Dense Fog Advisory 3, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
600 PM AKST Thu Feb 18 2021

.ANALYSIS AND UPPER LEVELS...
A large occluded low with at least three frontal wraps is in the
eastern Gulf of Alaska this afternoon. An inverted trough extends
out of this low and into the Cook Inlet region which is the
source of the snowfall there. GOES-17 satellite also shows strong
outflow winds off the Alaska Peninsula in cold air that reaches
across a good swath of the north Pacific south of this low. The
winds can be inferred by the cloud streaks out of the gaps in
terrain along the Alaska Peninsula. The fact that it is cold air
can be seen by the appearance of the cumulus clouds present across
the North Pacific.

Farther west, a warm front is moving into the Bering Sea ahead of
a low that is centered just south of the Kamchatka Peninsula. This
is bringing some increasing winds as well as some precipitation
to the Aleutians.

&&

.MODEL DISCUSSION...
Models remain in good overall synoptic agreement, but there is
still a bit of uncertainty with the development of a low along the
front moving into the Bering Sea Friday. Models generally all show
a low developing in the vicinity of the Alaska Peninsula and then
heading into the Gulf on Saturday, but changes in low structure
(mostly how much it elongates), may have some impact on the
forecast.

The last few discussions we mentioned the inverted trough moving
through Southcentral from east-to-west last night into today. The
big question was whether it would stall over Anchorage or the
Chugach mountains before reversing course and moving eastward to
dissipate. Models were all off on this one as the inverted trough
pushed west of Anchorage resulting in snow moving into to Kenai
and Homer.

&&

.AVIATION...
PANC...Conditions at the terminal will improve through the
evening as the snow moves out of the area to the east. At this
time expecting some clearing toward morning, but there will remain
the risk of low stratus or fog development when skies clear out.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3...Thursday
Afternoon through Sunday)...

The highlights...Snow moving east into the Copper River Basin
tonight and Friday. Outflow winds developing tonight and Friday
and colder for the weekend. Snow likely for Kodiak Island this
weekend.

Light snow is ending from west to east as we speak with snow
ending across the Kenai, Anchorage and MAT Valley by this evening.
The upper level shortwave over western Alaska will move east
tonight pushing the light snow to the Copper River Basin, snow
will continue through Friday for this area with up to 2 inches of
snowfall. Outflow winds will increase tonight and spread east on
Friday. Friday night the trough moves east of the area ushering in
colder air. It may get cold enough Friday night for a wind chill
advisory for Thompson Pass, but opted to wait to issue wind chill
advisory. Friday night and Saturday another frontal system enters
the Gulf this will bring some light precipitation to coastal
locations with snow likely for Kodiak City on Saturday. Saturday
night and Sunday a storm force low moves across the southern Gulf.
These two events may bring upwards of 6 inches or greater of snow
to Kodiak City, however there is uncertainty on snow amounts and
duration may be longer than 12 hours. Will need to closely watch
this system and monitor Kodiak City for possible winter weather
advisory snowfall for Saturday into Sunday. Otherwise increased
winds across offshore zones to storms for Sunday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3 Thursday
afternoon through Sun night)...

Generally quiet but cold weather is expected to continue for
Southwest for the forecast period. The main story will be the
continued breezy conditions through this evening especially for
the Kuskokwim Delta. Winds will gradually subside to between 5 to
10 mph out of the north for the overnight and throughout the day
Friday as the pressure gradient eases over the region. Cold
temperatures will be the main story for Friday with single digits
below zero highs for the Kuskokwim Delta and Middle Kuskokwim
Valley with single digits to low teens for coastal Bristol Bay.

Friday night into Saturday, the pressure gradient is expected to
build over the area. A strong gale force front moving up through
the Bering will be hitting up against strong high pressure over
the Alaska Interior leading to increasing winds yet again for much
of Southwest, especially Bristol Bay and the Kuskokwim Delta.
Saturday morning doesn't appear to be as cold as Friday morning
but colder air is expected to move into the area Saturday evening
along with breezy conditions with northeast winds sustained 20 to
25 mph. Breezy conditions combined with the very cold
temperatures in the teens below zero will yield wind chill values
of -30 to -35 degrees below zero. There remains some uncertainty
as to if wind chills reach -40 degrees below zero for an extended
time period as with the wind blowing it will help to keep the
temperatures a bit warmer than if it were clear and calm. Stay
tuned for the latest info as a Wind Chill Advisory may be issued
in subsequent packages for the Saturday night into Sunday time
frame.

Besides the cold and wind, lower Bristol Bay and along the Alaska
Peninsula, a frontal system is expected to bring snow to villages
such as Pilot Point and Port Heiden. Generally 2 to 4 inches of
snow are expected for this area beginning Friday evening and
continuing through Saturday evening. The peak of this event will
be Friday evening through Saturday morning. Light snow is then
expected to persist through Saturday evening before ending.

&&

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

The active storm track is expected to continue for the Bering Sea
and the Aleutian Island Chain for the forecast period. A gale
force front will continue moving from the western Bering to the
eastern Bering Friday afternoon. The front will come to a halt
though as it bangs up against the strong Arctic high pressure over
the Alaska Interior. As this front moves across the Bering, it is
expected to bring snow and blowing snow to the Pribilof Islands
beginning Friday morning and continuing through Friday evening. As
a result, a Winter Weather Advisory was issued for this area
beginning Friday morning and continuing through Friday evening for
blowing snow. This same front will bring some rain and snow mix to
Dutch Harbor and the Sand Point/Cold Bay areas during the same
time frame (Friday morning through Friday evening).

Heavy freezing spray will continue to be a hazard for mariners
near the ice edge for the next 3 days as the same general pattern
remains in place with the northeasterly flow off the Mainland.

Another low pressure system is expected to bring another round of
rain and snow mix to the western and central Aleutians Friday
night through Saturday. Generally small craft advisory level winds
are expected with this system mainly south of the Aleutian Chain.

Lastly, another frontal system is expected to bring another round
of unsettled weather to the same areas for Sunday/Sunday night as
the active pattern remains due to the jet stream remaining south
of the Chain.

&&

.MARINE (Days 3 through 5: Sunday through Tuesday)...

Bering Sea: Sub-gales and below 20 ft seas are likely Sunday.
Model agreement degrades into early next week. Multiple low
centers will interact over the western Bering. Winds exceeding 35
kt are possible but their location remains uncertain.

Gulf of Alaska: Westerly gales and seas greater than 20 ft are
likely in the southern Gulf on Sunday. These will expand northward
Monday and Tuesday, potentially strongest between Kodiak Island
and the Kenai Peninsula.

&&

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

A low in the eastern Gulf will bring light precipitation to
eastern Prince William Sound and the Copper River Basin on Sunday.
Northwesterly offshore flow looks to persist elsewhere across
much of the mainland through Monday with weak high pressure over
the interior. This will bring clearer skies and cooler
temperatures across Southwest and Southcentral Alaska. There
remains some disagreement into Tuesday as the ECMWF displaces the
upper level ridge faster than the GFS or Canadian and overspreads
snow well into the Middle Kuskokwim Valley. In any case,
precipitation will track eastward across the state Wednesday
before an upper ridge builds in on Thursday. As usual, the Bering
looks to remain active under a series of lows and fronts.
Confidence remains good regarding the overall longwave pattern,
but details are still lacking.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory: 195.
MARINE...Storm Warning: 130.
Gale Warning: 120 125 131 132 137 138 160 175 177-180 185 351 352
              411-414.
Heavy Freezing Spray Warning: 121 127 129-131 138 139 160 165 180
185 414.

&&

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

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