesday morning under polar high pressure, calm
winds and mostly clear skies.

A pattern change will commence by late Monday evening into Tuesday
as a negatively-tilted upper level trough sweeps across the
Southwest and weakens the upper level ridge in place out ahead
over Southcentral. Near the surface, a front will move into the
Gulf and approach Kodiak Island, sending across a band of snow
that will quickly mix with or change to rain as warm air streams
northwest and pushes temperatures near sea level above freezing by
early Tuesday morning. Looking to the north across the rest of
the outlook area, it's becoming clearer that weaker waves and
moisture casting ahead of the front and main trough will move far
enough north to spread precipitation well inland on Tuesday. All
but the far northern Copper River Basin and Susitna Valley are now
expected to see a period of accumulating snow from Tuesday
afternoon to Wednesday morning. Current thinking is for a broad
swath of around 1 to 4 inches to fall across the eastern Kenai
Peninsula and over most of the MatSu Valleys and Anchorage area,
with higher amounts possible in the Prince William Sound and
eastern Turnagain Arm region. In addition, the tightening east to
west pressure gradient ahead of the trough and frontal wave will
help drive gusty easterly gap winds through the Turnagain Arm,
Knik and Matanuska Valleys, mainly during the day on Tuesday just
before the onset of snow later in the day. There is some lingering
uncertainty for how strong the associated cross-barrier flow will
be, which could hurt snowfall totals near Anchorage due to
downslope drying.

By Wednesday morning, the upper trough will advance east towards
the AlCan border, ending the support for any continuing snowfall
from west to east.

&&

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

Expect another night of Arctic cold temperatures accompanied by
mostly clear conditions. A tightening pressure gradient over
Southwest especially across the Kuskokwim Delta will provide for
breezy conditions to accompany the Arctic air resulting in wind
chills of 15 to 30 below zero. The coldest wind chills expected
across the Delta. Sustained northeast winds of 15 to 30 mph are
expected with gusts as high as 40 mph.

Next, a decaying front (frontolysis) is expected to continue
moving northward across the Bering towards Nunivak Island and the
Kuskokwim Delta Coastline tonight. Basically, only expect some
increase in cloud cover overnight with at best some
flurries/light snow shower particularly for Nunivak Island.

Then a more rigorous upper level shortwave will move northward
from the North Pacific during the day Monday. This system is
expected to spread moisture northward crossing the Alaska
Peninsula Monday evening with the effects of this front continuing
through Tuesday night. Accompanying this system will be a great
deal of warm air aloft that will bring a myriad of precipitation
types to Southwest. As a result, Winter Weather Advisories are in
effect for the Kuskokwim Delta and the Bristol Bay area from
Dillingham north and west. Precipitation is expected to begin
overspreading Southwest Monday evening. The best chance time for
mixed precipitation of snow and sleet or freezing rain will be the
Monday evening into Tuesday morning time frame. Some slightly
cooler air aloft may move into the Delta and Bristol Bay area
Tuesday afternoon through Tuesday night to perhaps result in a
change back to snow, though, this is uncertain. Conditions begin
to improve Tuesday night into Wednesday as the front continues
inland while weakening. Another front moving towards the Southwest
coast on Wednesday may reinforce precipitation Wednesday potentially
resulting in more precipitation type issues.

&&

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

A compact surface low continues moving northward over the central
Aleutians while its front pushes eastward across the Aleutian
Chain towards Southwest. A strong southerly push of warm air
associated with this system is currently keeping a majority of
the precipitation across the Aleutians and Bering as rain.
However, a freezing rain/snow mix is possible for the Pribilof
Islands and eastern Aleutians tonight before the precipitation
completely transitions to rain by Monday morning. The surface low
tracks into the central Bering on Monday during the day and
proceeds to move northwestward out of our forecast area. At the
same time, another surface low approaches the Aleutians from the
North Pacific. The low passes over the eastern Aleutians early
Tuesday morning and into the eastern Bering where it dissipates
late on Tuesday. This system will bring another shot of rain to
these areas, although southerly flow across the Aleutian Range
will have a downsloping effect along the lee of the mountains.
Further west, a stronger North Pacific low tracks over Adak
starting late Tuesday night into early Wednesday morning. The
front extending from this low will push across the Aleutians and
Bering through Wednesday night. Strong winds are expected to
develop with this low, and precipitation type will be mostly rain
with the possibility of a rain/snow mix for some areas.

&&

.MARINE (Days 3 through 5 - Wednesday through Friday)...

Gulf of Alaska:

Gale force winds in the northern and western Gulf with localized
storm force winds in favored barrier jet areas diminish quickly on
Wednesday as a low exits into the eastern Gulf. The forecast for
Thursday and Friday is less certain with a series of systems
moving up from the Pacific. Portions of the Gulf may see another
round of gales on both days, with a similar but weaker barrier jet
setup possible on Thursday giving way to some potential for more
widespread westerly gales and gap winds Friday.

Bering Sea/Aleutians:

A strong low over the central Aleutians will produce widespread
gales across the central Bering/Aleutians Wednesday morning.
Storm force winds will also be likely near the center of the low.
Hurricane force gusts appear less likely now as the low is
expected to occlude and start weakening before reaching the
Aleutians. Southerly gale force gap winds can be expected east of
the low through the eastern Aleutians/AKPEN. Winds move with the
low into the western Bering late Wednesday into Thursday and
diminish below gale force by Thursday afternoon as the low exits
westward. Seas may exceed 20 ft in the vicinity of the low. The
northern Bering will likely see persistent northeasterly gales
before they decrease in coverage beginning Thursday afternoon.

Confidence is lower regarding the evolution of other features
which will impact wind and wave across the Bering/Aleutians.
A swath of easterly gales will be possible in the eastern Bering
and Bristol Bay from Wed afternoon into Thursday morning as
another low lifts northward across the AKPEN. By Friday morning,
another North Pacific low may impact the eastern Aleutians/AKPEN,
bringing the possibility for gales with some chance for storm
force winds and seas in excess of 20 ft near its center.

&&

.LONG TERM FORECAST (Days 4 through 7: Thursday through
Sunday)...

The prominent feature in the long term will be the 300 mb North
Pacific jet and placement south of the Aleutians; an ideal
location to generate multiple lows that will cycle through the
Bering. On Thursday, model solutions show a broad 500 mb trough
over the Bering and an upper level ridge over Siberia. A second
upper level ridge over the Alaska Panhandle early Thursday quickly
moves east as the first of several upper level shortwaves drive
across the Bering from west to east. Supported by the 300 mb jet,
multiple surface lows will lift from the North Pacific into the
eastern Bering Thursday into Friday, keeping an active weather
pattern in place for Southwest Alaska through the end of the
week. Southcentral will also see an active pattern with this setup
as each low gradually tracks eastward. By late Friday, model
solutions are hinting at the possibility for a deep fetch of
southerly flow lifting into Southcentral, however confidence is
low as agreement between solutions remains poor. In summary, the
synoptic setup is trending toward a more active pattern for the
southern half of the state for the end of the week and into next
weekend.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 155 161.
MARINE...Storm Warning 170 179.
         Gale Warning 131 138 150 160 165 170 171 172 173 174
                      175 176 177 178 179 180 185.
         Heavy Freezing Spray Warning 139 165 179 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...AP
SOUTHCENTRAL ALASKA...AS
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AF/MV
MARINE/LONG TERM...KO