Small Craft Advisory 212, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
456 PM AKDT Sun Mar 11 2018

.ANALYSIS AND UPPER LEVELS...
Snow from a low moving northward across the Gulf has developed
along the North Gulf Coast from Cordova to Valdez, and is beginning
to spread further westward toward the Kenai Peninsula. This low
is also bringing a brief round of gusty outflow winds to areas
such as Resurrection Bay and Passage Canal, but these should all
diminish this evening as the low makes landfall. Just to the west
of this low, a narrow area of high pressure has allowed for the
development of widespread stratus and some patchy fog over much of
the Southwest Mainland, which is beginning to break up as a gale
force front over the Eastern Bering begins to make landfall. This
front will be the focus for snow and blowing snow that will spread
into the Southwest this afternoon and evening. This front has
pushed through the Aleutians and AKpen today, bringing rain and
snow (which has been heavy at times) along with gusty southerly
winds. Behind the front, strong cold air advection can be seen
moving from the Western Bering into the Central Bering with broad
northwesterly flow.

&&

.MODEL DISCUSSION...
Now that the models have come into better agreement with the track
of the low into Prince William Sound this evening, confidence has
begun to increase that much of the Anchorage Bowl and Matanuska
Valley (especially as you move further east) will see a brief
period of light snowfall late this afternoon/early this evening.
Afterwards the models are in very good synoptic agreement through
Monday, and have improved significantly in the track of the next
trough crossing the Gulf late Monday-Tuesday, and as a result
confidence is increasing that another round of rain and snow will
spread back into the North Gulf Coast and Copper River Basin.
Confidence is much lower however over Southcentral as differences
begin to arise in how quickly a front over the Southwest Mainland
will lift northward into the interior, and how far west this will
allow the precip to reach. Overnight the models were in good
agreement that precip would struggle to reach any further west
than Whittier, but the daytime runs today are beginning to trend
slightly further west.

&&

.AVIATION...
PANC...Potential for MVFR snow will be the primary concern this
afternoon/evening as snowfall moves into the area from the east.
The terminal will likely be on the very western edge of this
snowfall, so at this point it appears that the greatest impact
of the MVFR conditions will be for operations to the east of the
airport.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The low moving rapidly into Prince William Sound will continue
to produce snow across the North Gulf Coast tonight, with snow
eventually spreading as far west as Anchorage and the Mat Valley
through late evening before the low dissipates. Accumulations
should remain minimal for most areas, although several inches will
be possible in the Valdez area. Precipitation should continue to
fall as mostly rain towards Cordova where surface temperatures are
a bit warmer. A rather quiet day is on tap for Monday, with a
just a few showers lingering along the Gulf Coast in the wake of
this system. A somewhat complex scenario then looks to take shape
Monday night into Tuesday as a weak upper level trough and a
dissipating front approach the Cook Inlet region from the
southwest while another stronger front moves into the Gulf from
the southeast. This stronger front will spread snow back into
Prince William Sound and the Copper River Basin, with rain once
again mixing in closer to sea level. Meanwhile, the passage of the
upper trough looks to be mostly dry for Cook Inlet and the Mat-Su.
The primary forecast question then becomes the extent to which
the eastward moving trough impedes the westward progress of the
more robust precipitation associated with the front over the Gulf.
Trending towards keeping Anchorage and surrounding areas dry
through Tuesday for now, but there is lower confidence in the
forecast moving into the end of the short term.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Snow will spread over the mainland from the coast this evening.
While the the system is weakening as it moves ashore, winds will
remain just strong enough for blowing snow and reduced visibility
along the coast, where an advisory for blowing snow has been
issued through early Sunday morning. The front slowly falls apart
as it moves inland, lingering light snow through late Monday for
Bristol Bay and the Kuskokwim Valley.

Another system is in the progressive flow as a trough from east
Asia impinges on the frontal boundary draped over the Bering Sea.
The interaction will develop another compact quick moving system
that will move ashore the mainland late Tuesday, with another
round of snow.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The warm front currently over the Alaska Peninsula will continue
to moves eastward, ushering in a short period of showery northwest
flow for the Bering Sea. However, another small and compact system
will move from the Adak/Atka area late tonight toward the mainland
with another shot of snow for the Pribilof Islands. Winds should
remain below gale-force, but not far off.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The long term forecast begins Wednesday with a broad upper level
trough extending down from Northeastern Russia across the Bering
Sea. Short-waves rotating around the base of the trough will spin
up several systems through the end of the week and into next
weekend, continuing the active pattern seen recently across the
region. At the surface, a short-wave trough will push east across
mainland Alaska Wednesday, bringing accumulating snowfall to the
southwest mainland before crossing the Alaska Range and weakening
some as it makes its way into Southcentral, where the prospect of
accumulating snowfall is less certain. On Thursday, a stronger
upper level trough digs down across the Western Aleutians which
looks to spawn the development of a surface low south of the
Central Aleutian chain. The models begin to heavily diverge at
this point with the Euro/Canadian models keeping the low embedded
within the main SW'ly jet, bringing the system, and a decent
amount of precipitation and warmer air, NE across the AKPEN and
into southcentral by Friday. The GFS cuts the low off from the
SW'ly jet by the time it reaches the Aleutians and retrogrades the
system back into the Bering Sea on Friday. Obviously, these two
solutions will have wildly differing impacts of the sensible
weather seen on the ground, especially over Southcentral.

The deterministic models have been struggling lately with the
progressive pattern, and the predictability of individual systems
has been around 120 hrs or less due to poor upstream initialization.
Therefore, the long range forecast has trended more toward the
ensemble means past day 5, which have been more consistent with
the Euro model.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 155 161.
MARINE...Gale Warning 180 185.
         Heavy Freezing Spray 180 181.

&&

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