Gale Warning 312, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
511 PM AKDT Sat Apr 8 2023

.ANALYSIS AND UPPER LEVELS...

The dominant feature remains a broad upper-level low centered near
Kotzebue Sound this morning. Multiple shortwaves are rotating
around the low, with the most prominent of these moving through
Southwest Alaska. A vorticity lobe, coupled with weak deformation
and southwesterly flow aloft, is producing bands of snow from the
northern Kenai Peninsula to Knik Arm. Intensity of these bands
has fluctuated through the morning hours; however, a general 1 to
2 inches of snow has fallen across the Anchorage Bowl through noon
today. A bit surprising given the antecedent dry airmass below
4,000 ft this morning as indicated from the 12z PANC sounding.
These bands are moving slowly to the east as the aforementioned
wave continues to move toward the AKPen and drier air begins to
advect across Cook Inlet (RH of 50% or less north and west of
PANC).

To the east, a low complex sits west of the Alaska Panhandle and
is beginning to lift slowly to the northwest. For the eastern half
of Southcentral, conditions are clear and dry due to weak ridging
between the two features.

To the west, temperatures remain well below average across
Southwest Alaska as the polar airmass moves south behind the
digging shortwave. Cold, northwesterly wind gusts and snow
showers persist across the eastern Aleutians and AKPen. Farther
west, a ridge of high pressure is entering the Bering from the
west, ahead of the next storm system spinning up south of the
Kamchatka Peninsula.

&&

.MODEL DISCUSSION...

Models are finally coming into better consensus with respect the
timing and track of the inverted trough as it moves across the
northern Gulf and Kenai Peninsula through Sunday. The 12z GFS
remains the quickest solution; however, it has trended a bit
slower and father north, similar to that of the NAM, EC, and GEM.
The 12z NAM was still a bit of slower, northerly outlier; however,
the 18z NAM has trended toward the 12z GEM and RGEM, which have
both been a consistent middle ground solution. As such, the latest
forecast weights more toward those solutions.

Of note, the 12z NAMNest was the only model to have any
indication of the development of accumulating snow this morning
for PANC, although still not to the extent that was seen via
observations. The NAMNest is also quite similar in its evolution
of this initial wave of snow and of the track and timing of the
inverted trough to the GEM and RGEM.

&&

.AVIATION...
PANC...MVFR conditions and northwesterly winds with very light
snow in the vicinity of the terminal are expected through the
late Saturday night. There may be a period of VFR ceilings later
this afternoon and evening as the northwesterly winds ushers in
drier air near the surface. These winds will also likely keep the
bulk of any steadier snow over east Anchorage and away from the
terminal. MVFR ceilings and visibility return Sunday morning as
the next round of snow moves back over the terminal. IFR ceilings
and visibility due to snow are also likely from mid to late
Sunday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...
An interesting late-season snow is setting up for Southcentral
Alaska tonight and Sunday. A weak deformation zone and upslope
cold advection are continuing light snow from the Matanuska Valley
southward through the Kenai Peninsula. Light snow will continue
through the evening and overnight hours, especially closer to and
over higher terrain. Early Sunday morning, a surface trough and
front will swing across the northern Gulf bringing a period of
snow to most areas from Cordova and Valdez and westward. As that
front meets westerly winds and colder air coming through the
Chugach, snowfall rates will rapidly increase, especially for the
Portage Valley through Whitter. The combination of snow intensity,
density, and winds should create low visibility conditions in the
area.

For the Cook Inlet side of the mountains, the Anchorage Bowl,
Eagle River, and the Hillside look to see the greatest snow
amounts through Sunday. The heaviest snow looks to be late Sunday
morning through early afternoon, however a couple recent models
have trended slightly later in the day, something to keep an eye
on. Light snow could continue through late Sunday night into
Monday. The Matanuska Valley should see slightly less snow than
places to the south with light snow continuing through Sunday
afternoon.

The system keeps an unseasonably cold air mass overhead, though
the April sun is still fighting to keep temperatures warmer than
they would otherwise be. These cold temperatures should make for a
low density snowfall that blows around easily, especially during
the overnight hours. On Monday that system will slowly weaken in
the Gulf as clouds begin to diminish and temperatures aloft warm
and gap winds diminish as well.

&&

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

A deep upper-level low will remain anchored by a cold, Arctic
air mass over northwest Alaska through Tuesday. This feature, in
combination with a weak surface high pressure over the eastern
Bering Sea will bring colder air to Southwest Alaska on
northwesterly flow through Monday. Temperatures will continue to
cool tonight as the coldest portion of the airmass surges
southward with a shortwave dropping into the North Pacific, with
skies remaining mostly clear. This is expected to bring another
night of low temperatures at or below freezing, with temperatures
nearing 30 below zero for some interior Kuskokwim Valley
locations. Temperatures recover on Sunday, while a weak shortwave
brings a chance of snow through the Y-K Delta region through the
evening. Another cold, mostly clear night can be expected Sunday
night as the shortwave weakens and precipitation chances
evaporate. Temperatures continue to moderate for Monday as an
incoming ridge starts to push the cold air mass into retreat.

By Monday evening, a front extending off a Kamchatka low pushes
into the Southwest Coast, bringing roughly 2 to 4 inches of snow
going into Tuesday. Blowing snow will be possible with this front,
but there is uncertainty in the extent and severity of the
threat. The forecast currently depicts the most likely scenario in
which blowing snow is most likely to be confined to the coast on
Monday night with winds rapidly diminishing as the front pushes
inland by Tuesday. However, there is some potential for a triple
point low to develop along the front in the eastern Bering Sea
which has the potential to enhance winds and spread the blowing
snow threat inland. Stay tuned as this becomes more refined in
future forecast packages.

&&

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

Northwesterly flow and a cold air mass will gradually be replaced
by warmer air and southwesterly flow as a low near Kamchatka
pushes a series of fronts into the Bering Sea. Several weak meso-
lows will traverse the central and eastern Bering Sea through
Sunday, promoting periods of light snow showers for the Pribilof
Islands, eastern Aleutians, and the Alaska Peninsula for the rest
of the weekend. A warm front will push from the western Bering Sea
this evening across the central and eastern Bering Sea by Monday,
bringing a band of light snow and rain to the islands. A stronger
front will sweep into the western Bering/Aleutians Sunday night.
The front will traverse the Bering/Aleutians through Monday night,
reaching the Alaska Peninsula and the Southwest Coast. Southerly
winds below gale force bringing in warmer air will mean
precipitation with this front is expected to be all rain for the
Aleutian Chain and Pribilof Islands. Snow showers and gale force
winds move in behind the front on Monday for the western Aleutians
and Bering Sea and continue through Tuesday.

&&

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

Overall synoptic flow appears to be in relatively good agreement
for the long term, but consensus begins to breakdown toward the
weekend. Zonal flow appears favorable across southern Alaska as a
polar low continues to flattened a ridge near the mainland. A
western Bering low will propagate shortwaves along this flow,
helping to break down any amplification of the upper level
pattern. This paves the way for embedded troughs to travel a more
southerly track across the Gulf of Alaska. While the prevailing
pattern appears to be unsettled overall, the bulk of the moisture
plumes is nudging more toward Southeast Alaska.

The Bering low and the polar low appear to have some interaction
for Thursday that will help to destabilize the cooler airmass over
the western and northern portions of the state as the bulk of the
Bering low energy attempts to shift into the Gulf of Alaska.
There is a high probability for the odds to favor colder than
normal temperatures across southern Alaska. Northerly and offshore
flow is possible as the low nudges southward and the cold airmass
points toward Southwest. Winds with cold air advection for the
last half of the week could lead to accelerated outflow winds for
the Alaska Peninsula and as far west as the Pribilofs and the
eastern Aleutians. The exact location and timing of the polar low
movement, along with interacting troughs from the west, increases
forecast uncertainty.

&&

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...ML
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CQ
LONG TERM...VR