Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 201234

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
434 AM AKDT Fri Oct 20 2017


A mature low pressure system continues to track along the Aleutian
Islands with storm force gusts being observed on the back side of
the low pressure system. This low pressure system is quickly
shifting eastward as it interacts with a strengthening jet stream
over the northern Pacific Ocean. Colder air is beginning to
filter in across the northern Bering Sea and is evident by a
rather agitated cumulus field developing in response to increasing
instability and steepening lapse rates. In addition to the colder
air over the Bering Sea, colder air continues to filter in across
the state of Alaska as the upper level trough amplifies southward
and an upper level low embedded in the trough digs over Southwest
Alaska. This is leading to the coldest temperatures of the
"winter" being observed across much of southern Alaska.



Models are in really good agreement with the evolution and track
of the low pressure system near Atka this morning as it moves into
the Gulf of Alaska late tonight. From here, models begin to really
struggle as models show a triple point low developing over the
southern gulf Saturday morning. It`s difficult to tell how the two
lows will interact with one another (will they merge or will one
low win out) and each model shows a different story. However, the
one consistent theme shows the low pressure system(s) moving
toward the Panhandle which is a change from 24 hours ago when
models showed the system moving toward Prince William Sound. This
will drastically change the precip pattern across Southcentral
Alaska with the latest runs favoring a drier weekend for Anchorage
and Southcentral Alaska. The GFS looks too strong as it tries to
consolidate the energy, the NAM looks like an outlier with 4+ low
centers and the GEM/EC show a semi-similar solution over the
eastern gulf. With all of the uncertainty, the forecast package
trends toward the GEM/ECMWF drier solution due to better run-to-
run consistency.



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



Southcentral Alaska remains under mostly clear skies with cold
temperatures this morning with numerous areas in single digits.
The Copper River Basin is keeping some cloud cover in it as the
upper level trough moves eastward over it.

The overall pattern is beginning to change today with a cold upper
level low digging southward into Southwest Alaska tilting the
upper level flow to the south and southwest. A weak surface low
east of Middleton Island will begin to push clouds up over the
Prince William Sound region today and toward Anchorage and the
Mat-Su region tonight. There is a chance for some snow showers as
far northwest as Anchorage and the Matanuska Valley late tonight
and into Saturday with an upper level wave moving through the
area, but any snow amounts are expected to be minor.

Sunday WAS looking like a day that could see some significant
snow accumulations, but it is looking less and less likely with
each passing model run. The low that was being indicated to move
into the Prince William Sound area has shifted its track much
farther east which removed the chance for any significant
snowfall from Southcentral Alaska this weekend. There may be some
weak impulses that kick off some light snow showers in the
Anchorage area, but it does not look like anything more than that
for the weekend.


An arctic trough currently over Norton Sound will continue slowly
descending down the west coast today, bringing snow showers
primarily to the Delta coast. This trough will produce a weak
front near Western Bristol Bay late tonight, but there remains
some uncertainty in how far east precipitation from this front
will develop, likely in the form of snow or a cold rain. At this
point the best chance for accumulating snow exists to the west of
Dillingham, however light accumulations from Dillingham northward
into the Kuskokwim Valley are possible if the front sets up
further east on Saturday afternoon-evening. This arctic trough
will be then primary driver for the regions weather as it slowly
weakens just off to the north this weekend. This will keep
generally cloudy skies, a chance for snow showers and continued
below normal temperatures for most areas through Sunday.


A weakening gale force low will continue to bring rain and gusty
winds to the Aleutian Islands this morning, becoming snow towards
the Pribilof Islands. As this low moves south of the area, broad
northerly flow behind it will allow cold air to spread throughout
the region, producing widespread small craft winds and snow
showers that should spread as far south as the Aleutian Islands.
Accumulations are however expected to be very light as SST`s in
the 40s will prevent any significant accumulations from developing
in the lower elevations. This pattern will hold through Sunday
when it will begin to break down in advance of the next low
expected to move into the Western Aleutians for Monday.


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

An active pattern is expected through the long range forecast
through the middle of next week. The main challenge surrounds a
storm system which slides east into the gulf this weekend bringing
the potential for snow across Southcentral into early next week.
Confidence remains higher in cold air advection keeping
temperatures below normal across much of the southern mainland
through the weekend. During this time, the area will remain under
dry offshore flow conditions as low pressure tracks south of the
Alaska Peninsula to the Gulf. For locations along the Bering,
including the southwest coast, precipitation chances increase
through the upcoming weekend, as an upper low pressure system in
the eastern Bering slowly propagates south toward Bristol Bay. A
colder air mass advecting from the northern Bering changes precip
type mainly to snow except along the Aleutians where a mix of rain
and snow prevail under moderate temperatures.

The biggest forecast challenge remains the track and placement of
the low tracking south of the Gulf, and the associated frontal
boundary over the northern Gulf this weekend. Guidance continues
to struggle with the placement and track of this system, as well
as some upper level disturbances peeling off of the upper level
low over the western Coast. Confidence in snowfall over the Cook
Inlet region and inland areas of Southcentral has decreased with
this forecast package given the drier trend and more southerly
low track. However, due to the inconsistency and variation of the
models recently, we can not count snow completely out of the
forecast just yet. With high uncertainty during the mid to long
range forecast, kept changes minimal through the middle of next

Early next week there is better agreement on the remnants of
Typhoon Lan tracking into the Western Aleutians/Bering Monday
morning, and becoming the dominant weather feature over the
Bering/Aleutians and southern Mainland through the end of next
week. This system is showing the potential to develop into a
strong system, capable of bringing gusty winds, warmer
temperatures, and a fair amount of rain to the area.


MARINE...Gale...170 172-178



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