Heavy Freezing Spray Warning 73, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
454 AM AKST Sat Feb 18 2017

.ANALYSIS AND UPPER LEVELS...
A large upper trough extends from the central Bering to the
Canadian Rockies this morning, with weak impulses moving across
the Kenai into Southcentral AK, and also through the eastern
Aleutians.

At the surface, low pressure is weakening over Southcentral AK
with snow showers moving into Prince William Sound, and across the
Kenai Peninsula into Southcentral. Some fog is developing behind
the showers. High pressure is building across the Bering, while
a weakening low is spreading some shower activity across the
eastern Aleutians. A front associated with a Kamchatka low is
approaching the western Aleutians with wind and light snow.

&&

.MODEL DISCUSSION...
Models are in agreement that the broad area of weak low pressure
over the mainland will consolidate over the Prince William Sound
area Sun into Mon. This will occur as the building ridge over the
Bering reaches across the Alaska and Aleutian ranges. The ridge
will bring colder air to just west of the PWS low, which is
conducive to producing snow. In the west the front will be slowed
by the ridge until a second low moves into the western Aleutians
Sun night. This system will push the front across the Bering Mon
into Tue. The forecasts utilize some nam and gfs, with mesoscale
input in the east.

&&

.AVIATION...
PANC... Locally dense fog remains the main challenge. A calm
surface wind, a shallow inversion near the ground, and convective
snow showers passing above this layer will make for variable
conditions through mid-morning, when the fog is expected to
dissipate with solar heating. The next chance of snow comes in
Sunday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Upper levels have dried out in the wake of a short-wave exiting to
the north early this morning. With light winds and weak surface-
based inversions, low levels remain moist. The combination of
very weak impulses moving overhead combined with continued weak
instability above the surface inversion will produce isolated to
scattered snow showers through tonight. Conditions are also
favorable for patchy fog in between showers.

A slightly stronger upper wave will ride up the eastern periphery
of a deep arctic trough sitting over western Alaska Saturday night
through Sunday. This will lead to cold advection across the
Western Gulf, Cook Inlet, and Susitna Valley. With low level
moisture still in place expect development of some very light
snow on Sunday from the Western Kenai to Anchorage and the Mat-Su.
Accumulation should generally be very light, with a half inch or
less most places. A more substantial short-wave moving down the
back side of the arctic trough will dig southeastward into the
western Gulf Sunday Night into Monday, bringing arctic air and
strengthening winds.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The focus today will be light snow for the Kuskokwim Valley,
followed by a period of frigid temperatures for the mainland as a
whole. A trough/frontal boundary stretched from north to south
over the Kuskokwim Valley will continue to bring light snow to the
area with minor accumulation today. To the west of the frontal
boundary, arctic air is already streaming into the Kuskokwim
Delta. The question for the next couple of nights will be how low
do the temps fall? The answer should be in the negative teens for
most places including the Bristol Bay area, but the Kuskokwim
Valley could see temperatures down into the -20s. A clearing trend
will continue as the front moves away today, and colder air
advects over the mainland from the west. Some snow showers are
possible along the coast and around Nunivak Island as the arctic
air interacts with any moisture it can pick up from ice leads.
Expect a downward trend in winds, and below normal temperatures.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Snow showers will be the story for much of the area for the next
couple days. Arctic air streaming southward from the Bering Strait
will pick up moisture from the warmer water below in the form of
convective snow showers. These will affect mainly the central and
eastern Bering Sea away from the ice edge. Conditions remain
favorable through Sunday night before the wind profile in the low
levels changes. On late Sunday, a developing north Pacific low
will bring a storm force front to the western Aleutians.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Beginning the long term forecast Monday, a large arctic trough
will be in the process of digging into Southcentral Alaska.
Rather cold 850 hpa temperatures as low as -25 C will push into
Southwest Alaska with those colder temperatures infiltrating into
Southcentral slowly through Tuesday. This will push temperatures
below average region-wide until temperatures begin to warm
Wednesday. Meanwhile, over the Bering, a powerful storm force low
will move into the Western Bering Sea Sunday night and Monday
followed by a another large low Tuesday through Wednesday night.
Eventually the associated fronts will move east into southern
Alaska by midweek, bringing increased chances for snow...then
eventually rain and snow as temperatures warm dramatically by
the end of the week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Heavy Freezing Spray Warning 179.

&&

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