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fxak68 pafc 210125 

Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
425 PM akst Sat Jan 20 2018

Analysis and upper levels...
the upper level pattern consists of a strong blocking ridge over
the central Bering Sea, and a north-to-south oriented trough over
the central Alaska Mainland. There is also a ridge over northwest
Canada and a negatively tilted trough southwest of the western
Aleutians and southwest Bering. Strong northerly flow over the
eastern Bering and Alaska West Coast is pushing cold air into the
area. An upper level low is forming near Kodiak Island on the
southern end of the Alaska trough. The atmosphere is generally
quite dry, but there are a few showers over the Gulf of Alaska and
Kodiak Island. There is also some moisture sneaking westward
along the Gulf Coast with the leading edge just east of Cape
Suckling. A fairly strong front stretches from the western
Aleutians northwestward across the southwest Bering Sea.


Model discussion...
the numerical models are in good agreement with the synoptic
features and they continue to exhibit good run to run continuity.
The biggest forecast challenge continues to be the duration and
the areal coverage of the fog and stratus in the Copper River
basin, which the models typically do not handle very well. This
will also have an impact on the temperature forecasts for that


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


Short term forecast southcentral Alaska (days 1 and 2: Sunday &

Calm, clear, and quiet conditions will continue over much of
southcentral tonight as weak upper level winds keep any
disturbances away. The biggest forecast challenge tonight is how
strong the winds across portions of the region become and where
there is little wind, how low will the temperatures drop. A much
drier 00z Anchorage sounding suggests that fog and low stratus
should not be much issue at all around Anchorage. Some of the
more sheltered areas like the interior valleys between Portage and
Seward, the Knik River Valley, and the Copper River basin may
still see some patchy fog. Much of the Susitna Valley, east
Anchorage, and the Sterling Highway through Kenai and Soldotna
should get below zero tonight, especially if the winds stay nearly
calm. West Anchorage should see enough of a northerly breeze to
keep temperatures warmer tonight than east Anchorage.

On Sunday, a weak low that is currently over the Gulf will track
northwestward towards the north Gulf Coast and Prince William
Sound. With southeasterly upper level flow, some of the moisture
will intrude inland from the coast. This will increase the clouds
from east to west, followed by a threat for a little snow. The
most snow will fall from Girdwood through Whittier and the Kenai
Mountains starting Sunday afternoon and continuing through the
overnight. General accumulations of 1-3 inches with isolated
higher totals are possible along the Seward Highway. The snow
threat Sunday evening will even approach Anchorage. There is still
quite a bit of uncertainty as to how far west the snow gets, but
enough of the model guidance suggests some flakes reaching the
Anchorage bowl Sunday evening that a chance of snow was introduced
into the forecast. If any snow does fall in Anchorage, a dusting
at most is all that is expected in The Bowl, with perhaps up to an
inch possible at higher elevations of the hillside. Plentiful
cloud cover Sunday night should prevent most locations from seeing
much colder temperatures as Anchorage continues to wait for its
first below zero reading of the winter.

Unsettled conditions with chances for snow showers will continue
across the Gulf and Prince William Sound through Monday with
plenty of cloud cover area-wide. Better potential for snow returns
to Anchorage and the surrounding areas on Tuesday. More on that
in the long term discussion below.


Short term forecast southwest Alaska (days 1 and 2)...
over the next couple of days the story remains the same for the
southwest Mainland, as Arctic air pours into the region and causes
persistent offshore flow and declining temperatures. This will
continue to push both daytime highs and lows downward with
increasing threats for bitter wind chills approaching -40f. This
will be most likely where the winds are the strongest, so it is
likely the Kuskokwim Delta will realize the coldest wind chills.
The one change in this forecast is it seems more likely that a
weak low will move into the Kuskokwim Delta, bringing some light
snow to the coast this evening. Otherwise, cold and windy
conditions with clearing skies will resume on Sunday and carry
into early next week.


Short term forecast Bering Sea/Aleutians (days 1 and 2)...
high pressure settling over the central part of the Bering Sea
will induce more of an easterly flow component as cold air pours
over the eastern Bering Sea and a gale force front moves into the
western Aleutians then stalls. This front looks like it will not
make further progress than the western Aleutians, so increasing
winds to gale force will be the biggest impact with this front.
With cold air pouring over the Bering Sea, expect freezing spray
to be a threat from the Pribilof Islands eastward.


Long term forecast (days 3 through 7)...

Beginning the extended period forecast Monday night, the main
feature of interest will be a very cold upper vortex situated over
the North Slope of Alaska. Another low will be in the vicinity of
the Gulf of Alaska. A deep Arctic air mass will be in place over
all of Alaska, although the bitterly cold air will still be north
of the Alaska Range. The first item of interest will be the
potential for snowfall across southcentral. As the upper trough
digs south and interacts with the Gulf low, this will set up a
potential favorable setup for light snow. However, the degree of
phasing between the two upper troughs is rather low confidence at
this point with some runs of the GFS, NAM and European model (ecmwf) showing the
possibility of weak deformation banding from the Kenai Peninsula
northward. Run-to-run consistency is low, so stuck with a
continuation of broad chance snow probability of precipitation until there is more clarity
with regards to the interaction of the two troughs.

Eventually, the cold upper vortex will move south into Bristol Bay
and dig into the southwest Gulf of Alaska, bringing down all of
the deep Arctic air with it. This will bring the coldest
temperatures to southwest Alaska, but southcentral will also drop
well below average region wide by Wednesday and likely lasting
through Thursday. As the low forms over the Gulf, this will
introduce more of an easterly flow component aloft, and with
offshore flow, will likely keep precipitation predominantly along
the coast with much lesser chances inland. Temperatures will
moderate some by the weekend, but will generally run below average
across all of southern Alaska.


Afc watches/warnings/advisories...
gale 130, 138, 150, 155, 165, 177, 178, 180
heavy freezing spray 121, 130, 138, 139, 150, 160, 165, 180, 181



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