Blizzard in Alaska, December 31, 2019

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
805 AM AKST Tue Dec 31 2019

.ANALYSIS AND UPPER LEVELS...

An impressive weather pattern regime is in place this morning as a
subtropical warm front is merging with a polar front. Satellite
imagery depicts two well defined upper jets, one a polar jet
situated over the north-central part of Alaska, and a subtropical
jet stream south of the Gulf of Alaska. A very warm and moist air
mass has wedged into Southcentral with the Western and Central
Alaska Range acting as the barrier between the interior polar air
and the warm Pacific air mass to the south. A latitudinal deep upper
level potential vorticity anomaly (PV anomaly) stretches from the
Chukchi Sea to the North Pacific, approximately situated south of
the Alaska Peninsula. This stable warm air advection pattern in the
low levels is forcing a strong pressure gradient up along the
Chugach Mountains this morning, and this is being realized in the
form of strong southeast winds blasting through Turnagain Arm and
through parts of Anchorage, Eagle River, and the Matanuska Valley.
This has pushed some of the overnight temps up into near record
territory for daily highs with temps near the mid 40s (Anchorage has
already broken the record high for Dec 31 with a temp of 45 at 3
AM). On the cold side west of the Alaska Range, classic warm air
deformation banding aloft is overspreading the deep polar air below,
resulting in heavy snow which continues to pound parts of Southwest
Alaska, favoring a line from Pilot Point to King Salmon to
Sleetmute. Even farther west, the deep cold air along with a
strengthening pressure gradient continue to support dangerously cold
wind chills as well as an off coast growing ice edge, which can be
seen on satellite imagery moving south. The remainder of the Bering
Sea remains under the influence of cold air advection snow showers
as ocean-effect convective showers dominate the regime.

Over the next 24 hours, weather is going to
shift dramatically as a powerful cold front moves across the
region, one of the strongest in many years for Cook Inlet.

&&

.MODEL DISCUSSION...
The numerical weather models have settled into very good agreement
with both the synoptic scale pattern and mesoscale pattern for the
next two days across Southwest and Southcentral Alaska. The only
slight discrepancies revolve around how quickly the arctic front
moving up Cook Inlet progresses. The Hi-Res ARW and NMM are outliers
and move it rapidly north as it clears through Anchorage into the
Matanuska Valley tomorrow night. The remaining models depict
something potentially more substantial, and that is convective
forcing from the Gulf, along the arctic front, feeding ahead of the
powerful PV anomaly moving northward. Should this verify as is, this
classic positive feedback mechanism (latent heat release aloft which
slows the overall progress of the PV anomaly aloft) will be
responsible for potentially significant and rapid convectively
enhanced snow accumulations for the New Year's festivities across
parts of northern Cook Inlet, including Anchorage. In conjunction
with the most extreme cold air advection this forecaster has seen in
his 7.5 years in this office (8-9 degrees C in 3 hours at 850 hpa),
things are shaping up for an impressive rapidly developing snow
event as the warm Pacific air mass is immediately replaced by arctic
air. The NAM Nest and GFS were favored models for this forecast
package.

&&

.AVIATION...
PANC...Turnagain Arm gusty southeast winds have blasted through the
airport this morning and will likely continue on and off through the
day today before winds go due southerly and cold air moves in. This
will develop late this evening, and will likely bring in heavy snow
squalls and rapidly deteriorating conditions to the airport with a
high possibility of IFR conditions in snow at times during the late
evening and early overnight period before lightening up toward
morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A very strong cold front will move across the area today and
Wednesday. The front will push through Kodiak Island early this
afternoon, and through Kachemak Bay by late afternoon. Blizzard
conditions are expected for the Kachemak Bay area from late this
afternoon through tonight as strong southwest winds combine with
heavy snowfall and rapidly decreasing temperatures. The front will
reach northern Cook Inlet in the late evening hours. This will
bring heavy snowfall to the Anchorage area beginning in the late
evening and continuing into Wednesday morning as moisture from the
southeast rides over the front. Temperatures will drop from near
40 in the early evening to the lower to mid 20s by late morning,
then into the teens in the afternoon. Snow will taper off during
the day as the front continues north through the area.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
(Today through Wednesday night)

The current forecast remains on track with only minimal changes
made. Currently, the edge of the deformation snow stretches from
Dillingham to just east of Aniak, with the highest radar returns and
lowest visibilities running along and east of a line from King
Salmon to New Stuyahok. In this area additional snowfall
accumulations of 2 to 7 inches are anticipated. Further north
towards the Lower Kuskokwim Valley, the heaviest snow will continue
to fall from Sleetmute east, where an additional 5 to 13 inches are
expected.

Heading into tonight, as the low pressure system moves east, its mid-
level reflection slowly moves from eastern Bristol Bay to King
Salmon, a vorticity ribbon (mid-level front essentially in this
case) stretches from it to the Lower Kuskokwim Valley, then back
across the Cook Inlet. This will provide an additional source of
lift, allowing the deformation snowfall to continue into Wednesday
morning, before snowfall decreases in coverage by the afternoon.
Further west, colder air will continue to move from the Kuskokwim
Delta into Southwest Alaska on the backside of these systems.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
(Today through Wednesday night)

Cyclonic flow will continue to pour cold air into the region, with
mostly cloudy to cloudy skies, accompanied by frequent snow
showers/snow squalls for the central Bering and Aleutians east.
Gale warnings and heavy freezing spray/freezing spray will be
common, with stronger wind gusts out of bays and passes likely.
Overall, similar conditions to the past 24 hours will continue.
For the western Bering and Aleutians, snow shower coverage will be
somewhat less, as this region is more removed from the cold air
advection and better instability.

&&

.MARINE (Days 3 through 5/Thursday through Saturday)...

...Bering Sea and Aleutian Coastal Waters...

Northerly gales and seas 15 to 20 feet will continue across the
central to eastern Aleutians into Friday morning as a front
associated with an area of low pressure in the northwestern Gulf
remains over the area. A front moves into the western Aleutians
Thursday morning which will bring southerly gales to the area and
seas around 15 feet.

...Gulf of Alaska...

Broad westerly to southwesterly small-crafts will persist across
the southern Gulf through Thursday night. Seas will range from 15
to 25 feet as the low south of Kodiak Island lifts north through
Friday morning.

&&

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

The focus on Friday will be on a 995 mb low south of Kodiak
Island. Most of the models keep the low over the southern Gulf
with the primary impact to Mainland Alaska being strong outflow
gap winds. High pressure will remain in place in the central
Bering throughout the long term. This will keep cold air moving
over the warm waters of the eastern Bering which will continue
producing widespread snow shower activity across the eastern
Aleutians and the AKPEN. That same high pressure system will keep
the western Aleutians in an active pattern as a series of fronts
move across the area through the weekend and block those fronts
from moving into the eastern Bering.

For Southcentral, Saturday keeps cold temperatures and dry
conditions across the Mainland. Models show significant
disagreement into the late weekend/early work week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Advisory 145.
         Winter Storm Warning 152 161.
         Blizzard Warning 121.
		 Winter Weather Advisory 101.
         Wind Chill Advisory 155.

MARINE...Gale 131 132 138 139 150 155 160 165 170 172 178 179 180.
         181 185 352 414.
         Storm 130.
       Heavy Freezing Spray 138 150 160 165 179 180 181 185 414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...Ahsenmacher
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
MARINE/LONG TERM...CK

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
942 AM AKST TUE DEC 31 2019

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

0935 AM     SNOW             SLEETMUTE               61.70N 157.17W
12/31/2019  E6.0 INCH        BETHEL             AK   PUBLIC

            SPOTTER REPORTED 6 INCHES OF NEW SNOW SINCE
            LAST NIGHT.

&&

ED