Blizzard and Heavy Snow in Alaska, December 28 to 31, 2018

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
630 AM AKST Fri Dec 28 2018

.ANALYSIS AND UPPER LEVELS...

The first synoptic feature to note is a negatively tilted long
wave trough that extends over the central and eastern Bering. An
increased band of absolute vorticity runs the length of the trough
axis. The increased vorticity coincides with a northwesterly
oriented arm of the jet where wind speeds are reaching 220 kts.
Cold, polar air is wrapping around the trough and advecting over
the entire Bering. This cold air is creating a polar front
positioned over the AK-Pen with an associated surface low just
south of the AK-Pen. The influx of cold air near the surface and
the warm air aloft is creating an inversion layer over the western
mainland. This has aided in fog development throughout the area.

A second and more intense front is moving over the western
Aleutians, which marks the start of the next storm in the Bering.
This front is associated with a deepening North Pacific low that
is advancing northward towards the chain. Scatterometer wind
detection has already observed storm force winds and a few
hurricane force gusts just offshore of the western most islands.

In the east, a short wave runs the length of the northern Gulf
coast. The deformation zone that brought snow Thursday afternoon
to Southcentral continues to to propagate eastward leaving much
of Southcentral dry. Similar to the western side of the state, an
inversion layer, which can be seen in the Anchorage morning
sounding, is creating fog through Cook Inlet, the western Kenai
Peninsula and through the Susitna Valley.

&&

.MODEL DISCUSSION...

Synoptically, models are in decent agreement while when it comes
to the more important details, there is a lot of discrepancy. They
are handling the low south of the AK-Pen well through Saturday
when it moves into the Gulf and dissipates. However, with regards
to the next intensifying low, models differ greatly. In the
initial stages, models are fairly consistent with the low
location as it moves towards Shemya, but the strength of the low
still remains vastly different. Through Saturday however, the
solution is fairly consistent both in location and strength. In
general, the GFS is the strongest of these solutions producing the
highest wind speeds while the EC is the weakest. This pattern
continues through the next few days. A second low system forms
along the frontal edge of the initial storm force low Saturday
night. The models diverge even More so with the GFS taking a much
stronger route than any of the other solutions. Due to the
differences between all of the solutions regarding this system, a
blend was used of all of the models.

&&

.AVIATION...
PANC...The low stratus deck currently over the Anchorage area
should help lower the chances of surface fog forming this morning,
however there is still of chance of visibilities dropping more as
well. A frontal wave lifting north late afternoon through this
evening will bring the chance of some light snow, however this
should primarily be west of the area with conditions likely
remaining VFR at PANC. A second trough with strong cold advection
moving through after midnight will bring a more widespread cover of
snow to the area, dropping conditions to IFR or lower at times
through Saturday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Today
through Saturday night)...

As one storm exits across the Copper River Basin this morning,
another is beginning to impact Kodiak Island and the western Gulf.
Behind the first one, there are some lingering areas of light snow
across the southeastern Copper River Basin as well as eastern
Prince William Sound. Further west, low clouds with some embedded
fog extend across much of the Cook Inlet to Susitna Valley corridor.
Expect improvement later today as clouds and precipitation move in
from the south.

The next system consists of a strong upper level short-wave trough
over the eastern Bering Sea, a low level Arctic front just ahead
of the trough, a weak low lifting northward along the Alaska
Peninsula, and a separate weaker short-wave tracking northward
into the western Gulf. The interaction of all of these is quite
complicated and results in a challenging precipitation forecast
for Southcentral this afternoon through this evening. Precipitation
with the first short-wave will largely be of a showery character,
with steadier and locally heavier precipitation right ahead of
the short-wave. With cold air in place, expect precipitation type
to be all snow except for Kodiak island and the immediate coastline.
Snow showers will spread northward this afternoon and evening to
all but the Copper River Basin. A band of heavier snow will then
develop overnight as the much stronger trough becomes negatively
tilted and approaches Southcentral Alaska. It will move relatively
quickly, so storm total snow amounts through Saturday will
generally be on the light side.

Strong cold air advection across the Alaska Peninsula and into the
western Gulf will produce gale to storm force winds, with very
strong gusts through gaps in the terrain. Winds coming through
Kamishak Bay will bend northeastward to Kachemak Bay and Homer.
With cold advection kicking in there before the upper trough
passes, this looks like a great scenario for the combination of
falling snow and strong winds in Homer and vicinity. Thus, have
issued a Blizzard Warning for overnight tonight. With such strong
cold advection, would also expect low level showers to persist
behind the trough, so snow and blowing snow will remain a threat
through at least midday Saturday.

This system will exit to the north by late Saturday while a
frontal boundary originating from a deep Aleutian Low approaches
the western Gulf and Kodiak Island. With multiple systems forming
out over the bering Sea, there is quite a bit of uncertainty in
the details of this front. What we do know is that the shift in
pattern will lead to a warm-up in temperatures as we head to early
next week.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Today
through Sunday)...

A very busy weather pattern has begun. An area of low pressure has
crossed the Alaska Peninsula and is currently near Pilot Point.
As the precipitation associated with the low meets up with an
Arctic front pushing inland across the Kuskokwim Delta, snowfall
rates are expected to increase from Dillingham northward today as
the two systems combine forces. Accumulations of 6 to 11 inches
are expected across this area. The low will dissipate this evening
as the Arctic front continues to slide eastward across Bristol
Bay and the Lower Kuskokwim Valley. This will lead to diminishing
snowfall rates as the entire area of snow moves east into Iliamna.
The snow will dissipate altogether later tonight. Behind the
Arctic front, as the name implies, very cold air will set up shop
for a brief time across Southwest Alaska, as temperatures once
again fall into the teens below zero tonight and on Saturday
night. The front should have some success, especially across
inland areas, with clearing out the clouds as the winds diminish
behind the front. Lots of sunshine is expected away from the coast
for at least the first half of the day on Saturday.

A strong front originating from a strong low over the Bering is
expected to begin making its presence known over Southwest Alaska
by Saturday afternoon in the form of increasing cloud cover
starting at the coast and moving inland Saturday evening. Snow
will begin at the coast on Saturday night, and will spread inland
through the overnight hours. Meanwhile, the strong front will also
feature strong winds with it, which will have some success moving
inland, especially across the Kuskokwim Delta. The combination of
the snow and the wind are expected to make for near-blizzard
conditions across the area Saturday night into Sunday. This front
will also bring warmer air along with it, so that will end the
threat of below zero temperatures for all of Southwest Alaska for
the time being. The areas that are most likely to have impactful
weather will be the Western Capes of Bristol Bay west of
Dillingham, and the Kuskokwim Delta. The Western Capes will likely
see winds of 35 to 50 mph, while winds of 20 to 30 mph with higher
gusts are expected in the Kuskokwim Delta. All the while, the
moisture-laden front may at times produce moderate to heavy snow
in those same areas. The models are still having a very hard time
with this system, and as such, pin-pointing further where
blizzard conditions can be expected is not prudent at this time.
This is largely in relation to where a new low forms, where it
tracks, and how strong it will be as it moves off the coast of
Southwest Alaska. The front pushing inland across Southwest Alaska
will lead to another widespread snowfall for most areas away from
the Alaska Range, though the most snow will be in the
aforementioned areas. Meanwhile, strong downsloping and warming
will lead to some above freezing temperatures and possible rain
along the Alaska Range through Iliamna and King Salmon. The warm
temperatures look to spread to large portions of Bristol Bay by
Sunday night as yet another strong low with even warmer air looks
to affect the area. Some areas of Bristol Bay may only have a
brief pause in the precipitation between the systems Sunday
morning and Sunday night. Stay tuned.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3: Today
through Sunday)...

A strong front currently over the western and central Aleutians
is currently causing storm force winds across the islands and
their adjacent waters this morning. This front is just the start
of a period of very active weather for that area. The front will
slow to a crawl and slowly weaken during the day today, so many
of the same areas seeing strong winds now will continue to do so
throughout the day. A secondary low is approaching the front from
out of the Pacific. This low will rapidly intensify as it merges
with the front late this afternoon and this evening. Thus, the
winds will reintensify across the western Aleutians tonight, as
they turn more into the northeasterly direction. Hurricane force
winds associated with the low are expected both north and south of
the western Aleutians, and as such, Hurricane Force Wind Warnings
have been issued for the western Aleutians and the central
Aleutians west of Adak. The strong winds affecting Shemya will be
on the north side of the low. On the south side of the low,
strong cold advection and the strong winds associated with the
low itself will cause even stronger gusts south of the islands
out of the southwest. With a long Pacific fetch included, waves on
the Pacific side of the islands may approach 50 feet with the
strongest winds in the portion of the low south of the center
known as a "sting jet", which is known to be associated with very
strong winds. Adak proper looks to be right on the
leading edge of the sting jet. As such a small eastward shift in
the storm track may bring hurricane force winds to Adak itself,
but there is a lot of uncertainty if that will happen. Thus, the
High Wind Watch has been maintained for now until there is more
certainty whether Adak will be impacted with the strongest winds.
The low will stall out with the center near the islands through
the day on Saturday, slowly weakening with time through the day on
Sunday.

Heavy snow is possible with the leading front ahead of the low
over the western Aleutians as it tracks eastward across the
Bering. Cold air is firmly in place across the Pribilofs, and as
the front approaches and it begins snowing, the strong winds and
snow will combine to result in periods of reduced visibility as
the front goes through. A Winter Weather Advisory for blowing snow
has been issued for the Pribilofs for this. Fortunately this
looks to be a relatively short time when the worst conditions are
expected across the islands since the front will continue its
march eastward towards Southwest Alaska.

&&

.LONG TERM FORECAST (Days 4 through 7)...

A wet pattern is on tap for this forecast period. The forecast
period commences New Year's Eve day with a broad trough over the
Bering and Aleutians and a high amplitude ridge from the Eastern
Pacific to the Yukon Territory. Meanwhile at the surface at 12z
Monday, both the GFS and the ECMWF have a potent low west of Adak
and another low over SW AK/Bristol Bay with cyclonic flow
dominating the entire Bering and much of the North Pacific. At 500
mb at 12z Monday, both the GFS and the ECMWF have closed lows in
support of the surface lows with an amalgamation of embedded
shortwaves aloft.  Complicating matters, the GFS has an
additional closed low at 500 mb north of St Paul whereas the ECMWF
has a minor ridge axis. While the timing of the precipitation
shield may vary as it moves from west to east, what's noteworthy
is that warm air will be advected into the Aleutians, Southwest
Alaska and portions of Southcentral. By Tuesday, a new surface
low will spin up south of the AKPEN and this storm is headed to
the Gulf Of Alaska. By 00z Wednesday the GFS and the ECMWF progs
this new surface low 200 NM apart from the other at opposite ends
of the GOA. The timing of the sensible weather highlights are
dependent the placement of the lows that develop and on the
ensuing track.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning: 121.
         High Wind Warning: 191.
         High Wind Watch: 187.
         Winter Weather Advisory: 152 161 195.
MARINE...Hurricane Force Winds: 177, 178.
         Storm: 130 175 176 411 412 413.
         Heavy Freezing Spray: 160 179 180 185 412 414.
         Gales: 119 120 131 132 138 139 141 150 155 165 170 172
                173 174 179 180 185 351 352 414.

&&

SYNOPSIS AND MODEL DISCUSSION...RA
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JPW
LONG TERM...PS