Storm Warning 58, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
503 AM AKST Mon Feb 27 2023

.ANALYSIS AND UPPER LEVELS...

A large ridge extends from the north-central Pacific north into
the Bering Sea. Upstream of the ridge a strong low is in the
process of developing over the Western Aleutians this morning,
with the deepening system expected to amplify the jet over the
coming days. Over the southern mainland, mostly clear skies and
cold temperatures are widespread other than a few small pockets of
fog/stratus over the Lower Kuskokwim Valley, southern Copper
River Basin, and Anchorage area. Cold air continues to spill out
of the gaps and passes from the Alaska Peninsula, through Kodiak
Island and around the Prince William Sound and Gulf coast. Cloud
streets are visible on satellite imagery where the cold air is
streaming southeast over the Gulf.

Most of the active weather for the time being is over the Aleutian
Chain, Pribilof Islands and southern tip of the Alaska Peninsula.
A warm front that brought snow and wind to the Pribilofs is slowly
moving north and will get enhanced by the system near Shemya.

&&

.MODEL DISCUSSION...

Models are in good agreement in the evolution of the system as it
impacts the Bering Sea/Aleutians today and Southwest Alaska
overnight and into Tuesday morning. Even as the front moves into
Southcentral on Tuesday, models are in relatively good agreement
with the initial snowfall, other than a few minor differences in
the magnitude of QPF. By Tuesday afternoon models begin to
diverge significantly, particularly in how a shortwave and
developing surface low in the North Pacific Tuesday interacts
with the upper level low over the Chukchi Sea, and how strong the
resultant surface low becomes. The GFS has held strong in it's
westerly and deeper progression of the system and maintains the
development of a negatively tilted trough on Wednesday as it moves
into Southcentral. However, in comparison to yesterday's model
runs, it has shifted slightly to the east, resulting in generally
less snow and QPF. The NAM solution is the most progressive,
pushing this system well east into the eastern Gulf before the
upper trough becomes negatively tilted. The GEM and ECMWF
solutions are somewhere in between the two American solutions.
Should the GFS solution pan out, a more prolonged snowfall event
would take place, with snow lingering into Wednesday, whereas the
NAM solution would be a quick hit of snow with less accumulations
across the board. Stay tuned.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist, but there is
once again a slight chance fog and/or low stratus moves over the
terminal again briefly this morning. For now it has been confined
to the east side of town.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3:
Monday through Wednesday night)...

Outflow winds will persist through the day today through favored
gap locations along the Southcentral coast due to a tight pressure
gradient between a surface trough along the coast and a ridge of
high pressure over the interior of the state.

A strong area of low pressure moving from the western Aleutians to
the central Bering will allow an upper-level ridge downstream to
amplify over the western Gulf later today. The ridge will kick the
aforementioned trough east, relaxing the pressure gradient and
allowing the outflow winds to finally diminish.

Conditions will remain generally clear and cold across
Southcentral through the day and into the evening and overnight
hours. Patchy fog and low stratus, however, will linger across the
Anchorage Bowl this morning before scouring out after sunrise.

Attention then turns to the Bering low as it drives an occluded
front across Southwest Alaska and toward the region for Tuesday.
Strong easterly winds will develop ahead of the front along with
snow, first for Kodiak. The snow will changeover to rain across
the island through Tuesday morning as the front lifts north and
warmer air filters in. A triple point low is then expected to
develop along the front northeast of Kodiak Island and move toward
Prince William Sound by Tuesday afternoon. The combination of the
front and new low, along with a strong upper-level shortwave and
copious moisture streaming into the region with a southwesterly
flow aloft, will allow for widespread snow across Southcentral
through Tuesday and into Wednesday. Strong southeasterly winds are
expected to develop through favored gap locations, with gusty
winds likely for Whittier, Portage Valley, and into Turnagain Arm.
Gusty northerly winds are also likely for Cook inlet and along
the western coast of the Kenai Peninsula. The combination of snow
and winds will likely reduce visibility significantly through the
day Tuesday for these locations. Significant snow accumulations
are also likely across the Kenai Peninsula before the snow tapers
off some time Wednesday.

Snow is also likely for the Anchorage Bowl, Mat-Su Valleys, and
eventually across eastern Prince William Sound and the Copper
River Basin. There is still a fair bit of uncertainty with how
this system evolves and the track of the low. Guidance is trending
more progressive, moving the low east faster. This solution would
bring an end to the snow from Cook Inlet to the Mat-Su Valleys
sooner, perhaps as quickly as late Tuesday afternoon or evening.
Some of the model solutions, however, are trying to keep the low
and the best dynamics farther west, allowing snow to persist
into Wednesday. If this solution works out, snow totals would be
higher for many locations. Right now, the forecast is a blend of
the two, but weighted more toward the progressive solutions. As
such, snow amounts have been lowered a bit from the previous
forecast. Still, it looks like the highest amounts will occur over
the Kenai Peninsula and, perhaps, the southern Susitna Valley,
with widespread 6 to 12 inch amounts and locally higher amounts
for the immediate west coast of the Kenai Peninsula and across
Portage Valley and Whittier. Anchorage and the Matanuska Valley
may see upwards of 6 inches of snow, but this could fluctuate
based on the storm track. Cordova, Valdez, and the portions of the
Copper River Basin could also see storm total amounts ranging
from 6 to 12 inches.

A return of colder air and gusty outflow winds is then expected
for the latter half of the week as the low exits the region.

&&

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

Clear, cold, and dry conditions will continue today across
Southwest Alaska ahead of a frontal boundary currently tracking
across the Bering Sea. Active weather returns as early as this
evening along the immediate Kuskokwim Delta coast and then further
inland tonight into Tuesday. A rather impressive influx of
moisture being pulled out of the Pacific is expected to dump
multiple inches of snow across Southwest, tonight into Tuesday
morning. Locally higher amounts are expected, but broadly,
anticipate between 6 to 12 inches of snow accumulation. Two Winter
Weather Advisories (lower Kuskokwim Valley and Bristol Bay) have
been issued for snow and blowing snow, as well as a Blizzard
Warning (Kuskokwim Delta) primarily affecting the immediate
coast but may push as far inland as Bethel.

&&

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

A strong low pressure system moving into the Bering Sea early this
morning is set to bring storm force winds and a healthy amount of
precipitation to both the Bering Sea and Aleutian Chain. The main
area of concern is the Pribilof Islands, who will remain cold
enough for precipitation to remain as snow through this morning
before transitioning to rain early this afternoon. Expect areas
of blowing snow and additional snow amounts of around 2 inches
through the rest of this morning.

Additionally, extreme freezing spray is likely along the ice edge,
with heavy freezing spray extending further out towards the
Pribilof Islands.

&&

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

Model discrepancies for this forecast period appear to have
minimized since yesterday. As such, forecast confidence for the
extended period is currently strong. By Thursday morning,
lingering showers across Southcentral from a low pressure system
in the Northern Gulf of Alaska will diminish through the day, and
by Friday morning the only light showers to be expected will be
from Cordova east. Gusty northwesterly offshore flow expected to
shift northerly as the low drops to the southern Gulf Thursday
evening. By Friday afternoon the leading front of a northern
Bering low approaches Southwest Alaska. This front will bring
widespread light showers from the Kuskokwim Delta, to the Lower
Kuskokwim Valley and Greater Bristol Bay region through Saturday
afternoon. This wave is expected to break-up along the Alaska
Range, and not extend into Southcentral. Late Saturday evening a
North Pacific low will surge towards the Western/Central
Aleutians, bringing widespread light precipitation and southerly
gales from Adak to Unalaska lasting until Sunday morning.
Precipitation associated with this system will likely be blocked
to the east, with only a few showers possibly reaching the
Southwest coastline late Sunday morning. A defining feature to
expect during this period is a building ridge over eastern
Mainland Alaska. This feature is keeping much of the activity out
of Southcentral Alaska during this period.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning 155
Winter Weather Advisory 152 161
Winter Storm Watch 125 121.
MARINE...Heavy Freezing Spray 119 125-129 138-141 161 181 185 412
414
Storm Warning 176 177-179 181 185 411-413
Gale Warning 351 352 131 150 155 161 165 170-174 176FIRE
WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...CJ
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KM
LONG TERM...CL