Blizzard in Alaska, December 26 to 27, 2020

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
937 PM AKST Fri Dec 25 2020

.ANALYSIS AND UPPER LEVELS...A quieter pattern continues for most
of Southcentral while things start to become more active over the
Bering. The jet is flowing over a ridge across the AK Range at
about 70 kts (300mb or 30,000'). Out west, the jet is pushing a
strong front towards the Western Aleutians as the jet axis moves
from south to north with max winds at 140 kts. It then flattens
out as it moves across the Central Aleutians with core winds of
160 kts.

The 3 primary systems affecting the sensible weather today are the
front approaching the Western Aleutians, a polar trough over the
Central Bering, and a weak system falling apart over SW AK.
Southeasterly winds are just starting to pick up over the Western
Aleutians. The front will bring a big pattern shift across the
Bering. A push of polar air across the Central Bering associated
with a stacked low near the Pribilof Islands continues to make
for very showery conditions. Finally, a weakening system over SW
AK continues to produce snow showers inland with some rain/snow
showers further south (near King Salmon and Iliamna).

&&

.MODEL DISCUSSION...Models are now in very good agreement and
forecast confidence is increasing from moderate to high. There
remain a few areas of uncertainty that we will continue to watch.
There is strong model agreement on the front moving over the
Western Aleutians and into the Bering. They all generally agree
that the surface low will drop to about 935 mb as it tracks
towards the Rat Islands. The exact track of this low is key as the
area of worst conditions will be in a corridor northwest of the
low center. Models are all now hinting at a polar low developing
in the cold air behind the system, and the exact track of this
feature remains highly uncertain. The timing of the front moving
into SW AK late Sat into Sun is being handled well by the models.
Meanwhile, there is a potent surface low tracking across the
northern Gulf of AK through early Sat. Models have now come into
better agreement with this feature. The remaining uncertainty
revolves around the timing of the warm air and whether or not much
snow makes it across the Chugach into Anchorage.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist. A period of
light snow is possible late in the period beginning late Sat
morning to early afternoon. This should start to lower cigs and
vis towards MVFR conditions. If heavier snow develops, cigs and
vis could drop to IFR by Sat evening.

&&

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

A surface low is expected to move into the northern Gulf of Alaska
tonight and then into Prince William Sound Saturday morning. This
low will produce gale force winds across the northern Gulf with
localized areas of storm force winds. Due to cold air and high
pressure over the Copper Basin at the same time this low moves
into Prince William Sound, pressure gradients will be favorable
for enhanced winds in Thompson Pass, Valdez Arm/Narrows and the
Copper Delta. A Blizzard Warning has been issued for Thompson Pass
from 5am AKST Saturday morning through 1pm AKST Saturday
afternoon for blowing snow reducing visibility to one quarter mile
or less with northeasterly gusts up to 45 mph. Total snow
accumulation for Valdez and Thompson Pass will range between
6-12". Some of the light precipitation associated with this low
may reach Anchorage Saturday afternoon and evening as cross-
barrier flow remains weak and thus minimizes the potential for
downslope off the Chugach Mountains. Total snow accumulation for
Anchorage will range from a light coating to an inch of new snow.

A weakening front will move in from the west Sunday night into
Monday morning. This front will bring rain and snow to Kodiak and
the southern Kenai Peninsula. However, there is low confidence on
the magnitude of the winds as well as the precipitation amounts as
guidance shows significant differences in solutions with respect
to the placement and intensity of this front.

&&

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

An upper level trough across Southwest Alaska will keep showers
lingering through early Saturday. Behind this feature, ridging
builds in across the region, giving a brief break in snowfall
before the pattern shifts Saturday night into Sunday morning.

A Winter Storm Watch for blizzard conditions has been issued for
the Kuskokwim Delta starting Sunday morning as a strong front
approaches the Southwest coast. Strong winds and blowing snow will
accompany this frontal system as it initially moves inland.
Expect reduced visibility to one quarter mile at times in blowing
snow from Kipnuk to Hooper Bay, including Nunivak Island. By
Sunday afternoon, warmer air will begin to usher in, allowing for
snow to quickly change over to all rain.

Areas across the Bristol Bay coast will also see strong winds and
snowfall on the onset of the front, but temperatures will be warm
enough for precipitation to switch over to rain or a mix of rain
and snow much earlier on Sunday. Thus, no winter weather products
have been issued for this area. By late Sunday, the frontal system
will begin to weaken through Monday, with the precipitation
transitioning to a mix of rain and snow showers as the system
tracks eastward across Southwest Alaska.

&&

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

Conditions across the central and eastern Bering and Aleutians
will improve tonight as two surface lows weaken. The bigger story
is a storm force low beginning to move over the western Aleutians
this afternoon. Winds will continue to increase overnight for the
western Aleutians. The potential for a combination of significant
falling and blowing snow by late tonight has lead to the issuance
of a Winter Storm Watch for Shemya. Blowing snow and high winds,
including hurricane force gusts, are possible late tonight
through Saturday evening as cold air filters in behind the
occlusion. The GFS is also trying to develop a polar low, which
would increase the chance for blowing snow and low visibility to
one quarter mile at times for this area.

The leading edge of this frontal system will continue its track
eastward, crossing over the central Aleutians and Pribilofs by
Saturday morning. The Pribilofs will also likely see blowing snow
as the front initially moves over the islands, which will then
transition to rain as warmer air moves in. Strong storm force
winds and widespread precipitation will accompany this system as
it works towards the eastern Bering and Alaska Peninsula through
Saturday evening and then to the Southwest coast for Sunday
morning.

&&

.MARINE (Days 3 through 5: Monday through Wednesday)...

Gulf of Alaska: There is increasing confidence in a low tracking
from the North Pacific into the western Gulf on Monday, with a
front crossing the Gulf waters. This will likely produce at least
gale force winds Monday through Monday night, with good potential
for at least a small area of storm force winds. Confidence is low
for Tuesday through Wednesday, though the pattern will remain
favorable for one or more additional lows to move up from the
Pacific.

Aleutians/Bering: Expect a series of gale to storm force lows to
track across the region Monday through Wednesday. It's hard to
say where individual storms will track, but most, if not all, of
the Bering Sea/Aleutians/Southwest Alaska waters will be impacted.

&&

.LONG TERM FORECAST (Days 3 through 7: Monday through Friday)...

Upper level troughing extending from the Bering into the North
Pacific moves through the Gulf next week, fizzling out as it
pushes to the North Gulf Coast. Multiple shortwaves propagating
off a strong Bering low cross south to north across the Southwest
Monday through Wednesday, resulting in bands of precipitation
across the AKPEN and the Mainland. By midweek, the gradient
weakens, transitioning from a closed low to a high amplitude
trough feeding into a North Pacific low. There is weak surface
reflection with this, thus, impacts are expected to be minimal.
The next system of concern will be for a strong low developing
West of Shemya late next week. There is high model discrepancy
with this, where the EC/Canadian models want to move the low to
the north near the Kamchatka Peninsula and become relatively
stationary, while the GFS takes a straight shot eastward into the
Southern Bering. The GFS solution is also significantly stronger,
bringing widespread strong, gusty winds to much of the Bering,
Aleutian Chain and Coastal Southwest. The GFS also shows a
leading front that could result in widespread precipitation across
much of the Mainland with heaviest precipitation likely located
along the AKPEN.

High pressure will dominate the upper levels across Southcentral
through late next week. A short-lived weak surface low tracks
north across the Western Gulf early next week, dissipating as it
crosses the Barren Islands, resulting in some possibly gusty down
Inlet flow.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning 131 155.
         Winter Storm Warning 191.
MARINE...Heavy Freezing Spray Warning: 160,180,181,185.
         Storm Warning: 119,155,170,172-178,180,185,411-413.
         Gale Warning: 150,160,165,171,179,181,414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...MSO
SOUTHCENTRAL ALASKA...ED
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AH
MARINE/LONG TERM...MK/KM