Area Forecast Discussion
Issued by NWS Missoula, MT

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FXUS65 KMSO 202141

Area Forecast Discussion
National Weather Service Missoula MT
241 PM MST Fri Jan 20 2017

.DISCUSSION...Morning fog has largely dissipated across valley
areas, although central and southern sections of the Bitterroot
Valley are still experiencing areas of fog. Aside from some high
clouds, mostly clear skies this evening should allow freezing fog
to redevelop overnight in many of the same areas that experienced
it this morning. Additional cloud cover by early Saturday morning
may limit the extent of dense fog but certainly it may become
locally dense, especially in the Bitterroot and Missoula valleys.
Travelers should watch for limited visibilities and patches of
black ice as roadways refreeze.

A low pressure system off the Pacific northwest coast will
send a few waves of energy and moisture into western Montana and
central Idaho beginning Saturday. The first wave for Saturday into
Saturday evening will result in areas of light snow through
central Idaho and west-central/northwest Montana. A second, more
prominent wave is expected Sunday afternoon into early Monday
morning. This wave will likely produce more widespread
accumulating snow, with the focus of the best amounts through
central Idaho into northwest Montana. Accumulations will generally
range from 2-4" in these areas, with roads likely becoming snow
covered and slick for any planning travel. Between these two
systems, there may be enough clearing for some freezing fog to
develop once again Sunday morning, though how dense and widespread
will depend on amount of lingering cloud cover.

On Monday, the low pressure system along the Pacific northwest
coast will begin to shift southward. As this occurs, forecast
models have been showing a mid level circulation spinning off of
the parent low towards the Northern Rockies region. Today`s model
runs have begun shifting this feature further north than
previously expected. If this more northward track continues,
several inches of accumulating snow would be possible through
central Idaho eastwards into the Bitterroot valley and southwest
Montana. A period of winter weather driving due to snowcovered
roads would be possible in this scenario Monday into Tuesday

Northerly flow is expected behind this feature through at least
mid week. This type of flow would maintain cool temperatures,
cloud cover and periods of light snow.


.AVIATION...Fog is expected to impact area terminals later this
evening continuing through Saturday morning, with the greatest
impact at KGPI and KMSO. KBTM and KSMN may experience brief
periods of fog, while KGPI and KMSO stand a better chance of extended
periods of fog, dense at times. Expect this to continue through
the morning hours until filtered sunshine has a few hours to mix
up the boundary layer and break up the fog. Also, a shortwave
trough will arrive Saturday causing stability to decrease and help
the fog mix out.



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