Small Craft Advisory 347, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
536 AM AKDT Sat Apr 28 2018

.ANALYSIS AND UPPER LEVELS...
The upper level pattern early this morning consists of continued
zonal westerly flow across the Aleutians, Bering and into
Southwest Alaska. An upper trough and accompanying surface low
over the eastern Bering is pushing an accompanying frontal system
over the Alaska Peninsula and into Southwest Alaska while a second
developing frontal system approaches the western Aleutians from
the southwest. A cool upper level trough is exiting Southcentral
to the east with a flat transient ridge building in behind it.

&&

.MODEL DISCUSSION...
While models are handling the initial frontal system fairly well,
the next developing low which will cross the Bering this weekend
is presenting more of a challenge. By Sunday when the low is over
the northern Bering, there is a significant west to east spread on
low position, though interestingly the strength of the low is
pretty consistent in the mid to lower 970s. Currently the EC and
GFS keep the low further west and offshore while the NAM and the
Canadian bring it in closer to the Y-K Delta coastal waters. The
NAM was chosen as the preferred solution to trend towards for
both domains.

&&

.AVIATION...
PANC...Predominately VFR conditions with breezy south to
southeasterly winds will continue through the weekend. A frontal
system swinging through this afternoon will bring the potential
for some brief periods of MVFR during showers, otherwise PANC
should stay mostly downsloped.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...

The active weather pattern continues across Southcentral through
the weekend. A surface low heads into western Alaska as its
associated frontal trough moves into Southcentral today. These
systems will bring generally windy conditions, with strong gap
flows across Turnagain Arm, Knik River Valley, and along the
Copper River today. The brunt of the precipitation once again
will be along the coasts and upslope areas of the Susitna Valley.
Warmer air will accompany this system so lower elevations not
shadowed by cross-barrier flow, will trend toward rain. That
said, higher elevations and northern valley sections will likely
see some snow or mix.

Except for the coasts, there will be a brief break in
precipitation overnight before the next system moves in on Sunday.
The weather front on Sunday brings another round of precipitation
with generally rain, but mixed or snow potential at higher
elevations and northern valley sections. Winds, especially gap
flows, may briefly diminish tonight, but that will be short lived
as the front coming in on Sunday will bring stronger winds. Winds
in many areas will peak in the late afternoon to early evening
hours Sunday before diminishing Sunday night.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2: Saturday and
Sunday)...

An area of low pressure moving into Kuskokwim Bay this morning
will continue to weaken as it tracks toward the coast. The cold
pool of air aloft associated with this system has also tracked
farther onshore, resulting in a push of warmer air into the
region and a changeover to a mix of rain and snow along the
southwest coast. Behind this system, persistent westerly winds and
vorticity advection in association with an upper-level trough
will keep scattered showers of mixed precipitation throughout the
region through the afternoon hours.

A brief ridge of high pressure will slide across southwest AK
this evening before the next disturbance approaches from the
western Aleutians. This system will advect more warm air across
the eastern Aleutians and into southwest AK as it strengthens and
tracks northeastward across the Bering Strait. Precipitation type
should only be a factor in the mountains as the coldest air will
remain north of the region. Precipitation will overspread the
region by midnight with the heaviest occurring Sunday morning.
With the progressive, long-wave pattern continuing, this system
will exit the region quickly, with showers diminishing by
nightfall. Southwesterly winds behind this system could keep
scattered snow showers continuing for higher elevations through
Monday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2: Saturday
and Sunday)...

Two areas of low pressure bookend the Aleutians this morning. The
first is continuing to weaken as it tracks eastward into Kuskokwim
Bay. This system will continue to bring a few widely scattered
rain showers to the eastern Aleutians and AKPen through the
afternoon hours. The second is situated south of Shemya and is
poised to strengthen as it tracks northeast across the western
Aleutians into the central Bering Sea this afternoon. This will
bring gales to the majority of coastal zones throughout today and
into the first half of Sunday. Storm force gusts are also possible
in the offshore zones north and west of the Pribilof's. The
associated front will slide eastward along the Aleutians today
advecting plenty of warm air from the south. This will produce
widespread rain across the chain. The front quickly moves toward
the coast tonight as the pattern remains quite progressive. Colder
air behind the departing low could trigger a few widely scattered
snow showers north of the Aleutians Sunday.

&&

.LONG TERM FORECAST (Days 3 through 7)...

Tuesday through Wednesday a slow pattern shift begins to evolve
in the upper level jet stream, initially a ridge amplifies across
the Alcan border with troughing across the north Pacific and
Aleutian Chain. This jet stream shift cuts off the mid level low
over the northwest mainland, which weakens in response as it
slowly spins along the Brooks Range. Models are all in descent
agreement with a slow progressive change of the jet stream through
the end of the week; however, there are significant differences
between the low and mid level dynamics which lead to less
confidence in the extended forecast through the end of next week.
Overall, the storm tracks are expected to move at a slower pace as
amplified ridging separates each system. Also, storm systems are
expected to track along or just south of the Aleutian Chain as the
jet stream migrates further south.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales...170 173 174 175 176 177 178 179 180 181 185.

&&

SYNOPSIS AND MODEL DISCUSSION...JR
SOUTHCENTRAL ALASKA...BC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TM
LONG TERM...KH