Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 200024

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
424 PM AKDT FRI AUG 19 2016

The upper level low responsible for active weather the past few
days is still draped over the area. It stretches from the north
gulf coast southwestward well into the northern Pacific Ocean. The
jet associated with it is pointed at the Panhandle, while the last
disturbance aloft is sliding eastward toward that area as well. A
stout closed upper high pressure system is sitting over the
northwest Pacific with a ridge axis stretching northeastward into
southwest Alaska. The more active jet stream is coming in from
the southwest over the Kamchatka Peninsula, complete with a
bountiful stream of moisture all the way into northern Alaska. A
frontal boundary draped from the Bering Strait southward through
the Central Aleutians. This front is the boundary between the
airmass left over from the previous trough, and the much warmer
airmass underneath the ridge.

Southcentral Alaska currently is between all these features with
the upper level low exiting and the ridge not quite here yet.
Plentiful moisture is left over in the low to mid levels to help
with shower activity today, but activity is in the isolated range
so far today.


Guidance remains in lockstep with the evolution of the upcoming
pattern both aloft and at the surface. The general setup is the
incoming east Asian jet will push the Bering Sea frontal boundary
across the eastern Bering onto the mainland. While the large scale
pattern is well agreed upon some of the small scale disturbances
and fronts moving into the gulf differ in timing/strength by the
weekend and beyond.


The weakening upper level trough stretching from the southwest to
northern Gulf will continue east tonight with flat ridging
building in over southcentral Alaska from the west. With the
building ridge, convection should die down pretty quickly later
this evening with partial clearing overnight. The question,
however will be what fills in the areas where the skies do
clear...patchy valley fog or a low stratus deck. Southerly flow up
Cook Inland and into the Susitna Valley with some very marginal
lift would tend to favor stratus...especially near the water, while
more protected inland valleys would be more susceptive to
radiative fog if they get enough clearing.

The next rather wet frontal system will approach from the west
Saturday with clouds and the leading edge of the precipitation
spreading into the northern Susitna Valley in the afternoon. Rain
will become widespread overnight Saturday night then heavy at
times Sunday. The heaviest rain will likely fall along Kodiak
Island, coastal areas of the eastern Kenai Peninsula, eastern
Prince William Sound and up the Susitna Valley where southerly
flow aloft will funnel moisture. Southeasterly gap winds will
increase through Turnagain Arm and other similarly oriented
valleys as will as into the Copper River Basin Saturday night and


The low that has been near the Alaska Peninsula will slowly drift
off to the east as weak ridging builds in from the west. This
ridging will allow for fog formation around Bristol Bay tonight
that will be aided by weak onshore southwest flow. The ridging
will also help briefly clear out some of the rain but that will be
short lived as the next system will move in Saturday and bring
steady rain through the weekend.


Ridging over the western Bering will swing south and east
throughout the day as the next system makes its way into the
Bering from the west. This system has a strong push of warm air
that has helped enhance the inversion over the western Aleutians
bringing dense fog to the area. As the warm air eventually moves
south conditions will improve, but patchy fog will remain through
Saturday. This system will also bring more rain to the Aleutians
Saturday that will last through the weekend.


.LONG TERM FORECAST (Days 3 through 7)...
An active weather pattern continues to headline the extended
forecast for later this weekend through the later next week. A
broad upper level trough will dig southward and encompass the
Central and Eastern Bering/Aleutians, and eastward over Southwest
Mainland beginning Sunday. This trough will begin to elongate and
stall on Monday, with its trough axis persisting over the
Southwest Coast and Alaska Peninsula through mid week, and briefly
splitting the energy north and south on Tuesday night. Models are
now coming into better agreement with the GFS`s initial solution
of bringing the southern portion of this energy back northward and
reinforcing the broad upper level trough over the Eastern half of
the Bering and Southern Mainland by Wednesday night. The GFS and
EC and now both showing a stronger system forming at the surface
and coinciding with the northward moving energy mentioned above,
but given the consistent shifting in the model solutions over the
past few runs, confidence still remains lower. The preferred GFS
solution shows this surface low and associated front tracking into
the Southern Mainland and Gulf Wednesday night, bringing more
rainy and windy conditions to the Southern Mainland through the
end of the week.

A second system will enter the Western Bering early Thursday
morning, flattening out the ridging over the area and bringing
another round of wet weather to the area as it crosses through
the Bering and into the eastern Aleutians and Alaska Peninsula by
the end of the week.




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