Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 210024
AFDAFC

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

.ANALYSIS AND UPPER LEVELS...

Another clear and cold day is being observed across most of
southcentral Alaska. The exception is over the Copper River Basin
and the eastern portions of Prince William Sound where clouds and
limited snow shower activity is occurring. These clouds are
slowly making progress westward and should approach the Kenai-
Anchorage-Palmer corridor by late tonight. Meanwhile, a low
pressure center is evident on satellite imagery south of the
Alaska Peninsula this morning. The low has an associated warm
front stretched out to the east, and the entire system is moving
almost due east. This was the storm that was potentially going to
impact southcentral Alaska but is now expected to head instead to
the Panhandle.

Meanwhile, a very strong upper level low is dropping down the west
coast of the state accompanied by a renewed push of cold air at
the low levels and very cold air at the upper levels. This is
resulting in unstable conditions and widespread shower activity across
the Bering Sea. In addition, areas of light snow have developed
from the Kuskokwim Delta over into the lower Kuskokwim Valley as
improved dynamics move into the area and squeeze a bit of snow out
of the airmass. This is generally expected to continue for the
next day or two as the upper low remains in the area.

&&

.MODEL DISCUSSION...

The models are still struggling with the repeat-triple point
development of the low currently south of the AKPEN, but
fortunately these differences will not impact the weather across
our forecast area in a significant way. Similarly, the upper low
on the west coast of Alaska along with the associated surface low
is in pretty good agreement and no strong preference is noted
today. There are discrepancies among the models with a basal
shortwave rotating around the bottom of the big upper low and then
moving northward either through Bristol Bay or up Cook Inlet on
Sunday/Sunday night. Confidence is not high enough to include this
feature strongly in the forecast today, but wherever it ends up
should see an increased chance of some light snow as the feature
moves through.

&&

.AVIATION...
PANC...Some mid-level clouds will likely move into the area after
midnight tonight. There may also be some snow flurries, but
conditions are expected to stay VFR with light winds.

&&

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

The cold, sunny weather of the past few days will be coming to an
end across Southcentral for the weekend. A weak disturbance over
the northern Gulf and Prince William Sound will creep westward
overnight tonight, bringing cloud cover back to the area from
about midnight onward. It will be aided by some mid-level warm
air advection and a subtle wave in the upper atmosphere. However,
a strong negative for anywhere west of the Chugach to pick up snow
from this system is the continual influx of dry air moving
southward out of the interior, along with how dry the atmosphere
already is. Thus, at most, just flurries can be expected for the
Cook Inlet region, but really most areas shouldn`t see any snow at
all. Mostly cloudy skies with the potential for flurries will
persist through the day Saturday and into the evening.

Then late Saturday night through the day on Sunday, a piece of
upper level energy rotating around an upper level low over
Southwest Alaska may be the catalyst to bring the first snowfall
of the season to portions of Southcentral. With the previously
mentioned low now forecast to track well east of the area into
the Panhandle, this upper level energy won`t have nearly as much
moisture to take advantage of as it would have had the storm
tracked closer. Thus, once again, moisture will be the limiting
factor for how much snow the area sees. Unfortunately for snow-
lovers, it will be a major limiting factor, essentially limiting
any snow seen to less than an inch. If the wave produces snow at
all, it will be in the form of scattered snow showers tracking up
the Inlet and into the Susitna Valley. Thus, one should expect
that for the vast majority of the period, there will not be any
snow falling in any one location. The uncertainty with any snow
currently lies with both timing and amounts. How much snow will
be wrung out from a still quite dry atmosphere and when the
strongest forcing will make snow the most likely late Saturday
night and Sunday remain questions with no clear answer at this
time.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...

The main weather driver over the next few days will be a strong
upper-level arctic low moving southward. This feature is
currently centered over the Seward Peninsula and will drop south
to the Kuskokwim Delta tonight, where it will stall through Sunday
afternoon. Significant cold air aloft will move into Southwest
Alaska with this feature, creating an increasingly unstable
airmass. Ahead of this feature, diffluence aloft is providing
enough lift to wring out some scattered snow showers over the
Upper Kuskokwim Valley. This area will remain just downstream of
the upper-low through tomorrow, so the chance for some scattered
snow showers in the Upper Kuskokwim Valley will continue
overnight. Moisture is limited in this area however, so
accumulations will be very light. A weak front will develop
tonight over the Southwest Coast and move to Bristol Bay by
Saturday evening. This will bring some light snow to the Bristol
Bay area Saturday afternoon through Saturday night. The front will
weaken as it moves inland Saturday night.

Sunday looks more uncertain as models are having difficulty with
timing and placement of upper-level waves that rotate around the
arctic low and into the Mainland. Sunday will feature more
widespread snow showers than Saturday over the Kuskokwim Coast
down to the Interior Bristol Bay area, but confidence in the exact
location of the precipitation is low.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...

The low pressure system south of the Alaska Peninsula will
continue to weaken as it exits the area to the east tonight.
A small swath of gale force winds to the south of the eastern
Aleutians will move into the North Pacific as the low departs.
Small craft force winds remaining over the eastern Aleutians and
surrounding waters will weaken overnight. Strong cold air
advection will preside over the Bering Sea through Sunday as an
Arctic low over the west coast pushes an Arctic airmass southward.
The atmosphere over the Bering Sea will become increasingly
unstable, which will support widespread gusty small craft winds
and snow showers through Sunday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
An active pattern is expected through the long range forecast
through the middle of next week. Confidence remains higher in
cold air advection keeping temperatures below normal across much
of the Bering/Aleutians and southern mainland through the
beginning of next week. During this time, the area will remain
under an unstable and showery regime as the colder air circulates
around the upper low parked over the western coast. This colder
air mass will change precipitation types mainly to snow over the
Bering, southwest Mainland, and interior portions of Southcentral
Alaska. Areas along the Gulf Coast, Aleutian chain, and southern
Cook inlet remain the exception, showing higher potential for a
rain/snow mix under moderate temperatures.

Our focus then shifts to the remnants of Typhoon Lan tracking
into the Western Aleutians/Bering early next week, becoming the
dominant weather feature over much of the area through the end of
next week. Models remain in good agreement with tracking the
strong leading warm front into the western Aleutians/Bering Monday
morning, and spreading eastward through the central and eastern
Aleutians/Bering Tuesday. This will usher in a warm moist air mass
into the area from the north Pacific, along with strong gusty
winds and rain. As the parent low tracks through the western
Aleutians Tuesday and continues to occlude, the colder air
wrapping in behind the system should bring in some strong gusty
winds and a rain/snow mix to the western Aleutians through
Wednesday. This system will eventually spread the warmer air and
rain into the southern Mainland Tuesday evening through the end
of the week.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 172.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...AD
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...RD
LONG TERM...TP



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