Heavy Freezing Spray Warning 4, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
435 PM AKST Mon Jan 16 2017

.ANALYSIS AND UPPER LEVELS...
An active weather pattern continues across Southcentral Alaska.
A large upper trough over western Alaska is becoming more negatively
tilted as it digs southeast toward the Gulf. This upper trough is
supporting a surface low over Prince William Sound which is
providing an injection of low level moisture. As a result, snow is
falling across much of Southcentral Alaska with a mix of rain and
snow along the coast. Strong pressure gradients to the west of the
low center caused fairly strong westerly flow across western
Prince William Sound and to a lesser extend through Thompson Pass
and Keystone Canyon. Blowing snow accompanied these winds
especially near Whittier where visibilities were reduced below one
quarter mile at times. Gusty southwest winds developed across
Kachemak Bay bringing blowing snow and snow showers. Storm force
westerly winds were also observed across the Barren Islands.

Out west, partly to mostly cloudy skies and cold temps were evident
across much of Southwest Alaska. Satellite imagery showed open
cell cumulus clouds with snow showers across the Bering sea.

&&

.MODEL DISCUSSION...
Models in the east are in generally good agreement with the higher
resolution NAM/GEM Regional preferred. Out west, the ECMWF/Canadian
NH and GFS/NAM continue to battle over the evolution of a surface
low developing near the central Aleutians Tuesday. The American
models are developing this low and moving it north toward the
Pribilof islands Wed while the ECMWF/Canadian NH keep most of the
energy much further south. Will continue to lean toward the ECMWF
which did adjust its low track a little further north and stronger
on the 12z model cycle.

&&

.AVIATION...
PANC...snow will diminish this evening with IFR/LIFR conditions
slowly improving to MVFR. As low level flow shifts to northwest
on Tuesday this should allow ceilings to become VFR, though lower
clouds will likely remain banked up against the mountains.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The Southcentral AK storm will slowly weaken this evening as it
continues moving northeast. Advisories for snow in the Anchorage
area, the Matanuska Valley, and the Copper River Basin will
continue until 9 pm before snowfall tapers off. A new advisory
for blowing snow this evening near Kachemak Bay is being caused
by strong flow across the southern part of Cook Inlet. Conditions
there are expected to improve by midnight as the winds rapidly
diminish.

As the low departs the region lingering precipitation will end by
midday Tue across the area. High pressure with very cold and dry air
is already filtering into Kodiak island. This airmass will move across
the entire forecast area, with widespread subzero minimum temperatures
expected through Thu morning, and subzero or single digits above
zero high temperatures during the period.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Tue and Wed)...
A deep cold trough will remain entrenched over Southwest Alaska
tonight into Tue. This will keep temperatures running between 20
to 30F degrees below normal. Weak impulses will continue to rotate
down the west side of the trough from the Chukchi Sea. As they
dive down into somewhat modified air over the open waters of the
Bering, they could turn into compact polar lows. Models have a
very hard time depicting the details with these systems. But they
are trending toward showing one such low developing near Nunivak
Island late Tue and sliding along the coast of the Delta and into
Bristol Bay by Wed. This could bring some gusty winds and heavy
snow showers to places mainly along the coast, but could also move
inland a bit interrupting the cold and dry pattern that we have
been in.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Tue and
Wed)...
One of the more complex patterns that we have seen in a while is
about to unfold over the Bering. To start though, cold air
continues to dive through the Bering Strait and bring widespread
snow showers and gusty winds to the region. Beyond that, the stage
is being set by a weak low developing near Atka and Adak tonight
and early Tue. This low seems to drag just enough warm air up from
the North Pacific to intensify the thermal gradient in the region.
Then on Tue, models are depicting a series of very compact low
pressure systems developing right along the best baroclinic
boundary over the northern Bering. As these systems work their way
south, the thermal gradient sharpens even more for the remnants of
the low near Atka and Adak. A stronger shortwave picks the system
up and intensifies it as drags it back northward in the Bering.
This is where the models diverge and things get interesting. Some
models are showing a very impressive warm front feeding off the
arctic air to the north and lingering very near the Pribilof
Islands. If this were to happen, it could bring heavy snow and
strong easterly winds to the Pribilof Islands Wed afternoon into
Thu. There are so many variables that have to come together for
this to occur, but we will be watching it very closely over the
next several model cycles.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Beginning the extended period forecast Wednesday night, a very
deep arctic trough will be centered over the central part of the
state with the trough axis extending through Southern Alaska.
Strong offshore flow will be in place across the North Gulf Coast.
This pattern will be supporting 850C temperatures in the range of
-20C across Southcentral to -30C across Southwest Alaska. These
cold temperatures will settle in through Friday morning before
warmer air moves back in as a Bering low spreads clouds and warmer
air northward. This will increase snow chances for all of Southern
Alaska, although at this juncture the overall pattern does not
look very conducive to any significant storms. Temperatures will
moderate through the remainder of the weekend into the next week
as a series of weak Gulf lows linger near the Gulf coast, keeping
enough clouds and warm air advection aloft to keep bitterly cold
temperatures from settling into Southcentral.

Over the Bering Sea, a gale force low will be moving from the
Eastern Bering Sea into the Gulf of Alaska Thursday and Thursday
night. Strong cyclonic flow behind this low will keep widespread
snow showers and northerly winds over the Bering Sea through
Friday before a strong low potentially moves into the Western
Bering Sea later in the weekend into next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 101 111 121 141.
MARINE...Heavy Freezing Spray Warning 121 126 127 129 130 138 139
									  141 160 165 180 181 185.
         Storm Warning 130.
         Gale Warning 119 120 131 139 141.

&&

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