Area Forecast Discussion
Issued by NWS Anchorage, AK

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642
FXAK68 PAFC 050136
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
436 PM AKST Sun Dec 4 2016

.ANALYSIS AND UPPER LEVELS...

Arctic High pressure over Interior and Southwestern Alaska is
building eastward into Southcentral. In response to the clear
skies, diminishing winds, and deep snow pack in many areas,
temperatures have been dropping for much of the morning.
Temperatures have been hovering between 5 and 10 above in west
Anchorage and the Susitna Valley, with values from 5 above to 10
below in the Matanuska Valley, east Anchorage, and the western
Kenai. With the sun already setting, expect some of the coldest
air of the season tonight in Southcentral. More on that in the
Climate section.

An Omega blocking pattern is controlling the weather over much of
Alaska this afternoon. This is characterized by a highly amplified
ridge in the upper levels between two troughs to the east and
west.

While very cold air remains near the surface state-wide, warmer
air aloft associated with the advancing ridge is pushing eastward
over southwest Alaska. However, with such an arctic air mass in
place at the surface, it would be very difficult to tell at this
time as more areas than not remain below zero. A front delineating
the leading edge of the surface warm air extends north-south over
the Bering between the Pribilofs and the mainland. This is forcing
steady rain across the Pribilofs with gusty winds. A showery,
convective environment is west of the Pribilofs with upper level
cold air in place above surface warm air, thus causing the
instability.

&&

.MODEL DISCUSSION...

Models remain in great agreement for the short term as the ridge
over the Bering pushes eastward into mainland Alaska, with a sharp
trough on its heels. Models also are in pretty good agreement on
the consolidation of this trough into an upper level low as it
pushes across Kodiak Island.

&&

.CLIMATE...

With many areas across Southcentral expected to see their coldest
temperatures of the season tonight, we couldn`t help but wonder
how long it`s been since temperatures this low were seen? To
answer this question, this list was made showing the forecast lows
for select sites across Southcentral tonight, along with a table
of dates indicating when the last time temperatures this low were
seen at those respective sites.

City.........Forecast Low temp...Last time it was this cold
-----------------------------------------------------------

Anchorage.......3 below....November 18, 2015
Cordova.........8 above....December 25, 2015
Gulkana.........24 below...November 22, 2016
Homer...........6 above....November 17, 2015
Kenai...........10 below...February 5, 2015
Kodiak..........15 above...December 13, 2015
Merrill Field...6 below....November 18, 2015
Palmer..........5 below....November 18, 2015
Seward..........10 above...March 15, 2015
Soldotna........12 below...November 17, 2015
Talkeetna.......5 below....November 29, 2016

&&

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

Cloud cover kept temperatures across Southcentral Alaska quite a
bit warmer last night than they had the potential to fall to given
the very cold air mass aloft which has pushed to the area with the
arctic trough. Skies clearing with the departure of the Gulf low
to the southeast however, will allow temperatures to fall much
more sharply tonight. Also as the low pulls away, northwesterly
outflow winds will continue to diminish along the western Gulf.
Northerly flow out of the Copper River Basin supported by the cold
pool deepening inland, however will last longer, continuing
northeast winds through Thompson Pass and Keystone Canyon and
north outflow winds across the Copper River Delta.

An upper level ridge building in from the west will shift over the
western Gulf tonight beginning to warm temperatures aloft over
that area. A frontal system following behind will spread precipitation
across Kodiak Island on Monday and over the eastern Kenai Peninsula
and northern Gulf Coast Monday night. Once the associated low crosses
the Alaska Peninsula Monday night and Kodiak Island Tuesday morning
it will continue slowly to the southeast Tuesday with a variety of
troughs and small surface lows rotating around the periphery of
the Gulf. Further north and inland, the cold air mass and offshore
northerly flow will persist.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...

Clouds continue to increase tonight especially along the Kuskokwim
Delta as the Bering front approaches the region. The very cold
dome of air over the mainland will serve as a block for this
system, with a new triple point low developing near the Pribilofs
tonight and then sliding toward the Alaska Peninsula Monday. The
warm advection aloft, along with overrunning (isentropic lift) and
upslope flow will make the western half of the Bristol Bay region
the most susceptible to the highest snow accumulations, but no
advisories, watches, or warnings are anticipated with this system.
The biggest question mark with this system will be extent of snow
inland toward Bethel and the Lower Kuskokwim Valley. The 12z and
18z NAM runs continues to depict a slightly different upper level
pattern that allows for accumulating snow to reach Bethel. Some
deference was given to the NAM, resulting in the raising of some
POPs. Precipitation will begin to taper off once again by Monday
night as the low departs towards the Gulf, bringing a return of
drier northerly flow heading into the middle of the week.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The primary forecast concerns through Monday will be largely
confined to the southeast Bering as the strong gale-force front
moves eastward from the Pribilof Islands to the Eastern Aleutians
and the Alaska Peninsula later today, with the aforementioned
triple-point low forming late tonight and pushing through Bristol
Bay on Monday. This will keep a strong south-southeast winds and
a rain/snow mix over much of the region through Monday, with the
developing triple point then helping to focus stronger northwest
winds over the Akpen on Monday night as cold advection commences
on the backside of the low. This will also facilitate a change
back to snow as the predominant precipitation type as the event
begins to wind down. The next front will then enter the far
western Aleutians Monday night accompanied by more southerly winds
and potentially heavy precipitation.

&&

.LONG TERM FORECAST (Days 3 through 7)...
A continuation of below normal temperatures and little
precipitation will characterize the extended forecast. Day 3
begins Tuesday afternoon with a weakening upper level low sliding
into the Gulf. There is little frontal activity with this system,
but a decent pool of cold air aloft (-35C to -40C) in which to
promote shower activity. The pattern will keep snow showers along
the north Gulf coast through Wednesday, especially in the western
Prince William Sound area. The combination of high pressure in the
Interior/Yukon and low in the Gulf will keep gap winds going from
Valdez through the Copper River Delta. Overall the pattern will be
very dry and cold through mid-week, however, some moisture could
make it over the mountains in the form of flurries.

The extended forecast looks to be much of the same as well,
continued arctic air overhead with little threat for precipitation.
The two features that will shape the pattern are a developing
blocking high pressure over northwestern Alaska, and a large
anchoring Siberian low pressure. The east Asian jet working around
the south side of the trough will be split as the blocking high
sends part of the energy south of the Aleutians. Confidence is
higher that we will remain in the dry and cold pattern through the
end of the week.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Wind Chill Advisory 145.
MARINE...Gales 127 130 155 165 172 178 180 181 185.
         Heavy Freezing Spray...121 126 127 129 130 139 160 181.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...EN/JW
SOUTHCENTRAL ALASKA...JR
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CC
LONG TERM...ML



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