Area Forecast Discussion
Issued by NWS Missoula, MT

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FXUS65 KMSO 072127

Area Forecast Discussion
National Weather Service Missoula MT
227 PM MST Wed Dec 7 2016

...Prolonged periods of snow north central Idaho and Western
Montana Thursday night through weekend...

.DISCUSSION...Many areas are clearing out this afternoon, and most
of western Montana is expected to have mostly clear skies by
morning. With the very cold air mass and recent snow cover,
temperatures tonight will plummet. Many areas of western Montana
and Lemhi county Idaho will be sub-zero for temperatures, with
much of central Idaho in the low teens and single digits.

Lake effect snow is still a concern tonight and tomorrow along the
southern half of Flathead Lake, especially as the winds decrease.
So far today the snow has been enhanced well south of the lake
near Ronan and Nine Pipes by the warm lake, but the wind has
prevented it from organizing over the lake and producing heavy
snow. We will watch this closely tonight and may extend or reissue
an advisory if conditions warrant.

Weak high pressure will support stable and cold conditions most of
Thursday. The exception should be around Flathead Lake (mainly
central and south portion and shoreline areas) where light Lake-
effect snow is expected.

The first in a series of moist Pacific weather systems is expected
to shift over the northern Rockies Thursday night. This should
support widespread light to moderate snow, especially from
Orofino, Grangeville and Salmon eastward across west central and
southwest Montana (Missoula, Hamilton, Drummond, Anaconda, and
Butte). The light snow should gradually shift northward over
northwest Montana by Friday morning...but the precipitation
intensity will become less with time. Temperatures have also been
adjusted downward in most areas because cold air is expected to
persist, especially from Orofino and the Bitterroots of Montana
northward and eastward across the Continental Divide.

Moisture will continue to stream over the region Friday, but the
precipitation should be more scattered and on the light side.

The next significant moisture surge/weather system is due Friday
night into Saturday. Compared to the Thursday night event, this
new surge of moisture should support more widespread snow and a
few areas of occasionally heavy snow (especially mountain
passes...Lolo, Lost Trail, name a few). Again, the
heaviest snow areas should be from Orofino eastward across the
west central Montana. The atmosphere may also become more
supportive for organized snow bands, meaning local persistent
heavy snow areas. Salmon, William Summit and Bannock Pass, in
Lemhi County, should also see significant periods of snow. Impact
to travel could be very significant at times due to slick roads,
low visibilities and accumulating snow on roads.

Yet another Pacific weather system is expected Saturday night
into Sunday, supporting more widespread snow showers. As with the
previous weather system, intense snow bands are a possibility,
that would create very difficult driving conditions. The favored
areas for the bands to develop, would be along the I-90 Corridor
from Lookout Pass to Butte and south through southwestern Montana
and north-central Idaho, to Lemhi County.

Temperatures will slowly moderate Saturday and Sunday, but only to
around the freezing mark in the lower valleys of western Montana
and slightly warmer in the valleys of north central Idaho.

On Monday of next week, another weather disturbance is expected to
move through central Idaho and western Montana. Earlier model runs
were showing another significant Arctic front but, the latest
guidance have trended the cold arctic air further eastward. But
even with the coldest air further east, it does still look like we
will see moderate valley snow across the area on Monday.

Beyond Monday, we may see a decrease in precipitation (especially
north of the I90 corridor) on Tuesday, but by Thursday another
weather system is expected to push through with an active weather
pattern expected to continue in the long term.


.AVIATION...A weak shortwave moving overhead is interacting with a
cold, moist boundary layer causing mountain obscurations and
flurries across the northern Rockies. As the shortwave exits the
area tonight, expect clearing across most of the valleys affecting
area terminals. As a result of the clearing and the current cold
air mass, temperatures will plummet with most area terminals below
zero by 10-12Z.


MT...Winter Storm Watch from Thursday afternoon through Saturday
     morning Bitterroot/Sapphire Mountains...Lower Clark Fork

     Winter Weather Advisory until 5 PM MST this afternoon
     Flathead/Mission Valleys.

ID...Winter Storm Watch from Thursday afternoon through Saturday
     morning Northern Clearwater Mountains...Southern Clearwater


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