Winter Storm Watch 1, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
432 PM AKST Fri Jan 20 2017

.ANALYSIS AND UPPER LEVELS...
A broad upper level trough extending across Western Alaska from a
closed low over the North Slope is beginning to slowly lift to the
north and east this afternoon. A front is entering the Cook Inlet
region, bringing snow to portions of the Kenai Peninsula and
setting the stage for another round of snowfall for Anchorage and
the Matanuska Valley this evening into tonight. Meanwhile, a very
cold arctic air mass is once again filtering into the southwest
mainland on the backside of the trough as precipitation moves east
of the Alaska Range. Convective snow showers continue in broad
cold air advection across a large swath of the Bering as a weak
North Pacific front approaches the Western Aleutians.

&&

.MODEL DISCUSSION...
Models are in reasonably good synoptic agreement through the short
term as the trough lifts to the northeast across the mainland and
the strong front enters the Aleutians later this weekend ahead of
an intensifying North Pacific low. 18z runs have also come into
slightly better agreement regarding some of the smaller scale
details regarding the snowfall set to impact Anchorage and Mat-
Su regions tonight, however there is still some uncertainty with
respect to the strength and duration of main snow band and how
quickly snow will taper off on Saturday, which will have large
implications on snow amounts. Primarily utilized the NAM for the
afternoon package.

&&

.AVIATION...
PANC...The snow will drop prevailing conditions to MVFR through
Saturday morning with periods of IFR conditions as the snow
fluctuates in intensity. The main snow band will dissipate by
Saturday morning, but scattered snow showers and MVFR conditions
look to persist through Saturday afternoon.

&&

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

The front that crossed the Aleutian Range is beginning to stall
across the Kenai Peninsula, the Anchorage Bowl and Matanuska
Valley, extending north into the Susitna Valley. Snowfall will
stream north across this region overnight, with up to one foot
expected along the Seward highway through the Kenai peninsula.
Generally amounts up to 6 inches are expected tonight over
Anchorage and the Matanuska Valley and the Susitna Valley east of
the Parks highway. As the front weakens in place on Sat, snow
showers will provide an additional 2 inches of snow over the
region. On Sun the upper trough finally passes over Southcentral
with a last round of snow shower activity.

Areas from Prince William Sound to the north and east will remain
fairly dry through the weekend, with continued cold temperatures
inland. The area with snowfall will experience warmer
temperatures on southerly flow.

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Sat and Sun)...
Very cold temperatures will settle over the area again tonight
into Saturday. The trough from the Bering that brought snow and
blowing snow to the region is slowly moving off to the east. Light
snow continues along the Aleutian range and in parts of Bristol
Bay, but even that should end through the evening hours. In the
wake of the trough, another plunge of arctic air is diving down
from the Chukchi Sea. Saturday will be a relatively quiet day
with dry, calm conditions and some patchy fog in the valleys. A
strong system over the Bering will allow an induced ridge to build
over southwest AK on Sunday. This will keep conditions dry and
allow temperatures to modify a little as flow starts to shift to
southeasterly.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Sat and
Sun)...
Cold air advection snow showers will continue across most of the
Bering and from the Central Aleutians eastward through much of
Sat. Some of these showers will bring periods of heavy snow and
gusty winds causing reduced visibility. Over the Western Bering,
an initial warm front will move in tonight with increasing
southeast winds and some light snow, possibly mixed with rain at
times. Then we shift our attention to the next strong low that
will move towards the Central Aleutians from the North Pacific.

The first impact from this will be strong east to southeast winds
from Adak to Dutch Harbor late Sat night into mid-day Sunday.
Right now it looks like winds will top out in the 65-70 mph range,
but we will be watching how it develops. A bit more cold air or
upper level support and we might see things get close to warning
level. Beyond that, conditions look to be setting up well for snow
and blowing snow conditions over the Pribilofs later Sun. Cold air
will remain in place and east to northeast flow is a very
favorable set-up for reduced visibilities at the islands. We will
also watch this to make sure the storm track remains consistent on
the next few models.

&&

.LONG TERM FORECAST (Days 3 through 7)...
An upper level closed low is centered over the Central Aleutians
starting out next week. As this feature moves eastward the strong
meridional south to north jet streak tracks into the Southeast
Panhandle by Tuesday. This pattern continues through the the next
weekend as the longwave trough moves farther eastward. Meanwhile,
look for a warming trend with the southerly flow into the Gulf
extending through the Southcentral Alaska region. Therefore, the
temperatures in the aforementioned region rises above freezing due
from the warm air advection. This results in more moisture
streaming into the AOR with a wintry mix along the coastal
communities as the warmer temperature filter into the region.
While the interior sets up with a better scenario for moderate
snowfall in favored mountain regions. Meanwhile, the models are
struggling some with the complex low developing in the North
Pacific Ocean before tracking towards the Central Aleutians by
Monday. This system will bring warmer temperatures along the
Aleutian Chain, and the lower Bering. While the rest of the
Bering is influenced by easterly to northeasterly flow through
majority of next week. Since the jet stream remains south of the
Aleutian Chain the storm systems will continue to track along the
Chain before entering the Gulf of Alaska. The forecast confidence
remains average through next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 150 170 172.
Storm 173-178.
Heavy Freezing Spray 127 130 138 160 165 180 185.

&&

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