Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 190108

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
508 PM AKDT Fri Aug 18 2017


A broad cold upper level trough encompasses much of the Alaska
mainland, with an upper level low centered in the Chukchi Sea and
a second upper level wave that is centered near King Salmon. At
the surface a weak front is pushing in along the north Gulf of
Alaska coast. Both features are helping to support widespread
showers along the north Gulf coast and also across Bristol Bay
near King Salmon. Meanwhile a low is moving northwards from the
north Pacific Ocean into the Central Aleutians and bringing some
gusty winds and showers. Meanwhile high pressure dominates across
the western and central Bering.



Models are in generally good agreement in the short term but begin
to drift in the position and intensity of a low pressure system as
it pushes into the Gulf of Alaska late Sunday night into Monday.
This will have important implications for wind gust intensity
along the north gulf coast and Prince William Sound, and also for
rainfall across Southcentral.


PANC...A band of rain is moving towards Anchorage which will
begin to impact the area by afternoon. Currently southeast flow is
expected to persist into the evening hours, which will help to
keep higher ceilings even as the rain approaches. Gusty southeast
winds will slowly diminish by the evening hours.


Cool and rainy conditions will persist overnight as a vigorous
upper level shortwave trough swings through Southcentral bringing
a wide swath of precipitation up the Cook Inlet and into Prince
William Sound. A surface low will form off the east coast of the
Kenai Peninsula this evening and move northeast tonight. Winds
wrapping around the top of the low will temporally form a weak
easterly barrier jet along the Northern Gulf Coast with increased
winds extending into southern portions of Prince William Sound.
The upper level trough and associated surface low move off to the
northeast tomorrow morning, with precipitation tapering off across
the region after midnight, though rain will linger along the Gulf
Coast through Saturday afternoon.

The next system approaches the region on Sunday as a deep upper
level low dropping down from the arctic merges with the remnants
of Tropical Cyclone Banyan moving in from the Northern Pacific. A
large swath of moisture will slam into the Gulf Coast by Sunday
afternoon leading to periods of very heavy rain through most of
the day on Monday. The heaviest precipitation will be focused
along the Northern Gulf Coast, the Prince William Sound, and the
Northeastern Coast of the Kenai Peninsula. Precipitation is also
expected to make its way into the Anchorage and Mat-Su on Monday,
though there is still some uncertainty regarding the timing and
amount of rain for locations on the west side of the Chugach.



Expecting a fetch of moisture which is currently over the Bering
to get entrained into the back side of the midlevel low bringing
precipitation into the AK Peninsula tonight. A broad vortex over
western AK will continue to amplify enabling cooler air aloft to
work its way deeper into Southwest Alaska. Unsettled weather will
persist for the next few days as cooler air associated with the
trough moves southward. Between the embedded shortwaves and the
diurnal heating at the surface, the dynamics will be present to
support showers. Temperatures will hover near or slightly below
average for this time of year.



South of Kiska, there is a closed low that is progged to move
eastward and slowly deepen. In its wake, expect ridging which
will keep a stable and relatively warm air mass in place. The
Aleutian Chain and portions of the Bering can anticipate more fog
and stratus. The ridge axis will persist into next week keeping
the areas of pesky fog in place.


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

Beginning the extended period forecast Monday night, there will be
a closed upper low over the North Gulf of Alaska with a shortwave
ridge axis over the Eastern Bering Sea. Expect showery and mostly
cloudy/cool conditions to be over Southcentral with drier weather
over Southwest. Tuesday and Wednesday will be a transition period
as Southern Alaska moves into weak shortwave ridging as the upper
low weakens and moves east. Another trough over the Bering Sea
will move toward Southwest Alaska on Wednesday and eventually dig
south toward Kodiak Island through the remainder of the week. This
will keep a wet pattern in place across coastal regions with
clouds and rain chances for inland areas. The degree of downslope
flow for interior locations remains somewhat uncertain at this
point as the low track and placement will modulate that factor,
but it still looks wetter and cooler for most of interior Southern
Alaska for the foreseeable future.





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