Small Craft Advisory 187, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
450 PM AKST Thu Mar 1 2018

.ANALYSIS AND UPPER LEVELS...
A warm high amplitude upper level ridge is over Southcentral
Alaska northward to the Arctic coast today, with the deep upper
level trough in the western Bering Sea. The jet stream at 250 mb
runs south to north in the eastern Bering between these features,
cuts across the northern side of the ridge and then goes north to
south along the AlCan border. The front associated with the low is
moving on shore along the Kuskokwim Delta and will move over
Bristol Bay tonight. Showers are occurring in the western Bering
sea in cold advection behind the front.

&&

.MODEL DISCUSSION...
Models are remaining in surprisingly good agreement considering
the persistent progressive pattern. No significant synoptic
differences through the weekend. The only item to really discuss
are the smaller features such as the trough moving though the
area over the weekend. Timing looks good on all models with
heavier precipitation the farther north one goes. Anchorage looks
to be on the southern edge of the precipitation and the NAM model
has even shut off all snow in Anchorage. GFS continues to show a
light snowfall.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...Fri and
Sat)...
High pressure will keep weather relatively calm the next 36 hours
before the next weak system starts to move through Saturday. For
tonight, offshore winds, especially at Seward, Valdez, and
Thompson Pass, will start to dramatically drop off later this
evening as upper level support/mixing starts to shift east. The
ridge axis will move directly over top of Cook Inlet early Friday
morning. This could lead to some patchy fog around open water
(Kenai up through Anchorage). However, mid-high clouds will also
be on the increase tonight and this could be just enough to
disrupt any fog formation. A fairly compact low pressure system
will approach Kodiak Island from the southwest by Friday
afternoon. As this system runs into the ridge, it will struggle to
move north. In fact, there is only a chance for precipitation at
this time for Kodiak City. If any does make it in there, it will
be the only location around the area to have a chance to see rain
mix with the snow.

It will take a trailing upper-level trough to bring the next
chance for precipitation to the majority of southern AK on
Saturday. This trough will clip just north of the afore mentioned
low pressure center. It will have limited moisture to work with
and it will be weakening as it moves through. However, it still
does have enough dynamics and some cold air to bring light snow to
Anchorage. The best chance for a few inches will be north of the
Cook Inlet, over the Susitna Valley and the Talkeetna Mountains.
Temperatures will remain at or below normal for just about all of
the area, so any precipitation that does fall, will be safely all
snow.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA...
A frontal system will continue to track from the west coast
across the Kuskokwim River Valley and Bristol Bay area tonight
through Friday morning. Snow will move through the Kuskokwim
Delta and Lower Kuskokwim Valley before tapering off in the
morning as the front weakens after becoming detached from the
parent low and elongates. Friday morning, a low will skirt south
the of Eastern Aleutians and AKPEN bringing moisture north into
the Bristol Bay area. This moisture will combine with the remnants
of the previous decaying front over the area and a weak
deformation zone to produce steady snowfall over the Northern
Bristol Bay area and Lower Kuskokwim River Valley Friday through
Saturday morning. Total snowfall looks to range from 3-6" with the
heaviest amounts seen between New Stuyahok and Togiak, including
Dillingham. High pressure builds in over the SW mainland by
Saturday afternoon before the next system quickly approaches
Sunday morning.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS...
An upper level trough will dig south across the central Bering
tonight and Friday with cold westerly flow and embedded snow
showers developing over most of the area. Weak ridging will build
in out ahead of the next low Friday afternoon through Saturday
bringing a short period of rather tranquil weather to the area.
The next system moving in across the Western Aleutians will make
it to the central Aleutians Saturday morning and the eastern
Bering Saturday evening, bringing a swath of gale force winds as
the front sweeps across the region. It is also pulling up enough
warm air to bring rain or a mix of rain and snow to the Islands
as it passes.

&&

.LONG TERM FORECAST (Days 3 through 7)...
At the onset of the forecast period, the GFS ensemble members are
relatively grouped with the main synoptic features. There is a
low near Kamchatka, a dome of high pressure south of the Central
Aleutians and well-defined troughing north of the Alaska Range. As
the forecast period unfolds the ensemble members remain clustered
with the high in the North Pacific, but the ensemble members
diverge over the Bering pretty significantly. This does not bode
well for pinging into the precise timing of the fronts as they
cross the Bering, but at least two disturbances will push across
the Bering this forecast period and then move into Western Alaska.
Very confident in the progressive nature of the pattern. Monday a
new batch of wintry weather is anticipated to impact Kodiak with
moisture working its way up Cook Inlet later Monday impacting the
Anchorage Metro Area Tuesday and Wednesday. Late in the period,
the GFS and Canadian GEM bring additional moisture and dynamics
towards Southcentral from the north but the timing of the waves
has not phased yet. The bottom line is, the Anchorage Bowl and
the Mat-Su Valleys can expect more snow.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 155 172 177 178.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...MSO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KVP
LONG TERM...PJS