Winter Storm Watch 11, NWS Missoula, MT

Area Forecast Discussion
National Weather Service Missoula MT
402 AM MST Sun Dec 17 2017

.DISCUSSION...Dense fog has developed in Grangeville, ID and also
in Butte, MT early this morning. Other areas could see fog
develop near daybreak but should dissipate by late this morning.

Light snow will develop today across northwest Montana then spread to
most mountain locations tonight. With accumulating snow combined
with blowing snow expected along US-2 towards Marias Pass, we
have issued a winter weather advisory effective this evening
lasting through Monday evening. Those traveling along Interstate
90 should be prepared for snow- covered roads and slower travel
between Lookout Pass and St. Regis. Have kept the chance for light
freezing rain in the forecast for valley locations in western
Montana Monday morning, but confidence is low at this time. Those
traveling along Highway 95 between Whitebird Hill and Grangeville
should expect some reduced visibility due to blowing and drifting
snow this evening as southerly winds pick up and gust to near 40
mph continuing through tonight.

Moist and strong westerly flow will continue on Monday. Snow will
continue at mountain passes along the Idaho/Monday border and at
Marias Pass. In fact snow may not stop falling in those places
through mid-week. For valleys on Monday, expect mostly cloudy
skies with temperatures warming into the upper 30s to near 40
degrees for some locations with an occasional rain or snow shower
and breezy conditions at times.

The next surge of subtropical moisture is anticipated beginning
Monday evening, lasting into Tuesday. Widespread precipitation is
expected for the Clearwater Mountains in Idaho and across
northwest Montana from generally the I90 corridor and northward.
Snow levels will be tricky with this system as this moisture will
be accompanied by relatively warm air. Current thinking is that
most valleys along a line from Seeley Lake to Polson to Libby will
remain snow, amounting to several inches of new accumulations by
Tuesday morning. Locations south of this line will likely see a
mix of rain and snow, and thus potential snow accumulations look
more limited. Nevertheless, slick roadways should be expected.

By Tuesday afternoon, warmer air continues to work northward, with
most locations in northwest Montana briefly changing to rain or a
rain snow mix. The one exception will be for valleys near the
Canadian border and along the Continental Divide, where
precipitation may continue as snow. Areas that do not see a
transition to rain could end up with some fairly hefty snow
amounts, so stay tuned as we refine the potential snowfall
forecast.

From Tuesday night into Wednesday morning a series of cold fronts
will move through the region. Locations that briefly turn to rain
will quickly transition to snow as the Pacific front passes early
Wednesday morning. Then a much colder Canadian cold front will
cross the Continental Divide Wednesday afternoon and evening. This
second front will help enhance snowfall across the region, and
will be accompanied by gusty winds, lowered visibility and
potentially very slick roadways. A period of difficult travel due
to snow, slick roads and blowing snow is possible Wednesday,
especially from roughly Kalispell north and east to the
Continental Divide.

Drier air will settle in fairly quickly with the colder airmass
Wednesday night, bringing a fairly quick end to snowfall. Forecast
models are now showing a wave of energy moving through the region
on Friday; however, this may lead to a period of light but
widespread snowfall during peak holiday travel. This wave will be
accompanied by a secondary push of cold air, which is expected to
bring bitter temperatures to the region for the weekend. Just how
cold it gets will largely be a factor of sky cover, but highs in
the teens across western Montana valleys and 20s for central Idaho
valleys look possible by Saturday, with low temperatures in the
single digits.

&&

.AVIATION...A couple areas of fog and low stratus have formed this
morning, causing locally lowered visibilities and ceilings. The
biggest impact from this fog and stratus is currently being felt
through central Idaho, including in the vicinity of the Camas
Prairie of Idaho, across northwest Montana (including KGPI) and in
a few valleys across southwest Montana (including KBTM). For KBTM
in particular, visibility has been reduced to 1/4 mi and this will
likely continue through roughly 17/1600z. The next weather system
will begin to spread light snow into northwest Montana after
roughly 17/1600z, spreading further south into central Idaho and
west central Montana after 18/0000z. Low ceilings will cause
terrain to be obscured with periods of lowered visibility. In
addition, a period of light freezing rain will be possible from
KMSO northward to KGPI Monday morning.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Weather Advisory from 11 AM this morning to 11 AM MST
     Monday Lower Clark Fork Region.

     Winter Weather Advisory from 11 AM this morning to 11 AM MST
     Monday Flathead/Mission Valleys.

     Winter Weather Advisory from 5 PM this afternoon to 5 PM MST
     Monday West Glacier Region.

     Winter Weather Advisory from 11 AM this morning to 11 AM MST
     Monday Kootenai/Cabinet Region.

ID...None.
&&