Winter Weather Advisory 20, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
556 PM AKST Sun Feb 28 2021

.ANALYSIS AND UPPER LEVELS...

The low pressure system over the Gulf is continuing to move east,
although the eastward progression is slowing some. The precip
over eastern Prince William Sound and points east will continue in
favored areas but, in general, will weaken in intensity. An
amplified mid- to upper-level ridge continues to work its way
east and is currently moving into western Alaska and the eastern
Aleutians.

Deep low pressure continues over the western Aleutians and
western Bering. This system continues to produce widespread storm
to hurricane force winds, seas over 45 ft, and areas of heavy,
blowing snow. This system is also pulling some warmer air
northward, however, so periods of rain are also expected mainly
along the Aleutian chain.

&&

.MODEL DISCUSSION...

Models are still in good agreement through the middle of the work
week. The strong low pressure system over the Bering Sea will
continue to move east, crossing the Bering and eastern Aleutians
on Tuesday, then continuing into the Gulf of Alaska, gradually
weakening as it does so.

&&

.AVIATION...

PANC...VFR conditions will persist through today and much of the
overnight period. Expect light to moderate snow to move into the
area between 14-16z with periods of heavy snow possible.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3: Today
through Wednesday afternoon)...

Snow showers are tapering off and skies are clearing from west to
east as an upper ridge builds over the region. Temperatures will
subsequently drop to near or below zero across much of the region
by early Monday morning. Northerly gap winds will likewise
diminish as a Gulf low departs to the west.

An occluding front associated with a broad low in the Bering will
quickly approach Southcentral early Monday morning. An upper
level shortwave will allow for the formation of a surface low near
Kodiak. This will take a similar track to the previous low;
however, moisture associated with the aforementioned front will
allow for snow accumulations west of the Chugach Mountains and
minimize any initial downsloping. Thus, light snow will continue
from early Monday morning through the evening. Approximately 3 to
5 inches are expected across the Kenai Peninsula, Anchorage and
the Matanuska Valley. Similar totals are likely along the Prince
William Sound coast. More benign conditions are expected Tuesday
and Wednesday with offshore flow developing bringing clearing
skies.

&&

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

Recent clear and cold conditions across Southwest Alaska will
soon transition into an active stormy pattern as a front arrives
to the coast this evening. Gusty easterly winds will pick up,
especially across coastal locations as the front pushes inland.
One challenging aspect of the forecast has been temperatures.
After persistent cold temperatures at the surface over the last
few days, southerly flow in the midlevels is expected to erode
this cold air rather quickly. Snow is still expected across the
Kuskokwim Delta and Lower Kuskokwim Valley, however locations
across the Alaska Peninsula may see a rain/snow mix at times as
warm southerly flow lifts northward. This front is well occluded,
however, so most of the warmer air will be kept at bay south of
the Alaska Peninsula. Snow and blowing snow is still expected to
cause limited visibility overnight through early Monday,
especially across the Kuskokwim Delta Coast.

As the low center moves over the central Bering Monday afternoon,
gusty west to southwest flow is expected as colder air fills in on
the backside of the low. This will allow for a showery pattern,
with snow showers likely to last into Tuesday. Additionally, one
other thing we are keeping an eye on is a long westerly fetch
across the eastern Bering that may bring the potential for surge
along coastal areas of Bristol Bay sometime late Monday into
early Tuesday. Confidence remains low at this time, however we
will continue to monitor this.

As the low center continues to track southward over the Alaska
Peninsula Tuesday, winds will become northerly across the coast.
Snow showers are expected, and visibility may be reduced at times
across the Kuskokwim Delta Coast Tuesday morning. However,
precipitation is expected to end quickly from north to south, as
much colder, drier air arrives across Southwest Alaska from the
north for Wednesday.

&&

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

A hurricane force low centered north of Shemya continues to bring
impactful weather to the Bering Sea. The leading front continues
to move eastward, bringing gusty southerly flow to the Pribilofs,
central Aleutians, and eastern Aleutians. On the backside of this
low, westerly winds have filled in across the western Bering with
snow and blowing snow conditions. As the leading front continues
to track eastward, high winds will spread across the Chain
through Monday. A triple point forms along the initial front over
the Alaska Peninsula early Monday and then moves into the Gulf.
However the parent low moves from the northern Bering north of the
Pribilofs to the eastern Bering by Monday night. As a result,
gusty westerly winds over the Pribilofs will eventually shift to
northerly, with potential for continued snow and blowing snow
which may reduce visibility at times Monday through Tuesday. A
much colder airmass arrives over the Bering in the wake of this
low Tuesday night, while another front arrives over the western
Aleutians Wednesday morning.

&&

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

Bering: Upper level disturbances move over the Central and
Eastern Bering and Alaska Peninsula. Forecast confidence is good.
Widespread high end gale force winds and waves will diminish Thu.
Waves to 20 feet subsiding. A well developed low and front move
across the Bering from the West through Fri. The front brings
widespread high end gale force winds with areas of storm force
winds to the Central Bering, diminishing late Thu, while the wind
wrapped low holds onto widespread gale force winds through Fri.
Waves building to 26 feet through Fri.

Gulf of Alaska: A low exits the Southern Gulf for Thu, with
outflow winds from the Alaska Peninsula right behind. Models vary,
but confidence is good. Widespread high end gale force winds and
waves spread from the Shumagin Islands to the Barren Islands into
the Western Gulf through Thu. Waves to 17 feet across the Southern
Gulf, subsiding. The next low and front approach the Southwest
Gulf Fri.

&&

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

Generally quiet weather is expected for the long term forecast
period, especially across Southcentral. The period is expected to
mainly be dominated by a strong upper level low over northeastern
Russia that will continue to send rounds of Arctic/Siberian air
across much of Alaska especially early on. Upper level ridging
will periodically build north and try to cut off this cold air as
storm systems move north into the Bering and along the eastern
periphery of the upper level low. As a result, a frontal system is
anticipated to move into Southwest Alaska on Thursday with some
snow. However, this front won't have much progress into
Southcentral as another lobe of cold air Will quickly try and push
across southern Alaska behind it. The front will be quickly
pushed southward across the Alaska Peninsula and into the North
Pacific. The main weather concerns for Southcentral will mainly be
the winds through the typical gaps, bays, and passes.
Temperatures are also expected to remain well below normal for at
least the first half of the forecast period with some warming
heading into Saturday and Sunday though remaining either slightly
below or near normal.

Towards the very end of the period on Sunday, models are keying
in on an upper level ridge setting up across much of southern
Alaska as the upper level low over northeastern Russia moves
southward into the western Bering bringing colder air to this area
and starting to bring milder air north into Southcentral. The
storm track looks to also become more active starting Sunday as
another system traverses the Bering and eventually into the Gulf
early next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...WINTER WEATHER ADVISORY 155 195.
		 HIGH WIND WARNING 185 187 191.
MARINE...HURRICANE FORCE WIND WARNING 178.
		 STORM WARNING 170-177 185 411 413 414.
		 GALE WARNING 119-121 125 129-132 136-138 150 160 165
                      179-181 185 351 352 412.
		 HEAVY FREEZING SPRAY WARNING 130 139 150 160 165 179 180
									  185 412 414.
FIRE WEATHER...NONE.

&&

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