Blizzard and Avalanche in Alaska, February 7 to 10, 2020

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
429 PM AKST Fri Feb 7 2020

.ANALYSIS AND UPPER LEVELS...
There is an upper-level low situated over the YK Delta this
afternoon. At the surface, a low south of Atka continues to swing
and an occluded front over the AKPEN. This low continues to track
to the north and east. Strong winds across the Aleutians have
started and will continue to increase through the afternoon. On
the western side of this front is strong northerly flow.

Across the mainland, areas of light snow developed over the Kenai
Peninsula this morning, and continue to track east. Clouds
continue to linger in the moist southwesterly flow behind the band
of snow.

&&

.MODEL DISCUSSION...
Synoptic model agreement in the short term remains consistent.
All models handle the upper level low over Bristol Bay sliding
north, weakening and getting absorbed with a low across the Seward
Peninsula. Into the weekend, all models are in generally good
agreement with another strong western Pacific low and attendant
front moving across the Aleutians. While the general picture is
present with all of the major models, there are considerable
differences in timing and strength, causing confidence in the long
term specifics to remain low.

&&

.AVIATION...

PANC...Snow moved overhead this afternoon, but never really reach
the surface at the terminal. Thus, it appears VFR conditions will
persist. Expect increasing Turnagain winds tonight and into
tomorrow which will cause ceilings to rise. Wind sheer concerns
arise at the onset, as winds at 2k ft will be considerably
stronger than at the surface. Into Saturday, surface winds will
decrease some, but winds at 2k ft will shift to the south to
southwest, causing concern through the afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 2)...
(Tonight through Saturday night)

The main story in the short term will be a brief warming trend
with strong winds. First, the quick burst of snow that we were
expecting did develop as the hi-res models had forecast. It moved
through Kenai with visibilities having dropped to less than a
mile with 6 hour precip accumulations (liquid equivalent) of over
1/3 of an inch reported at Kenai Airport. However, as it continued
its northward trek a surface trough/convergence boundary, aided
by Turnagain winds, set up from near Sterling to near Ted Stephens
airport. This helped focus most of the precipitation out over
Cook Inlet, with just a few flurries having been observed here at
the forecast office.

Our attention now turns to tomorrow's warmer and windy weather.
The numerical guidance seems to be struggling with how warm we may
get, given the current 10 inch snow depth (measured here this
afternoon) and how much of a down-slope warming effect the
Turnagain wind will have. A weaker version of this today, on the
order of gusts around 25 mph were sufficient to spike temperature
10 degrees in a matter of minutes, with the airport topping out at
36 degrees. Similar conditions were felt across much of the city
and Eagle River areas, given favorable down-slope southeasterly
winds. A repeat scenario is expected tomorrow, but with winds
gusting to 40 mph west side of town, and around 66 mph for the
upper Hillside. Thus, with snowfall decreasing some tomorrow and
increased winds, most locations in the Anchorage Bowl should peak
near 40 degrees or in the lower 40s.

A warm front crossing the region will switch winds to southerly
with precipitation breaking out for much of Southcentral for the
day, with 6 to 11 inches likely north of Talkeetna in the
northern Susitna Valley, as this region will remain colder for a
longer duration with orographic lift giving precipitation totals a
boost. A similar scenario also exists in the Thompson Pass area,
where snowfall totals may be locally higher.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
An intense large low moving across the central Aleutians will
push a front through the Alaska Peninsula this evening then to
the Bering Sea coast tonight. In advance of the front strong wind
coupled with snow will produce areas of blowing snow and near
blizzard conditions across the Bristol Bay region to the Kuskokwim
Delta. The challenge for the Bristol bay region is how much
temperatures warm in advance of this front and how much down slope
drying on the lee of the Aleutian Mountains will reduce the
amount of snow or rain and snow. From Dillingham north the
precipitation should remain all snow. After the front moves
through, the return of southwest flow will usher in colder air
and snow showers Saturday and Saturday night across the southwest
mainland. Deep cold air returns to the Kuskokwim Delta and lower
Kuskokwim Valley Saturday night and Sunday bringing back the sub
zero temperatures and the potential for low wind chill. The other
issue of concern is the potential for storm surge coastal flooding
from Dillingham north to Kuskokwim Bay Saturday night as the wind
turns to west-southwest. The wind is expected to be up over 40 KT
with a log fetch and coincident to the high tides Saturday night.
The ice near the coast should be shoved into the inter-tidal zone
Saturday night with the surge.

Over the Kuskokwim Delta low wind chills will persist through
late tonight. Temperatures will warm late tonight, thus bringing
to an end the low wind chills. Snow in advance of the front
coupled with the strong east to northeast wind will produce
blowing snow reducing visibilities. The low will track to the
northeast across the Kuskokwim Delta Saturday evening. As
mentioned above, this will usher in the deep cold air and return
the region to the sub zero temperatures Saturday night and Sunday.

&&

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

The intense storm pushing through the Aleutians and the
associated front will bring strong wind and rain to the central
Aleutians to the Alaska Peninsula. Around the west side of this
storm will be gusty North to west wind coupled with snow showers
resulting in brief periods of blowing snow and reduced visibility
from the central Aleutians west through Saturday afternoon. As the
low moves northeast, the southwest and west wind around the low
will blow through the eastern Aleutians and Alaska Peninsula.
Frequent gusts around 75 mph can be expected early Saturday across
the Unalaska region and through Saturday evening across the
Alaska Peninsula. Following this low a trailing trough of low
pressure remains along the Aleutians that will support snow
showers through Sunday.

&&

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

A high end gale force storm will move through the central and
western Aleutians into the western Bering from late Sunday into
Tuesday. Storm force winds are possible along the frontal
boundaries. There is still considerable uncertainty for the timing
and strength of this system.

For the Gulf, Northwest gale force winds will increase across the
Barren Islands starting Monday afternoon and will continue through
Tuesday. Winds and seas should abate Wednesday and beyond.

&&

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

We start the long term period Sunday with high pressure over the
western part of the state, high pressure south of the Gulf of
Alaska and another storm on tap moving toward the Aleutians and
Bering Sea. Models are in generally good agreement with the
location of the low moving into the Aleutians and the frontal
placement on Monday. However, they differ significantly with the
intensity of this storm. The front is then expected to slowly
track east across the bering and Aleutians through the middle of
the week, before dissipating as it crosses the southwest mainland
Thursday.

This pattern and scenario support the Arctic air pushing back
across the southern mainland, persisting through Thursday, thus
returning the mainland to the colder air, similar to the past few
weeks. The Bering Sea and Aleutians will be in a highly active
weather pattern, with frequent periods of strong winds and
precipitation through the extended period.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning 155 195.
         Winter Weather Advisory 145 152 161.
         Wind Chill Advisory 155.
         High Wind Warning 181 185.
MARINE...Hurricane Force Wind Warning 172 174.
         Storm Warning 155 160 165 170 171 179 180 181.
         Gale Warning 119 120 125 128 130 131 132 138 139 140 141
                      150 173 175 176 177 178 185.
         Heavy Freezing Spray Warning 160 179 180 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...SS
SOUTHCENTRAL ALASKA...PD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...SA
MARINE/LONG TERM...SS