Brisk Wind Advisory 66, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
559 AM AKST Mon Dec 21 2020

.ANALYSIS AND UPPER LEVELS...

An upper-level wave is moving across the Copper River Basin this
morning. At the surface, a weak 1000 mb low over Prince William
Sound is slowly drifting to the northeast. These features are
allowing a few scattered snow showers to persist across the
immediate Southcentral coast, especially from Valdez to Cordova.

Farther west, the next storm system is taking shape as a frontal
system moves from Southwest to Southcentral Alaska. This front is
attached to a storm-force 948 mb low sitting nearly stationary in
the southern Bering. The first wave of clouds and precipitation
is pushing from Bristol Bay to the Barren Islands ahead of the
occluded front still draped west of the Southwest coast. Both IR
and Water Vapor imagery indicates a deep tropical feed associated
with this system with a moisture feed lifting from north of
Hawai'i and cooling cloud tops along the front. A shortwave is
also visible via the water vapor imagery with the beginnings of a
surface circulation along the front. This feature favored to
strengthen as it sits underneath the left exit region of a 180kt
jet streak.

Snow and blowing snow is also beginning this morning across the
Kuskokwim Delta coast as the front lifts toward the region. Winds
are winding down and cloud cover decreasing across the Pribilofs
in the wake of the aforementioned front. Farther west along the
Chain, scattered clouds and showers continue underneath an
elongated trough of low pressure.

&&

.MODEL DISCUSSION...

Models are continuing to converge on a common solution regarding
the passage of the initial front across the Gulf and the
subsequent areas of low pressure lifting north into the western
Gulf toward the Kenai Peninsula. There are still timing and
strength issues with the first low as it tracks to Kodiak Island
by Tuesday morning. The 6z GFS and 12z NAM have slowed the
transit of this low down by about three hours, with the NAM about
three hours faster in its timing then the GFS. The 00z GEM and
00z EC are similar in timing to the GFS, but weaker with the low.
All guidance have this first low moving toward the Barren Islands
by late Tuesday morning. There are still issues with the second
low for late Tuesday into Wednesday. However, the 6z and 12z NAM
solutions have come into much better agreement with the 00z and
6z GFS, albeit again faster in timing, but still lending a bit
more confidence in the stronger GFS solution, even if the GFS is
perhaps a bit too strong with the low when compared to other
guidance. Nonetheless, it appears that a deep low will track to
somewhere between Kodiak City and Seward by late Tuesday night and
up Cook Inlet Wednesday morning.

Farther west, models are in decent agreement with the evolution
of the storm-force low in the southern Bering through late
Tuesday. Guidance wants to drive it northeast to the Southwest
ALaska coast after that time; however, the 12z NAM is faster and
farther north with the low than the 6z GFS. Both the NAM and GFS
now have a low developing along the front associated with the
storm-force low, lifting parallel to the Southwest coast through
early morning Tuesday. The NAM is faster and weaker, but again
has come into better agreement with both the GFS and GEM with this
system.

Needless to say, however the details pan out, models are all
indicating a very active weather pattern across, not only the
Southern Mainland, but parts of the Aleutian Chain through at
least midweek.

&&

.AVIATION...
PANC...VFR conditions with ceilings at or below 5,000ft are
expected throughout the day. A period of light snow is possible
from mid-morning to around noon today as moisture moves up Cook
Inlet. Any snow that does develop may briefly drop conditions to
MVFR. The biggest issue in the near term will be winds, both at
the surface and aloft. Northerly wind are expected through early
Tuesday morning; however, strong southeasterly winds will develop
above the surface beginning late morning. LLWS is expected from
mid morning through early Tuesday morning. By Tuesday morning,
LLWS is expected to diminish as gusty southeasterly wind develop
over the airport complex.

&&

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

An incredibly strong and active weather pattern continues through
Wednesday for nearly all of Southcentral. Here is a breakdown of
the changes made to the forecast since yesterday:

Starting with Anchorage, the High Wind Warning issued yesterday
evening has been updated with further increases in the potential
wind gusts for the entirety of Anchorage. The main change, in
addition to the winds, is the increased potential for mountain
waves and rotors developing downwind of the Chugach, which will
take advantage of the favorable atmosphere to potentially produce
even stronger gusts. Thus, for Turnagain Arm and higher
elevations, peak wind gusts could reach 115 mph and for the
Hillside, peak wind gusts could reach 100 mph. For the Anchorage
Bowl, the downsloping from the mountain wave activity could drive
some of that wind into town, down to sea level. Thus, gust
potential across the Bowl has been increased to 60 mph. These
winds are most likely along Turnagain Arm, but could also occur
in the mountain gaps across east Anchorage, Eagle River Valley,
and Eklutna as well. The peak winds are most likely Tuesday
morning, during the commute. As the winds are ramping up Monday
night but before the warmest air arrives, blowing snow will reduce
visibilities along Turnagain Arm and higher elevations as well,
causing potentially treacherous driving conditions.

Across the Matanuska Valley, winds will increase down the
Matanuska and Knik River Valleys, resulting in gusts to 60 mph
Tuesday morning. A Special Weather Statement has been issued for
the gusty winds. Temperatures will warm into the 40s on Tuesday.
On Tuesday night into Wednesday morning, the cold air will return
in earnest behind a strong low tracking north up the Susitna
Valley. This could produce a brief period of potentially heavy
snow in addition to gusty winds out of the southwest leading to
blowing snow and potentially reduced visibilities.

For the Western Kenai Peninsula, the main threat will also be on
the back side of low as it tracks up Cook Inlet Tuesday Night into
Wednesday. Gusty southwesterly winds and periods of potentially
heavy snow will also cause reduced visibilities and gusty winds,
particularly from Homer Bluff north to Clam Gulch, though snow
will impact all of the Western Kenai Peninsula. See the Special
Weather Statement for more.

The Eastern Kenai Peninsula will have the worst weather of all. In
addition to winds gusting to 90 mph through Portage Valley, strong
winds will impact much of the mountains while very heavy snow
falls Monday night into Tuesday morning. Thus, the Winter Storm
Watch has been upgraded to a Blizzard Warning. Fortunately, the
latest guidance shows conditions will be a bit warmer on Tuesday,
thus expected lower elevations to Turnagain Pass level to change
over to rain by midday Tuesday. While this may improve travel
conditions, there will be no such luck for the avalanche threat.
The High Wind Warning issued yesterday evening remains unchanged.

For the Cordova area, an impressive hurricane force barrier jet
will develop offshore during the day on Tuesday. However, when a
strong low develops and moves north Tuesday evening, it will drag
a front with it that may push the barrier jet into Cordova for a
brief time Tuesday night, most likely around midnight. There
remains considerable uncertainty as to the timing and how strong
the barrier jet will be when this occurs, so for now have issued a
High Wind Watch to highlight the possibility of hurricane force
wind gusts through town. Stay tuned.

The Western Susitna Valley and Thompson Pass will not feel the
full force of the active weather, but periods of heavy snow are
likely, particularly along the Alaska range west of Skwentna. The
heaviest snow will be during the day Tuesday west of Skwentna. For
Thompson Pass, the heaviest snow will be Tuesday night into
Wednesday. The south to southeasterly flow through Thompson Pass
is not favorable for particularly strong winds, and warm
temperatures should keep the snow heavy, and thus not likely to be
resuspended.

I'm sure there's more that could be discussed but that's enough
for now. Take care and stay safe, Southcentral!

&&

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

The warmer feature of the front from eastern Bering and southern
AKPEN will spread across Bristol Bay and farther inland this
morning. The upper level ridge over Southwest Alaska will block a
couple shortwaves from penetrating eastward past the coastline
within the next day or two. This will keep hazardous conditions
including blowing snow, strong winds and higher gusts along
Bristol Bay through at least Monday evening and over Kuskokwim Bay
through at least early Tuesday morning. Issuance of Winter
Weather Advisories for Kipnuk to Hooper Bay and for the north and
west of King Salmon reflects this situation just under 24 hours.
Cold air advection will retreat once the warmer area of the front
tapers off from the region by Tuesday night.

The transition of snow to rain and back to snow will become
noticeable throughout today and tonight as warm temperatures
extend from AKPEN northwestward to Kusko Delta. The snow
accumulation is expected to be a lot higher along the higher
elevations of Wood River Mountains in the coming days, with the
12-hour snow amounts reaching up to 10 to 15 inches. Overall, mix
rain/snow and very strong gusty winds will impact the coastal
areas today and Tuesday.

&&

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

The upper low over the Bering is not in any rush to evacuate its
position yet until Tuesday morning. The upper level ridge ahead of
this system will delay the forward motion of the low across the
eastern Bering. This phenomena will steer developing shortwaves
across the Bering and Aleutians through the forecast period, in
which the potential for increasing snow is feasible across the
central and eastern Aleutians in the next two days. Essentially,
gales will persist over much of the Aleutians and Bering Sea
through the forecast period.

&&

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

Gulf of Alaska
There is good agreement on steady improvement of conditions across
the Gulf on Wednesday as the major Monday/Tuesday storm begins to
track inland and weaken. Southerly winds will be found across
much of the Gulf, with all areas falling below small craft levels
by late morning. Wednesday night into Thursday there is
significant disagreement on the strength and track of a North
Pacific low lifting into the Gulf. If the stronger and further
west solution indicated by the GFS works out, southerly gales may
return to the western Gulf by late Thursday. By Friday there is
better agreement on all areas once again seeing winds fall to
small craft or weaker.

Bering Sea:
There is good agreement on a weakening broad low across the
Bering on Wednesday which should result in fairly tame winds. On
Thursday there is disagreement on the placement of a compact
surface low near or just south of the central Aleutians. A small
areas of gales or even briefly storm force winds appear possible,
but the exact placement is difficult to pin down at this point. On
Friday uncertainty increases even further with the potential for
a Gale force front pushing into the western Bering/Aleutians, but
the GFS is significantly weaker with this front versus other
models.

&&

.Long Term (Days 4 through 7/: Thursday through Sunday)...

Active weather looks to persist right into the long term. On
Thursday a broad upper level low looks to extend across much of
the Bering. A shortwave rounding the base of this trough looks to
support the development of a compact storm force low somewhere
near the central Aleutians. Further east, a negatively tilted
trough lifting out of the North Pacific will aid in the
development of another surface low that will lift into the western
Gulf of Alaska and spread another round of precipitation across
the AKPEN, Kodiak and eventually much of the Northern Gulf Coast.

For Friday the details become much more murky, but the general
idea is slightly less active weather taking over for Southcentral.
Meanwhile another gale or storm force front looks to push into the
western Bering, bringing wind/rain to the western Aleutians.

For Saturday and Sunday a deep upper trough looks to dig into the
western Bering/Aleutians with the development of a strong system
likely out ahead of it. The placement of said system remains a
huge question mark with solutions split towards this being a
wind/wave generator for the Bering, or yet another strong low
moving into the western Gulf.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning 125
		 High Wind Warning 101 125
		 Winter Weather Advisory 155 161
		 High Wind Watch 135.
MARINE...Hurricane Force Wind Warning 119 125.
		 Storm Warning 120 130-132
		 Gale Warning 129 136-141 150 155 160 170 172-177 181 185
		 Heavy Freezing Spray Warning 181 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
MARINE/LONG TERM...BG