Heavy Snow in Alaska, March 7 to 9, 2018

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
440 AM AKST Wed Mar 7 2018

.ANALYSIS AND UPPER LEVELS...
The active pattern continues with multiple upper level waves
tracking through the area. One shortwave is pushing through
Southcentral Alaska and has brought snow to the area overnight.
Another wave is associated with a system over the western Bering
that is pushing its front into the central Bering and western
Aleutians. In between these two features is an area of ridging
that has a fair amount of warm air associated with it.

&&

.MODEL DISCUSSION...
The models are doing fairly well in the short term but continue to
struggle with a low forming near Kodiak Island late Wednesday
night. They all have the low developing but where they take the
low and how fast has been a challenge. The GFS is bringing the low
into Cook Inlet while the others move it into the northern Gulf at
varying speeds with the NAM taking it into Prince William Sound.
These different tracks will play a big part in what areas of the
southern mainland will see the most snow.

&&

.AVIATION...
PANC...Ceilings look to rise throughout the morning leaving VFR
conditions and light winds until early Thursday morning when the
next front moves in dropping ceilings and visibilities with snow.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
An upper level wave moved through southcentral Alaska overnight
and is heading toward the Copper River Basin. Stratus has again
developed over Cook Inlet and parts of the Susitna valley behind
the wave. The progressive ridge building in late today may clear
things out for awhile but the next system will sweep into the area
by late Wednesday night.

In conjunction with the colder air associated with the new
system, a lee side low will develop just north of Kodiak Island
wednesday night and then rapidly strengthen Thursday. This will
bring widespread Gales to the Gulf of Alaska region and some
areas of storm force winds as well. This low is a tricky one to
deal with as a small difference in track can make a large change
in weather over the western Kenai Peninsula northward through the
Susitna Valley including the Anchorage area and Matanuska valley.

Starting with what is most likely: Kodiak will see strong west to
northwest winds develop with the strongest winds over Afognak and
Shuyak Islands. Also likely is the eastern Kenai Peninsula will
see areas of heavy snowfall with some areas seeing blowing snow as
well. Temperatures will be close to or just above freezing so that
often limits the blowing snow. In spite of temperatures in the mid
30s along the coast the precipitation type is expected to remain
all snow at this time. However, this is something to keep an eye
on.

Farther east, Cordova and Valdez also look to be in for some
extended periods of snowfall with the southerly flow aloft and
temperatures remain cold enough for precipitation to remain all
snow. Accumulations could become significant due to the duration
and some periods of moderate snowfall.

Anchorage and the Matanuska valley will see the light snowfall
tomorrow morning shut off as downslope winds develop over the
area. This system is too strong not to cause the downsloping and a
weaker system would give better snow chances. While there is still
a great amount of uncertainty as to how this low will transit the
area through Friday, there is a chance that there could be some
significant snowfall Thursday night or Friday in these areas. The
main problem is the persistent downslope.

That leaves the area of biggest uncertainty the Western Kenai
Peninsula. The downsloping off the Kenai Mountains will mitigate
the snowfall west of the mountains but there will be a sharp line
where the snowfall will greatly increase depending on the
subtleties of the low track. Where this line occurs will
determine whether areas near the Inlet will receive just a few
inches of snow or closer to the 6 to 10 inch range Thursday into
Thursday night. Came very close to issuing the snow advisory for
tomorrow and tomorrow night, but the chance that the heavy snow is
just offshore and over the Inlet is great enough to dissuade me
until another model run can be analyzed.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The latest Bering Sea front will bring another round of snow to
the southwest mainland today as it tracks quickly eastward across
the region. The majority of the area can expect minor
accumulations around 1-2 inches, with rain possibly mixing in
during the afternoon from the Kuskokwim Delta to coastal Bristol
Bay. There is the potential for higher snow amounts along the
Alaska and Aleutian Ranges, and this will be the primary forecast
challenge for the region through Thursday night. There is higher
confidence that several inches of snow will fall along the
corridor from Iliamna to Lake Clark NP tonight as the front stalls
along the mountains and favorable southwest flow develops aloft.
The more uncertain portion of the forecast begins Thursday as a
triple point low develops along the front near the Akpen and then
rapidly intensifies as it moves into the western Gulf of Alaska.
There are stark differences among the model guidance which would
result in very different scenarios playing out near the
Alaska/Aleutian Ranges through Thursday night. The GFS solution
tracks the low into Cook Inlet and sets up a deformation zone over
the Aleutian Range, which would bring another round of significant
snowfall to the Iliamna-Lake Clark corridor through late Thursday
night. In contrast, the NAM takes the low farther to the east and
thus sets up the deformation zone farther to the east,
essentially bringing the snow to an end by Thursday morning with
much lower accumulations as a result. The current forecast more
closely resembles the snowier GFS solution, however given the wide
range of possible outcomes and the resultant lower forecast
confidence, will hold off on any decisions on product issuance
for now and continue to monitor model trends. Switching gears back
to the remainder of the southwest mainland, a more showery regime
is on tap for the end of the work week in the wake of the front.
A low over the northern Bering will drop southeast along the coast
Thursday night. This will usher in a much colder air mass by
Friday as a sprawling longwave trough digs over the region.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The trailing portion of the front moving across the mainland will
bring a rain/snow mix to the Pribilofs and Central Aleutians today
as it drops southeastward towards the Akpen. The focus then
shifts to the Eastern Aleutians and Akpen tonight into Thursday
morning as the developing triple point serves to enhance the
rainfall over these areas. Active weather will continue to impact
the region Thursday into Friday in the wake of this system as
strong cold advection associated with the digging longwave trough
ushers in widespread convective snow showers and gale force winds
encompassing nearly the entire Bering Sea by the end of the short
term.

&&

.LONG TERM FORECAST (Days 3 through 7)...

For the start of the weekend, a generally showery and unstable
airmass over the Gulf will keep showers mainly along the coast,
while much of the Bering Sea remains under high pressure and
quiescent weather. Another weather front moves into the western
Bering Sea Saturday evening but looks at this point like it will
fall apart significantly by the time in reaches the Southwest
Mainland on Monday. Model agreement falls apart significantly from
Monday onward, though in general it looks like the progressive
southwest-to-northeast storm track of the last few weeks will
continue, with periodic chances for snow/active weather across
most of the forecast domain.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales...120 125 130 131 132 138 139 150 155 165 170 171
				 172 173 174 175 176 177 178 179 351 352 411 412
				 413 414.
		 Storms...185.

&&

SYNOPSIS AND MODEL DISCUSSION...DK
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
LONG TERM...AMD