Small Craft Advisory 208, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
428 PM AKST Sat Mar 10 2018

.ANALYSIS AND UPPER LEVELS...
A broad upper level trough stretches from the South Mainland into
the Gulf, which continues to bring snow showers from the Southern
Kenai Peninsula into the North Gulf Coast. Under this trough are
two weak surface troughs in the Gulf, the first which is
beginning to make landfall over Western Prince William Sound. The
other is beginning to enter the Eastern Gulf which will also
track towards the Sound for tomorrow. This active weather pattern
continues further west over the Bering Sea as a gale force low is
beginning to enter the Northern Bering on its way to the Southwest
Coast for late Sunday. Ahead of this low, the gusty northwesterly
flow over the Eastern Bering/AKpen are beginning to slowly diminish,
though a broad area of cold air advection snow showers continues
throughout the area.

&&

.MODEL DISCUSSION...
The models are in generally good synoptic agreement for the first
24 hours, with the primary differences being with the placement of
snow showers over the North Gulf Coast today. Starting Sunday
afternoon there begins to be significant differences in the exact
track of the next weak Gulf low into Prince William Sound. All of
the models track this low somewhere in/near the Sound by Sunday
evening, but whether this low tracks into the western sound or
near Seward (as the 12z NAM/GFS indicate) or further east towards
Valdez (GEM global/regional) create significant differences in how
far west into Southcentral the associated snowfall will develop.
At this point it appears that the best chance for accumulating
snowfall should be from the Matanuska Valley east, however recent
models trends are beginning to bring precip as far west as the
Anchorage Bowl/Northern Kenai Peninsula, so will need to continue
to monitor model trends.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist into Sunday
afternoon. There is an outside chance that a few snow showers will
bring ceilings down to around 5000 ft this evening as the
remnants of a trough over the Gulf move through, however this
will likely be intermittent with only light accumulations
expected.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Sunday and
Monday)...
Snow showers will continue through much of coastal Southcentral
and the upslope areas of the mountains over the next couple days
as a series of weak disturbances take advantage of an unstable
air mass in place across the region. The disturbances will add
moisture to the atmosphere leading to more organized snow shower
activity. For the Cook Inlet region, the first of these will come
through late this evening. Since it will be brief and transitory,
the resulting snow should be less than a half inch for most of the
area.

The next stronger disturbance tracks northwestward out of the Gulf
through Sunday, affecting the Cordova and Valdez area Sunday
morning, Portage and Whittier Sunday afternoon, then potentially
impacting the Matanuska Valley and portions of Anchorage Sunday
evening. There remains significant model discrepancies as to how
far west the moisture gets to, so it remains very possible that
Anchorage gets very little snow Sunday evening.

Then on Monday, as the flow turns more southeasterly through
Southcentral, upslope flow along the coast and Alaska Range will
pick up, while Anchorage and the Matanuska Valley stays relatively
dry. Precipitation chances diminish area-wide going into Tuesday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Expect widespread cloudiness, periods of snow, and near normal
temperatures for the period. Lots of low stratus and flurries
linger around the mainland as low-level moisture remains after
the previous system. As winds wane tonight, stratus should thicken
up, possible reaching the ground as fog. The next low and front
will begin to impact the coast Sunday morning with some light snow
and increasing southeasterly winds. Some areas of blowing snow
will exist along the coast, but they should be fairly short lived,
with the lowest visibilities from Kuskokwim Bay through Togiak.
The system moves inland overnight Sunday into Monday. The next
system will be quick on it's heels on Tuesday, however, looks
rather disorganized, only bringing some light snow to the area.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A warm front progressing west to east across the Bering will bring
snow to the Pribilof Islands and Unalaska before turning to rain
early Sunday morning. A weak system will traverse the trailing
cold front through the central Bering Sea on late Monday/Tuesday,
with light precipitation and winds.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The long range forecast begins Tuesday with a relatively quiet
weather pattern over the southern mainland and most of the
Aleutians/Bering as the area lies in a transition zone between
systems. A nearly stationary surface low will be present over the
Northern Pacific/Southern Gulf of Alaska which could bring some
snow to the Gulf Coast on Tuesday before being kicked east by a
building longwave ridge, bringing pleasant mid-week weather to
southcentral.

A persistent long wave trough over the Bering will begin to re-
amplify Wednesday as waves rotating around the base of the trough
will kick off several low pressure systems through the week. The
upper level jet over the Bering will become more west/southwesterly
by mid week which will help to tap into subtropical moisture to
the south and advect it north towards mainland Alaska. At this
point, the long range models fall out of agreement regarding the
placement and even timing of the various lows tracking out of the
Bering, with models becoming 180 degrees out of phase with each
other by the end of the week (trough vs ridge), so there is little
confidence in the timing of specific systems late next week.
Regardless, there is consensus in an overall anomalous and active
pattern for mid March as the persistent upper level trough over
the Bering Sea drives the main storm track into mainland Alaska.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 119 172 174 175 176 177 178 180 181 185.
         Heavy Freezing Spray Warning 180 181 185.

&&

SYNOPSIS AND MODEL DISCUSSION...DEK
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MTL
LONG TERM...KVP