Small Craft Advisory 286, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
450 AM AKDT Thu Apr 5 2018

.ANALYSIS AND UPPER LEVELS...
The low that pulled a front inland across Southcentral Wednesday
is currently on a slow westward track from the southwest mainland
to the eastern Bering. The associated surface low lags behind and
is expected to lift north across Bristol Bay this afternoon. A
continuous stream of snow showers continues to develop along
easterly flow from the northern Gulf to Bristol Bay. To the west,
a gale force low is approaching the Aleutians with the frontal
band just south of Shemya. There is one other low to note from the
eastern North Pacific, this system is on a northward track toward
the Gulf.

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.MODEL DISCUSSION...
Models are generally in good agreement with the progressive low
pattern through the end of the week. Only minor adjustments were
made for the morning package using the higher resolution NAM
models.

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.AVIATION...
PANC...VFR conditions are expected through Friday morning. Wind
shear was kept through mid morning with southeasterly winds just
off the surface expected to slacken by the afternoon as low
pressure moves further west.

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.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
While the upper level trough and front responsible for snow
yesterday is pulling away to the northwest, an unstable low-level
air mass remains. Snow showers will continue, mainly along the
north Gulf coast and Prince William Sound area, through this
morning. By afternoon hours, most shower activity will have
diminished, bringing clear and dry conditions to most areas. A
weak front pivots toward the coast on Friday, returning some light
precipitation, likely rain, to coastal areas. There's a chance
some of the rain makes it over the mountains, but it should be
very light if it does.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Snow showers are expected to persist across much of the Bristol
Bay and the Lower Kuskokwim Valley today, as an upper level
disturbance pinwheeling around the upper level low south of
Kuskokwim Bay increases the atmospheric lift through the area
while at the same time the flow becomes increasingly south to
southwesterly, bringing in more moisture. These showers are likely
to form a narrow band today as the system stalls out over the
area, with the highest snowfall amounts lining up from Koliganek
north into Sleetmute.

The single greatest point of uncertainty in the short-term
forecast is where the band will set up exactly. The models have
generally been shifting westward with this feature over time, but
unfortunately a clear answer may not really be known until it's
actually formed. Right now our best forecast is that it will set
up near or just east of King Salmon, in essentially a north-south
band that will encompass Igiugig, New Stuyahok, and continue
northward through Sleetmute. Instability will be large enough that
the affected communities will likely see large changes in
visibility as individual embedded snow showers move through. There
may also be large variability in snowfall amounts as well, and
only a few miles may be the difference between less than an inch
of snow to as much as 6 inches where the snow band persists
longest. Since temperatures will be warm enough to keep snow
ratios down, and the expected isolated nature of the greatest
snowfall amounts, no advisory is thought needed at this time. A
few hit-and-miss snow showers are possible through the Kuskokwim
Delta today, but the main action as far as precipitation goes
will be further east.

By tonight, the snow band will diminish in intensity and fall
apart during the evening, though scattered snow showers will
continue through the night into Friday, as they generally drift
back eastward towards the foothills of the Alaska Range where
uplift will be aided by the topography. Elsewhere, Friday into the
weekend looks to be quiet area-wide.

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.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Ocean-effect snow showers ongoing through the northeastern half
of the Bering will continue through today as cold air moving over
the relatively warmer water picks up moisture. Over the western
Bering and Aleutians, fog is the primary weather hazard. This is
associated with a front that too is picking up moisture from off
the Bering, but unlike further east, a warm air mass above it is
trapping the moisture in the lowest levels of the atmosphere,
resulting in fog. A second much stronger front will move into the
western Aleutians this morning, ending the fog just in time for
more steady rain to move in. The front will not make much
northward progress since steady northeast winds in the northern
Bering will impede its progress, but the front will have an easier
time spreading eastward across the Aleutians and far southern
Bering tonight into Friday. So all of these areas can expect
gusty winds with steady rains. On Friday afternoon, the rain or a
rain/snow mix will reach the Pribilofs.

&&

.LONG TERM FORECAST (Days 4 through 7)...

The forecast period commences with a decaying upper level low in
the Gulf Of Alaska and a second low near the Central Aleutians.
The GFS and the ECMWF are in decent agreement with the placement
of both lows but the ECMWF is deeper with the surface low
pressure for both lows. There is a third low south of Kamchatka
that will move towards Attu Island. By Monday however, the
divergence in solutions gradually increases which impacts
confidence levels as to where the gale force winds will
materialize. By Tuesday, the GFS brings a fourth low into the
Western Aleutians while the ECMWF just brings a wave. These
differences get compounded with time. It is a progressive and
every global model depicts a train of lows from Hokkaido to the
Central Aleutians. But given the vagaries of the models, stay
tuned for updates on these systems.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Heavy Freezing Spray Warning 181.

&&

SYNOPSIS AND MODEL DISCUSSION...KH
SOUTHCENTRAL ALASKA...ML
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TP
LONG TERM...PJS