Area Forecast Discussion
Issued by NWS Missoula, MT

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FXUS65 KMSO 232118

Area Forecast Discussion
National Weather Service Missoula MT
218 PM MST Tue Jan 23 2018

.DISCUSSION...The rest of today will be fairly quiet weather-wise,
but later this evening snow will begin to spread across the
panhandle of Idaho into northwest Montana. Snow will become more
widespread overnight, impacting most of Lincoln, Sanders, and
Flathead counties in Montana, as well as Clearwater and Idaho
counties in Idaho. Elevations above 4500 feet along the MT/ID
border, including Lookout and Lolo Pass, will see accumulating
snowfall through Wednesday. Travel is expected to become gnarly
over Lookout Pass Wednesday morning around daybreak, so be
prepared for difficult winter conditions over the Pass. As
Wednesday morning wanes into Wednesday afternoon, snow levels will
rise and cause snow in valleys of north central Idaho and
northwest Montana to mix with rain and/or freezing rain at times
before changing over to all-rain in the afternoon.

For locations in west central Montana, including the Missoula and
Bitterroot valleys, strong upper level winds will shadow out the
valleys and prevent much precipitation from falling during this
timeframe. Some light snow accumulations might occur if
downsloping effects aren`t as strong, but will transition to rain
later in the morning. And lastly, for southwest Montana including
Homestake Pass and the Butte/Blackfoot region: very little precip
is expected at all until late Wednesday evening into Thursday
morning with the passage of the cold front.

Thursday night through Friday night: expect widely-scattered to
numerous snow showers to lasting over the entire Northern Rockies
under a cold and unstable airmass. Additional snow accumulations
are possible under a few stronger showers that may develop,
dropping a quick inch or two in localized areas. These areas are
difficult to pinpoint given the fickle nature of convective snow

Later Saturday afternoon through Sunday night: an atmospheric
river of moisture extending well into Pacific ocean will impact
the Northern Rockies, especially central Idaho and Northwest
Montana. An increasing westerly flow aloft with this moisture will
favor orographic lift (AKA: enhanced mountain precipitation).
Several inches of snow over Lookout and Lolo passes are once
again expected through Sunday evening (roughly a 36 hour period).
This stream of moisture will slowly be pushed northward with
increasing heights from a building ridge late Sunday. As heights
increase, so will snow levels. Precipitation will be focused
across northwest Montana, with valley locations expected to warm
nicely into the upper 30s, with precip remaining as mostly rain.

Monday evening through Tuesday, a cold front will push through
the region and lower snow levels back down to the valley floors.
A few inches of snow are quite possible across the valleys as
this front moves through. This frontal passage is likely to occur,
but we have plenty of time for finer details (such as timing of
the front and valley snow amounts) to work themselves out.


.AVIATION...A few mountain snow showers are possible the rest of
this afternoon. After 24/0300z periods of snow and rain showers
will begin to impact KGPI. Precipitation in the form of snow
becomes more likely at KGPI after 24/0900z where lowering ceilings
and vsby are likely through the morning hours. A series of
weather disturbances will impact the region through the rest of
the week. There is a slight potential KMSO to receive snow showers
after 24/1200z to lower ceilings/vsby. KBTM and KSMN may have some
lowering of clouds but will be dry through Wednesday.


MT...Winter Weather Advisory from 5 PM this afternoon to 5 PM MST
     Wednesday Lower Clark Fork Region.

     Winter Weather Advisory from 5 PM this afternoon to 5 PM MST
     Wednesday Kootenai/Cabinet Region.

ID...Winter Weather Advisory from 4 PM this afternoon to 4 PM PST
     Wednesday ABOVE 3000 FEET for the Northern Clearwater


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