Heavy Freezing Spray Warning 43, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
510 PM AKST Mon Feb 22 2021

.ANALYSIS AND UPPER LEVELS...
An upper level trough that was over the Cook Inlet/Susitna
regions overnight has now moved eastward to near the AlCan border
this afternoon. This leaves a ridge building in behind it and an
upper level low in the Western Bering Sea just north of Attu. The
trough over Cook Inlet, along with very light cold air advection
from the west at lower levels is mainly what was responsible for
the light snowfall overnight around Anchorage and Mat-Su.

At the surface, westerly flow is occurring over southern mainland
Alaska and the Gulf of Alaska. A meso low can be seen in eastern
Prince William Sound from the satellite imagery early this
afternoon with strong westerlies over the Gulf of Alaska.

The westerly flow is also keeping some low stratus along the
Kuskokwim Delta with the flow off the Bering Sea. The low in the
western Aleutians just south of Attu has a front ahead of it over
the eastern Aleutians and into the east-central Bering Sea. There
is some decent warm air advection in conjunction with this front
switching precipitation type from snow to rain in along the
ALeutians and AK Peninsula.

&&

.MODEL DISCUSSION...
Models are in generally good synoptic agreement the next few days.
The handling of the Bering Sea low complex has especially good
agreement for a low with this structure. The main model
uncertainty of impact will be for Wednesday through Wednesday
evening in the Gulf of Alaska region and extending to the coastal
areas of Southcentral. There is a low that tracks from southwest
to northeast, but how far north that track runs is the issue. The
ECWMF has a very consistent run-to-run track that will keep most
of the impacts offshore over marine waters. The GFS is even
farther south while the NAM is the one that has it far enough
north to impact the Gulf coast. Should the NAM verify, it could
increase winds and cause blowing snow in Thompson Pass as well as
snow for the Prince William Sound area. However, it is the only
solution that does this so we continue to be skeptical of the NAM
at this time, though it has not been completely dismissed.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist through
Tuesday afternoon. MVFR conditions are likely to develop Tue
evening when the next batch of light snow arrives.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...

Clear and cool conditions are expected across much of the
Southcentral through tonight and tomorrow morning. Gusty outflow
winds persist as a result of an upper level shortwave pushing west
to east across the eastern Kenai and the Gulf of Alaska tonight.
Winds out of Seward and Whittier, as well as out of the Copper
River Delta, should begin to diminish early tomorrow morning as
the wave exits to the east.

Active weather returns across most of Southcentral Tuesday
afternoon when a weak front pushes the North Gulf Coast followed
quickly after by a strong low entering the southern Gulf Wednesday
afternoon. The system continues pushing north through Wednesday and then
changes course, tracking east towards the Panhandle before
reaching the North Gulf Coast. Models have begun to come into
better agreement when compared to past runs, however,
discrepancies are still relatively large, with the GFS being the
biggest outlier.

Widespread precipitation, mainly in the form of snow, is expected
across much of Southcentral by Tuesday night with the heaviest
amount across Prince William Sound and along the eastern Kenai
Coast from Seward to Seldovia. Coastal areas along Kodiak Island
will see periods of rain and rain/snow mix. Several inches of snow
accumulations are expected across the Southcentral region Tuesday
night through Wednesday. There is good blowing snow potential for
Turnagain Pass Tuesday night and for Thompson Pass Wednesday
afternoon, which may significantly reduce visibilities at times.
The lower elevations of the Copper River Basin should stay
relatively dry and out of any heavy/widespread snowfall. Should
any precipitation end up falling between Eureka and Gulkana it
will likely remain light.

A period of storm force winds are expected north and south of the
low Wednesday afternoon and evening as the low begins moving
towards the east. Otherwise, widespread gales will dominate much
of the Gulf Wednesday and Thursday. Additionally, an area of gales
and small craft winds are likely along southern Cook Inlet
tomorrow night and Wednesday. Widespread precipitation is expected
across the Gulf, becoming heavy at times, and will be mostly rain
or a rain/snow mix with areas of snow confined to the coasts.

&&

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

An upper level ridge this afternoon will give way to a trough and
associated surface front later this evening. Snow will quickly
track eastward across much of Southwest Alaska. A Winter Weather
Advisory for blowing snow remains in effect this evening through
early Tuesday morning for the Kuskokwim Delta coast. Snow amounts
have remained generally unchanged from the previous forecast. The
snow growth layer is centered around 750 mb. However, despite
favorable temperatures, stronger winds below this level will cause
lower snow-to-liquid ratios and thus slightly inhibit
accumulations. A general 1 to 3 inches through Tuesday afternoon
remains likely from the Middle Kuskokwim Valley westward. Wind
chills in these regions could approach 30 below.

Precipitation will linger farther south as southwesterly flow
aloft will continue to allow for moisture advection into the
Bristol Bay region. Another couple inches are possible generally
north and west of Dillingham through Wednesday morning. A
rain/snow mix is expected south of and including King Salmon with
warmer surface temperatures. After a brief lull Tuesday afternoon
and evening associated with an upper ridge, snow will redevelop
overnight into Wednesday across the Kuskokwim Delta and Middle
Kuskokwim Valley bringing additional accumulations. Another ridge
will build in by Thursday, though scattered snow showers will
continue.

&&

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

Snow and blowing snow across the Pribilofs will gradually
diminish through this evening. A series of lows and their
associated fronts will move over the Bering/Aleutians through
Thursday. As such, rain/snow showers and generally unsettled
weather will continue.

&&

.MARINE (Days 3 through 5: Thursday through Saturday)...

Bering: There is good confidence that a low north of the western
Aleutians will produce an area of gales across the western Bering
for Thu. There is also good confidence that a low south of Attu
will move over the western Bering by Fri morning with a second
North Pacific low developing ahead of it and lifting north over
the central Aleutians and Bering. A widespread area of gales with
waves approaching 30 ft on the Pacific side and 20 feet or
greater on the Bering side look likely for the eastern half of the
Bering/Aleutians late Thursday or early Fri through Sun. However,
there is low confidence in the exact placement of these features
beyond Fri as they both move east.

Gulf of Alaska: There is good confidence in the development of
widespread gales and waves over the northwest portion of the Gulf
through late Thu or early Fri. There is also good confidence that
a low will develop near Kodiak Island by Sat morning and near
Prince William Sound by late Saturday. An area of easterly gales
is possible along the northern Gulf Coast with more widespread
northwesterly gales and elevated seas across the northwestern
Gulf by Sun as the low moves east.

&&

.LONG TERM FORECAST (Days 4 through 7)...

The long term forecast begins Thursday evening with a
continuation of an active and progressive weather pattern
transitioning to a more amplified pattern for the weekend. By
Friday, the outflow winds that developed along the Southcentral
coast and favored gap locations on Thursday will diminish as a
ridge builds over the Gulf. Farther west, a broad, complex low
will develop near the western half of the Aleutians. Widespread
gales are expected for much of the central and eastern Bering as
the low lifts to the central Bering then slides southeast over the
AKPEN. A secondary low developing along the front of the
aforementioned low then looks to move into the northern Gulf for
Saturday as the ridge retreats. These lows will bring plenty of
unsettled weather across much of the southern mainland late Friday
through late Sunday. By Monday, a ridge builds back over
Southcentral while a potential storm-force low near Attu pushes a
front toward the AKPEN and Southwest Alaska coast.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory: 155 195.
MARINE...Gale Warning: 130 131 177-180 185 351 352 412 414.
Heavy Freezing Spray Warning: 130 160 180 185.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...KM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AP
MARINE/LONG TERM...TM