Storm Warning 227, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
544 AM AKST Sun Dec 20 2020

.ANALYSIS AND UPPER LEVELS...

A vertically stacked low continues to slowly weaken as lifts
north from the Southcentral coast to the Copper River Basin this
morning. Any rain/snow mix that fell over the northern Gulf and
along the immediate Southcentral coast yesterday afternoon has
changed to all snow as colder air has wrapped back in behind the
exiting trough. Snow has tapered to flurries this morning from the
Kenai Peninsula to the Mat Valley as the deformation band weakens
in response to the weakening upper-level support. Farther west, a
surface low and associated upper-level shortwave are moving inland
from Bristol Bay this morning. This system is producing areas of
snow showers from King Salmon and Dillingham to Iliamna. Colder
air remains situated north of this systems, with early morning
temperatures well below zero from the middle Kuskokwim valley to
the Kuskokwim Delta.

Over the Bering, a weak ridge is keeping conditions rather
tranquil for this time of year from the Pribilofs to Adak and
Atka. A complex storm-force low southwest of Attu is lifting a
gale-force front over the Western Aleutians and Bering this
morning.

&&

.MODEL DISCUSSION...

The big takeaway from the various deterministic models is that
they are all keying in on what will be a change to a more active
weather pattern beginning late Sunday / early Monday. The models
are in very good agreement through 00z Tuesday (Mon evening) with
not only an initial trough moving through the Copper River Basin
Sunday, but the transit of another shortwave from Southwest to
Southcentral Alaska on the heels of the first. Guidance is well
clustered on the secondary upper-level trough moving across
Southcentral by early morning Monday, with areas of light snow
breaking out once again from Cook Inlet north to the Susitna
Valley as a weak surface low spins up along the eastern Kenai
Peninsula coast. Models have this feature exiting Southcentral by
late Monday as what will then become the main weather feature
takes shape.

Issues with the models center on what is currently a deep, complex
low southwest of Attu. There is high confidence on the forecast
through late Monday as this system and its associated front
slides east today. The GFS/NAM/EC all spin up a low along the
front south of Adak this afternoon and push it north as a sub 955
mb low (GFS is strongest at 945 mb). These three models cluster
the main low over the Southern Bering through Tue before tracking
it east, south of the Pribilofs by Tue night. The differences
appear along its front as it reaches the Southwest coast Monday.
It appears multiple lows will spin up along this front as upper-
level shortwaves lift north along it, and herein lies the issue.
The NAM is a bit faster with the eastward progression of the front
and spins up a deeper low west of Cape Newenham. The GFS and EC
spin up a weaker low later in time (00z Tue) and lift it quickly
north of Nunivak Island. These differences are even more
pronounced along the front draped south of the AKPEN. The NAM spins
up a low farther southwest of the GFS and EC which both deepen a
surface low south of Kodiak Island by Tuesday morning. By late
Tuesday, the NAM has this low southwest of Kodiak Island while the
GFS and EC has this low over Kuskokwim Bay. The Canadian is
somewhere in the middle.

Bottom line, these solutions will affect the timing of
precipitation and the strength and placement of the strongest
winds, with areas of storms likely across the Bering and even
hurricane-force winds across the northern Gulf by late Tuesday.
These differences will also have an influence on just how much
warm air pushes inland, affecting precipitation type for interior
Bristol Bay and interior Southcentral by Tuesday.

There is high confidence, though, that an atmospheric river event
will accompany the front as it moves from the AKPEN to Prince
William Sound by Tuesday. Guidance is keying in on the likelihood
of heavy mountain snow, with snow along the coast changing to
heavy rain. There is also a high likelihood of very strong gap
winds from Portage Valley through Turnagain Arm ahead of the
front.

&&

.AVIATION...
PANC...VFR ceilings and visibility will persist through mid-
morning before the cloud deck lowers to MVFR ceilings. Light snow
is again expected to develop over the terminal this afternoon with
IFR ceilings and visibility likely for a time. Ceilings are
expected to improve to MVFR by early morning Monday as any
lingering light snow tapers off. Satellite imagery this morning
shows the presence of low stratus over northern Cook Inlet. There
is low confidence that this moves over the complex this morning
either as very low stratus or fog; however, VCFG is present in the
TAF to denote this possibility.

&&

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

The extremely active weather pattern for Southcentral continues
through midweek. Seasonably cold temperatures and potential for
another inch or 2 of snow later today into this evening in
Anchorage the Mat-Su will give way to a powerful winter storm from
Monday afternoon through Tuesday night.

Starting with the present and recent past...the snowfall event
that resulted in a daily record setting 8.0 inches of snow in west
Anchorage yesterday is still ongoing across portions of Prince
William Sound and the Copper River Basin this morning. Light to
occasionally moderate snow will persist through the morning in the
Copper River Basin, as it ends over the next few hours in Valdez
and the rest of Prince William Sound.

A second upper level wave, taking advantage of the available
atmospheric moisture and the recent snowfall, will cause another
round of light snow to develop across the Cook Inlet region this
afternoon, which will continue through about midnight. There is
high confidence that this will not be a repeat of yesterday's
snow, as there will not be additional moisture being supplied by
the Gulf, like there was yesterday. Thus, an inch or two of
accumulation through midnight is all that will be squeezed out
for Anchorage and the Mat-Su this evening. Nuisance snow shower
activity is also likely through eastern Prince William Sound with
that same wave crossing the area. With Gulf moisture to work with,
accumulations in Cordova will be a bit higher, in the 2 to 4 inch
range.

Behind this upper level wave there will be a brief break for the
first half of the day Monday, then the next major storm system
will move in. Most of the hazardous weather will be associated
with the leading front, as the parent low stays well west of the
area in the Bering. However, the front will have a tropical tap
a.k.a. atmospheric river of moisture, and will be very strong
itself. Thus, as the front gets hung up along the north Gulf Coast
through Tuesday, it will produce very heavy precipitation. Around
Seward, it will start as snow, accumulating up to a foot Monday
night, then change over to rain, with up to 3 inches of liquid
falling. This may lead to flooding concerns there. Meanwhile, in
the mountains the precipitation should remain all snow at higher
elevations, and will fall heavily, peaking at rates as high as 3
inches per hour Monday night into Tuesday through the Kenai
Mountains.

The other aspect of this front will be the high winds. This will
result from a combination of the front orienting itself parallel
to the coast and the gaps in the mountains, and the front taking
advantage of the very cold air in place now inland contrasting
with the warm moisture feed coming in from the subtropics. Thus,
the front could produce gusts as high as 100 mph along Turnagain
Arm and higher elevations, 85 mph through Portage Valley, 80 mph
along the Upper Hillside, 75 mph through Turnagain Pass, and
55 mph through the Knik River Valley, and 40 mph in south and west
Anchorage Monday night into Tuesday. Therefore, High Wind Watches
have been issued for Turnagain Arm, higher elevations, and
Portage Valley.

A Winter Storm Watch for blizzard conditions has been issued for
much of the Seward Highway north of Seward, including Turnagain
Pass and Portage Valley. The very heavy snow combined with
hurricane force winds will cause severe whiteout conditions that
will persist for hours, in addition to the 2 to 5 feet of snow
expected. By Tuesday, warmer air moving in will change the snow
over to rain through Portage Valley, Seward, and Whittier, and
will warm temperatures well into the 30s to near 40 for Anchorage,
the Mat-Su, and the western Kenai Peninsula.

Uncertainty increases during the day on Tuesday into Tuesday night
further east for Valdez, Thompson Pass, and Cordova. Depending on
how strong and where a low that forms along the front is, high
winds could be pushed inland with the front into Cordova, and will
increase snowfall rates in Thompson Pass, potentially requiring
additional watches and warnings. Stay tuned.

&&

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

The main challenge in this forecast package was to discern the
precipitation type (P-Type) for Bristol Bay in response to the
approaching frontal system from the Bering between Sunday night
and early Monday morning. Though warmer temperatures from the
front should remain near but mainly south of the Aleutians and
AKPEN through at least Monday night, the potential for snow
transitioning to rain for Bristol Bay is credible and it may be
brief too. At 850 mb, very cold temperatures near -16 degrees
Celsius dips right over Kuskokwim Bay from the northern Bering by
this afternoon. This cold air advection advances eastward and
across Southwest Alaska through tonight until the warm boundary of
the frontal system steps across the eastern Bering tonight, which
will cause the departure of the cold temperatures from the teens
to near single digits as early as Monday morning. The southerly
flow mainly over Bristol Bay will also cause warm air to move
farther inland, and downsloping will take place as well over the
leeward side of Chigmit Mountains. In the meantime, steady light
snow continues over much of the coastal area and farther inland,
south of the Kusko Valley and Nushagak Hills. Strong winds
associated with the frontal system will also move across AKPEN and
Bristol Bay between Monday and Tuesday. There's a good chance for
increased blowing snow to occur over portions of Bristol Bay and
Kuskokwim Bay by overnight Sunday night, and should conditions
worsen it could lead to diminishing visibility into the early
hours of Monday morning. Will keep an eye on this situation and
provide updates in the next forecast package.

&&

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

The onset of the North Pacific storm over the central
Aleutians/Bering around noon today will elevate winds, seas, and
precipitation for the rest of eastern Aleutians and Bering in the
coming days. This storm and its strong frontal boundary would most
likely boost winds over Cold Bay just a bit higher by Sunday
night and Monday than the forecasted winds. Strong southerly winds
from the front will trend upward especially along the Gaps and
Passes around Cold Bay. For this reason, will monitor the
observations closely and issue updates should winds worsen there.

&&

.Marine (Days 3 through 5: Tuesday through Thursday)...

Gulf of Alaska
A storm force low will be present in the Gulf on Tuesday, with a
hurricane force barrier jet set up possible along the Southcentral
coast. This low is expected to exit early Wednesday, with
southerly winds remaining. There is moderate confidence in another
low entering the Western Gulf on Thursday. This low looks to move
north from the Pacific and looks to be at least gale force in
strength. Overall, an active weather pattern is in place for the
Gulf of Alaska.

Bering Sea:
A broad area of low pressure is expected to be in place over the
Bering Sea on Tuesday, with gale force winds possible. This low
should weaken and move north out of the Bering Wednesday, with
generally calm conditions present through Thursday. There is at
least moderate confidence in this forecast. A possibility of a
compact low moving right along the Western Aleutians exists on
Thursday, but most weather associated with this low looks to
remain south of the islands. However, there is still uncertainty
with this system so adjustments may be needed.

&&

.Long Term (Days 4 through 7/: Tuesday through Saturday)...

An active weather pattern will continue into the long term. A
storm force low with a hurricane force barrier jet will be present
in the Gulf of Alaska Tuesday. Broad low pressure will be present
in the Bering, also with storm force winds possible. As the week
progresses, another low looks to move north into the Gulf of
Alaska on Thursday. This low looks to be at least gale force in
strength, but its exact track is not determined yet. The Bering
looks to quiet some midweek. A large and powerful low then pulls
into the Western Bering on Saturday. While this low is a ways off,
it preliminarily looks to be storm force in strength.

It should be noted that models continue to have difficulty in the
long term, so confidence in the long term tops out at a moderate
confidence level. We are confidence in the presence of the major
low pressure systems mentioned in the previous paragraph, but
adjustments will need to be made to location and track. Due to
uncertainty, an ensemble approach was used in the long term with a
blend of the GFS and ECMWF, which was collaborated on with WPC.
Therefore, we anticipate leaning on WPC grids, especially with the
low in the Bering late next week. To sum it, active weather
continues with some major low pressure system anticipated for our
forecast area later this week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Watch 101 125.
		 Winter Storm Watch 125.
MARINE...Storm Warning 155 165 170-174 176 177 179 181 185.
		 Gale Warning 130 132 138 150 160 175 180 352.
		 Heavy Freezing Spray Warning 160 180 181 185.
FIRE WEATHER...NONE.

&&

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

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
906 AM AKST SUN DEC 20 2020

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0854 AM     SNOW             WASILLA                 61.58N 149.45W
12/20/2020  M1.1 INCH        MATANUSKA-SUSITNA  AK   NWS EMPLOYEE

            SPOTTER REPORTED 1.1 INCHES OF SNOW.

&&

MV

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
1136 AM AKST SUN DEC 20 2020

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0356 PM     SNOW             WEST ANCHORAGE          61.15N 149.93W
12/19/2020  M8.0 INCH        ANCHORAGE          AK   NWS EMPLOYEE

            CORRECTS PREVIOUS SNOW REPORT FROM WEST
            ANCHORAGE. SPOTTER REPORTED 8 INCHES OF
            TOTAL SNOW.

&&

MV

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
1137 AM AKST SUN DEC 20 2020

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0937 AM     SNOW             NORTHEAST ANCHORAGE     61.21N 149.78W
12/20/2020  M6.0 INCH        ANCHORAGE          AK   NWS EMPLOYEE

            CORRECTS PREVIOUS SNOW REPORT FROM NORTHEAST
            ANCHORAGE. SPOTTER REPORTED 5.8 INCHES OF
            TOTAL SNOW.

&&

MV

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
1138 AM AKST SUN DEC 20 2020

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0937 AM     SNOW             NORTHEAST ANCHORAGE     61.21N 149.78W
12/20/2020  M5.8 INCH        ANCHORAGE          AK   NWS EMPLOYEE

            CORRECTS PREVIOUS SNOW REPORT FROM NORTHEAST
            ANCHORAGE. SPOTTER REPORTED 5.8 INCHES OF
            TOTAL SNOW.

&&

MV

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
1139 AM AKST SUN DEC 20 2020

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1003 AM     SNOW             10 WNW ANCHORAGE        61.24N 149.44W
12/20/2020  M4.8 INCH        ANCHORAGE          AK   NWS EMPLOYEE

            SPOTTER REPORTED 4.8 INCHES OF NEW SNOW FROM
            4 AM SATURDAY TO 9 AM SUNDAY.

&&

MV