Area Forecast Discussion
Issued by NWS Missoula, MT

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FXUS65 KMSO 132154

Area Forecast Discussion
National Weather Service Missoula MT
254 PM MST Fri Dec 13 2019

.DISCUSSION...Moist northwest flow sets up over western Montana
and central Idaho this afternoon. We`ll stay in this pattern
through the weekend as a stream of Pacific moisture rides up and
over an offshore ridge. Tonight, snow showers will linger over the
higher terrain, especially the Mission and Swan Ranges. Dynamics
will not be quite strong enough overnight for widespread
precipitation. This changes Saturday as an embedded shortwave
trough dips into the Northern Rockies. The trough will provide
enough lift to interact with moisture already in place and bring
more widespread precipitation. Impacts will be concentrated on
central Idaho, including the Camas Prairie and Clearwater
Mountains extending eastwards to Lolo Pass. The mountains of
Northwest Montana will continue to see snow as terrain enhances
precipitation. Impacts will be primarily to travel along Whitebird
Grade, Greer Grade, and Lolo Pass.

Saturday night into Sunday, another wave of energy will enhance
precipitation in northwest flow aloft. With this second wave,
snowfall chances will be enhanced further east into west-central
Montana, including through the Missoula and northern Bitterroot
valleys as well as through the Seeley Lake and Swan valleys. A few
more banded type precipitation features are possible which could
lead to variable snow amounts over short distances. Overall
accumulations will be fairly light, but continues snowfall on
roadways will cause slick travel at times.

High pressure begins to build into the region Monday, though some
lingering snowfall will be possible in the higher terrain across
northwest Montana. The ridge will strengthen Tuesday and
Wednesday, leading to more stable, stagnant air over the region.
With recent snowfall, valley fog may become an issue. The ridge
does not look to be long lasting, with the next relatively warm
and moist system expected by Thursday into Friday.


.AVIATION...Transient showers (snow, rain, even graupel) will be
common into this evening across western Montana but impacts will
brief at terminal sites. Moist northwest flow becomes established
later today and lasts all weekend leading to more widespread
precipitation. This flow will lead to persistent mountain snow
showers with periodic snow showers in the valleys. Terminal sites
will experience lower ceilings and reduced visibility at times in
showers. Patchy fog and stratus are also possible if occasional
clearing occurs.


MT...ID...Winter Weather Advisory from 1 AM to 1 PM PST Saturday ABOVE
     2000 FEET for the Northern Clearwater Mountains...Southern
     Clearwater Mountains.

     Winter Weather Advisory from 1 AM to 11 AM PST Saturday ABOVE
     2000 FEET for the Lower Hells Canyon/Salmon River Region...
     Orofino/Grangeville Region.


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