Gale Warning 713, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
414 PM AKST Tue Dec 8 2020

.ANALYSIS AND UPPER LEVELS...

A colder airmass has worked into Southcentral through the day
today, changing early day rains over to some sloppy wet
accumulations of snow across the Anchorage Bowl. This round of
precipitation is mostly wrapping up across the interior, but
snow/rain persists across the north Gulf Coast. Further south and
west, snow showers and areas of blowing snow continue across
portions of the AKPEN this afternoon, but these too are starting
to wind down.

Tonight into tomorrow an upper level low over Southwest Alaska
will phase with an upper trough across Northwest Alaska and then
gradually progress through Southcentral into tomorrow night.
Ahead of this phased system, another round of snow will set up
across Southcentral on Wednesday, while Southwest Alaska and the
AKPEN see snow and blowing snow come to an end.

Further west, a storm force low and associated front will lift
out of the North Pacific into the western and central Aleutians
tonight into Wednesday morning. Ahead of the approaching front
snow is expected, but as the front lifts through the Aleutians
warmer air should work in eventually changing much of this
precipitation over to a mix or rain by later Wednesday.

&&

.MODEL DISCUSSION...

Model agreement is fair in the near term, but there are
disagreements on the overall orientation of the trough across the
area as the southwestern and northwestern AK shortwaves phase
with tonight into tomorrow. Ultimately the orientation of this
phasing will influence the placement of the heavier snowbands
across Southcentral tomorrow, so some detail adjustments are
likely. Otherwise there is good model agreement on the timing and
impacts associated with a front pushing through the
Bering/Aleutians tomorrow into Thursday.

&&

.AVIATION...
PANC... Improving conditions are expected this evening with snow
showers wrapping up. VFR conditions will take over tonight and
hold until the next round of snow showers works in Wednesday
evening, bringing back MVFR conditions.

&&

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

An active pattern is expected to continue for Southcentral over
the next couple of days as a low pressure system that is currently
centered over Bristol Bay gradually moves northeastward towards
the Kenai. This low will continue to bring snow to the southern
Kenai Peninsula through Wednesday evening. There is still
uncertainty on the exact snow amounts for Seward as any small
shift in wind can change the temperature and thus affecting the
snowfall. Observations in Seward show temperatures hovering around
the freezing mark with light winds. The uncertainty is due to the
sea surface temperatures in the Gulf of Alaska remaining in the
40s which could potentially warm the temperatures in Seward above
freezing if a persistent south wind develops. However, at the
same time there is cold air advection aloft as 850 mb temperatures
are expected to decrease which makes this synoptic set up even
more complex. Due to the uncertainty, the snowfall forecast was on
the conservative side, however should temperatures cool further
the Seward Highway area between Seward and Moose Pass could see
12" of snow. As of now the general range of what is expected for
the greater Seward area is 6-12" with the highest amounts in the
higher elevations.

A light Turnagain Arm wind will continue this evening through
Wednesday afternoon before gradually weakening Wednesday evening.
That being said, the chance for snow will increase for the
Anchorage Bowl Wednesday night as the southeasterly cross-barrier
winds weakens and thus decreases the potential for downsloping
off the Chugach Mountains.

&&

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

A vertically stacked low over Bristol Bay will gradually track
eastward through Wednesday. Gusty northerly winds are still
blowing along much of the coast, especially along the Kuskokwim
Delta coast. Snow and blowing snow will gradually taper this
morning as the low weakens and the majority of the precipitation
dissipates. Some snow showers are likely across the interior
Bristol Bay area, the Lower Kuskokwim Valley, and the foothills of
the Alaska Range this evening as residual moisture tracks
eastward with the low. Meanwhile for the Alaska Peninsula, snow
and blowing snow conditions are still expected through the day
today as gusty northerly flow persists. By Wednesday, clearer
conditions are expected as a much colder airmass arrives from the
north and an upper level ridge builds eastward through Thursday. A
pattern change is expected by Friday with a return of warm
southerly flow as a front associated with a storm force low in the
Bering approaches the southwest coast.

&&

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

A developing storm force low south of the western Aleutians is
visible on satellite imagery this morning. As this low tracks
eastward, its associated front will extend across the Aleutian
Chain, bringing storm force winds across the Bering waters through
Thursday as it travels east. Model solutions seem to consistently
agree in a rapid occlusion of this front, allowing precipitation
to initially fall as a rain/snow mix over the western Aleutians
tonight with a change to snow as colder air fills in on the
backside of the low. Given the strong temperature gradient along
the front, model solutions are suggesting several smaller lows
will develop along the front as it reaches the central Aleutians
by Wednesday night. Meanwhile the parent low will have evolved
into a broad area of low pressure centered over the western Bering
by Thursday. This system will allow active weather to persist
through the end of the week.

&&

.MARINE (Days 3 through 5 - Friday through Sunday)...

Bering: A broad area of low pressure over the western Bering will
produce some easterly gales across the Northern Bering early on
Friday before weakening late day. Friday night into Saturday there
is model disagreement as to how quickly a storm force low will
cross the Eastern Aleutians into the eastern Bering, with risks
being towards stronger easterly winds than currently forecasted.
While the positioning of this low is uncertain heading into Sunday
there is general agreement on winds falling below gales across
most of the Bering.

Gulf of Alaska: A North Pacific low lifting towards the eastern
Aleutians may generate some easterly gales across the
southwestern Gulf of Alaska on Friday before this system moves
west of the area and weakens. Later on Saturday into Sunday
another low in the North Pacific looks to lift into the eastern
Gulf of Alaska. Currently the winds look to remain below gale
force across the western/central Gulf, but if this storm takes a
further westward track there could be a period of gales late
Sunday before the storm weakens.

&&

.LONG TERM FORECAST (Days 3 through 7)...

On Saturday we kick things off with a storm force low just to the
south of the AKPEN. Most guidance points towards this low crossing
into the Bering Saturday morning and then eventually
weakening/being absorbed into a broader low complex. This storm
should bring a rain/snow mix to start, but a quick transition to
all rain for the eastern Aleutians and southern AKPEN.

Late Saturday another storm looks to lift out of the North
Pacific and into the Bering, ultimately aiming for the north Gulf
Coast by Sunday, but with jet support staying well south of the
area, this low looks to be weakening rapidly as it approaches and
it seems most of the precipitation impacts will remain confined
to coastal areas, with the interior staying quiet.

By Monday and Tuesday model agreement falls apart, with little
left in the way of coherent signal to work with. That being said
the primary jet looks to remain well to our south, so odds are the
pattern will remain relatively quiet across much of Mainland
Alaska, with the storm track generally remaining out over the
North Pacific.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning: 173-177 411-414.
Gale Warning: 155 165-172 178 179 185.
Heavy Freezing Spray Warning: 130 160 180 181.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...BG
SOUTHCENTRAL ALASKA...ED
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...RC
MARINE/LONG TERM...BG