Heavy Snow in Alaska, December 19 to 20, 2018

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
507 AM AKST Wed Dec 19 2018

.ANALYSIS AND UPPER LEVELS...

Since yesterday, the trough over Southwest Alaska extending over
Kodiak has now become a newly formed closed low over the western
Gulf east of Kodiak City. The remaining trough still persists
across Bristol Bay and extending northwestward over the Kuskokwim
Delta. This has made the entire upper level trough become very
negatively tilted, forming a boomerang shape. Meanwhile across the
Bering, a very pronounced deformation band has developed, which
extends southeast to northwest across the Cold Bay area and out
into the western Bering. This is a classic boundary signature of
cold air to the north and east of the boundary deflecting the
incoming warmth and moisture from the south into a line
perpendicular to the incoming moisture. In the upper levels, this
ridging is rather weak. To the west, heights actually drop due to
an incoming trough associated with an even deeper upper level low
centered over the southern Sea of Okhotsk. The ridge in between
the incoming trough and the trough over mainland Alaska is
essentially being squeezed out and elongated with time, a feature
common with distinct deformation zones.

The upper low over the Gulf slowly tracking northward with time
is enhancing lift and upper level divergence ahead of it. In this
case, that means along the Gulf Coast and the Kenai Peninsula.
Thus, those areas are continuing to see steady light snow. Even
Cordova has cooled enough to get all snow. The key is the
strongest lift, and thus, heaviest snow are concentrated along the
Gulf Coast and Prince William Sound, with a rather distinct
northern edge, which is approximately along an Anchorage to Valdez
line. The upper flow out of the east is helping keep Anchorage and
points north relatively drier due to cross-barrier flow across the
Chugach. Conversely, snow extends further north across the
southern Susitna Valley due to upslope.

Temperature-wise across Southwest, we are seeing conditions
broadly below zero at most inland locations. That said, those
sites with a significant wind, such as Iliamna are staying above
zero. A fair bit of stratus is moving southward across the
Kuskokwim Delta south of Bethel. The cold spots this morning are
Aniak and King Salmon, both of which are well colder than negative
20 with calm winds and mainly clear skies.

&&

.MODEL DISCUSSION...

As has been the case the past several days, the models are in
generally good agreement on the placement of the large scale
features, but how those translate to the surface is very much in
disagreement. Light steering level winds ultimately will make any
surface lows that form in the Gulf move very slowly. Thus, any
changes in the weather would be slow. Additional disagreements in
the models appear by Thursday regarding a front moving into the
western Bering and where any associated surface lows develop. In
addition, the models are having a difficult time deciding even by
this afternoon how quickly a stationary surface low south of
Seward begins to move eastward and how quickly any secondary lows
form. Confidence even with as early as today's forecast regarding
timing and location of snow and any associated wind shifts
remains unusually low for how short the time frame involved is.
Quieter weather returning Thursday and beyond to Southcentral will
help improve forecast confidence as concerns turn to gap winds and
how cold the temperatures get.

&&

.AVIATION...
PANC...Scattered snow showers are producing brief periods of MVFR
conditions, but conditions are expected to steadily improve over
the next few hours as the snow tapers off. A few more snow showers
may get as close as the Hillside later today, but otherwise
offshore winds will dry out the area with predominately VFR
conditions expected for the rest of the TAF period.

&&

.CLIMATE...

Anchorage International Airport tied its record consecutive days
with measurable snowfall this morning at 9 days. This is the first
time in the 21st century this has happened. Below are the other
streaks of 9 consecutive days of measurable snow and the total
snow that fell during those 9 days:

1) December 11-19, 2018: Total Snowfall: 21.7 inches
2) December 4-12, 1991: Total Snowfall: 13.0 inches
3) November 26- December 4, 1987: Total Snowfall: 19.3 inches
4) February 16-24, 1968: Total Snowfall: 23.4 inches
5) March 31- April 8, 1956: Total Snowfall: 10.6 inches

&&
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The upper level low over the western Gulf will slowly slide
northeast today and weaken. The occluded front associated with
this low is currently draped across the north Gulf coast into the
Kenai Peninsula, and will also move north and weaken today. This
front will cross the Alaska range this afternoon. As a result of
this, snow showers will diminish across inland portions of the
area, ending for most areas in the interior this evening. Snow
and snow showers will linger through tonight into Thursday morning
along the north Gulf coast and Kenai Peninsula as another system
wraps around the upper low and sends some more moisture into
those locations. The upper low will move inland into the Copper
River Basin on Thursday, with dry and cold conditions expected
through Thursday night.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3/Wednesday
through Saturday)...
Cold air continues for southwestern Alaska. Low level
northeasterly flow will keep the cold, dry air in place into the
weekend. Some areas of disorganized stratus continue pass through
the region, which will warm temperatures when they move over a
given location. Predicting exact movement of these cloud decks is
difficult, so have just gone with the coldest possible
temperatures at each location.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through
3/Wednesday through Saturday)...
The northerly flow pattern in the Bering Sea has finally ended.
A northwesterly push associated with a low moving over the eastern
Aleutians is subsiding. This low prompted a winter weather
advisory yesterday that was valid until midday today. However,
conditions have tapered off overnight, so cancelled the advisory
with this morning's forecast package. Expecting winds to continue
to diminish over the eastern Aleutians today, and that should be
the last of the northerly push.

The low that helped to end the great northerly push is currently
working its way through the western Aleutians. It is moving winds
ahead of it to a more southeasterly direction, which is providing
some warming along the island chain as well. It will move along
the southern side of the Aleutian Islands the next couple of days.
Warm air advection will make it as far north as the Pribilofs,
where rain will mix in with snow today and tomorrow. A weak low
spins up in the middle of the Bering tomorrow, with some gale
force winds on the north side of it. It is an interesting feature
that the models don't quite know what to do with yet, so will have
to keep an eye on it to see what happens.

&&

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

Friday's setup is looking similar to the model forecasts
yesterday with a low south of the AKPEN moving into the southern
Gulf, bringing gusty offshore winds to the north Gulf coast and
favorably aligned terrain gaps through Saturday afternoon. A
trough rotating through the Gulf Saturday night and Sunday may
bring some precipitation to coastal areas then lift north on
Monday bringing a chance of snow to the Copper River Basin.

There is more confidence with a front moving over the western
Aleutians and Bering on Friday due to better model agreement
today. Strong winds will remain possible at times in the western
Bering through Sunday. A deep low on Sunday will move north over
the western and central Aleutians, which models have come in
better agreement on since yesterday. Better confidence in this low
brings increasing chances for gales over the Aleutians and Bering
through early next week. The exact intensity and placement of
this low has decent agreement, but is less certain as it moves
eastward through Tuesday. Stay tuned over the next few days as
this feature continues to be looked at.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storm 130
         Gales 131 132 150 155 172 174 178 185 352 412
         Heavy Freezing Spray 130 138 160 180 181 185

&&

SYNOPSIS AND MODEL DISCUSSION...JPW
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BB
LONG TERM...AH/JR