Blizzard in Alaska, January 19, 2017

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
459 PM AKST Wed Jan 18 2017

.ANALYSIS AND UPPER LEVELS...
A closed upper low centered over northwest Alaska coupled with
surface high pressure over the interior is responsible for a
frigid arctic air mass in place over much of the mainland this
afternoon. A deep surface low impacting the southeastern Gulf of
Alaska is helping to maintain a tight pressure gradient over
southcentral AK, leading to gusty gap winds along the north Gulf
coast and wind chill concerns through some mountain passes.
Another compact surface low is deepening over the central Bering,
with the associated front currently pushing through the Pribilof
Islands accompanied by blowing snow.

&&

.MODEL DISCUSSION...
Guidance continues to struggle with the evolution of the Bering
low as it tracks eastward towards the mainland through Friday
morning. This looks to be the primary forecast concern in the
short term. There is still a large spread in the position and
track of the low as it passes over the Pribilofs late tonight,
with the GFS/EC taking more of a northerly track towards the
Kuskokwim Delta, and the higher resolution NAM/regional GEM
staying farther south in the vicinity of Bristol Bay.
Investigation of low level pressure and winds fields suggests that
the regional GEM (Canadian) model has initialized the best of the
12z guidance, lending some credence to a more southern solution,
however this still represents somewhat of an outlier compared to
the other models. Model trends will continue to be monitored
closely, as the track of this system will have large implications
on the potential for additional blowing snow and visibility
reductions for the Pribilofs on Thursday as strong north winds
develop on the backside of the system.

There is stronger model agreement over southcentral AK as cold
temperatures and outflow winds remain the primary short term
forecast concerns.

&&

.AVIATION...
PANC...VFR conditions and light northerly winds will persist
through Thursday.

&&

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

The coldest temperatures of the season will be widespread across
the area tonight. An upper level trough moving across the region
tonight will provide some enhancement to outflow winds. This will
provide another night of low wind chill values for Broad pass and
Thompson pass, and advisories have been issued. Winds will begin
to ease on Thu, but subzero low temperatures away from the coast
will persist Thu night. As a storm approaches the west coast of
AK, temperatures will begin to moderate on Fri.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA...

An elongated warm front aloft has moved over parts of the
Kuskokwim Delta coast and Nunivak Island this evening. Light snow
has already developed and will increase during the late evening
and overnight as a strong Bering low moves toward the coast. This
will be accompanied by an increasing offshore pressure gradient
which will lead to areas of moderate snow and reduced visibility
in blowing snow. Some areas along the coast may see up to 6-9
inches of snow by tomorrow afternoon, so there will be plenty of
snow in certain locations to pick up and blow. The front will
continue to move east and spread snow across all of Southwest
Alaska Thursday night and Friday. Snow accumulations will be
lighter as the low/front moves east away from the Delta with no
significant hazards anticipated except for the Kuskokwim Delta.
For that reason, a Winter Weather Advisory for snow and blowing
snow will be issued with this forecast package.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS...

Satellite imagery depicts a very impressive bent-back cold core
occlusion over the Central Bering Sea. The associated front is
moving through the Pribilof Islands with periodic heavy snow and
strong southerly winds. This near storm force low will move east
tonight and Thursday, with the powerful core of northerly winds
moving over the Pribilof Islands Thursday and Thursday night.
Along with this will be strong convective snow showers and wind
gusts as high as 60 MPH, so areas of blowing snow will be likely
for the Pribilof Islands. For this reason, a Winter Weather
Advisory for blowing snow will be issued with this forecast
package. The low will move inland Thursday evening with a broad
area of snow showers and north to northwest winds encompassing the
Bering Sea.

&&

.LONG TERM FORECAST (Days 3 through 7)...
There remains good general agreement in the models that the
current cold snap over much of the Mainland will begin to break
down beginning this weekend as the arctic trough begins to lift
northward. As this occurs, there are significant differences in
the track of low dissipating near Bristol Bay late Friday. This is
causing low confidence with this system, especially with regards
to chances of snowfall from the Southwest Mainland into Cook
Inlet.

Once this system dissipates on Saturday, there is good confidence
that a trough digging over the Bering will shift to the east,
bringing the North Pacific low track back into the Gulf beginning Sunday
and continuing into early next week. As is typical in the long
term forecast, there is enough uncertainty in the exact impact of
this pattern change. There is however reasonable confidence that
temperatures throughout the Southern Mainland will rise to near
or even above normal for the first half of next week, with the
greatest precipitation chances along the Gulf Coast.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Wind Chill Advisory 131 145.
Winter Weather Advisory 155 195.
MARINE...Gale 119-121 127-132 165 170 180-185.
Storm 179.
Heavy Freezing Spray 121-131 139 141 160 179 180 185.

&&

SYNOPSIS AND MODEL DISCUSSION...CB
SOUTHCENTRAL ALASKA...DS
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
LONG TERM...DEK