Winter Weather Advisory 77, NWS Missoula, MT

Area Forecast Discussion
National Weather Service Missoula MT
250 AM MST Fri Dec 24 2021

.DISCUSSION...GOES-17 Water Vapor imagery this morning depicts a
broad upper level trough digging southward along the Washington
coastline. Widespread precipitation is noted on regional radar
imagery across the Pacific Northwest. The pacific trough will
eject eastward across the Northern Rockies late this afternoon
through Christmas morning. Widespread snow shower activity will
develop later this afternoon, particularly across northwest
Montana. Strong cold air advection aloft will allow for convective
snow showers to develop, leading to brief periods of moderate to
heavy snowfall. Snowfall rates around one inch per hour are
possible under heavier showers.

Friday night into Christmas morning, confidence is increasing for
a period of heavy snow to impact the West Glacier region. The
arctic boundary will creep over Marias Pass into the US-2 corridor
during this time. A wave of pacific moisture will interact with
the arctic boundary, creating an ideal scenario for heavy
snowfall. Forecast soundings suggest a deep dendritic growth zone
will be present, increasing confidence for periods of heavy
snowfall, with rates exceeding one inch per hour at times.
Therefore, a winter storm watch has been issued through 1100 am
MST Christmas morning. Widespread accumulating snow is also
expected across the Lower Clark Fork and Kootenai/Cabinet regions
and across the Flathead Valley, where 2-5 inches is likely.

On Sunday, a broad upper level trough will deepen across the
Pacific Northwest and will slowly move eastward towards the
Northern Rockies. As the trough deepens, a surface low will
develop across eastern Washington and will move eastward across
central Idaho and into Wyoming late Sunday into Monday morning.
Model guidance is coming into better agreement with the
progression of the arctic front. Arctic air will push across the
divide and move into northwest Montana late Sunday night into
Sunday morning and will slowly move south towards the I-90
corridor by Sunday afternoon, likely stalling in the Mission
valley and in the Potomac/Seeley Lake region. Widespread light to
moderate snow will impact the Northern Rockies Sunday as the
arctic boundary interacts with pacific moisture embedded within
southwesterly flow. Gusty north to northeast winds will lead to
localized areas of blowing snow, especially along the US-2 and
US-93 corridors from Marias Pass to the Flathead and Mission
valleys. Snowfall amounts remain very difficult to pin down at
this time. Our forecast remains fairly conservative in line with
the NBM, which has 1-3 inches across western Montana valleys, with
higher totals in the West Glacier and Kootenai/Cabinet regions.

The arctic air will continue to track westward across north
central Idaho through Monday. Light snow will focus as north
central Idaho during this period, with north to northeast winds
expected across much of western Montana. Temperatures across
western Montana will fall into the single digits to below zero
along the divide. Wind chills will become a concern with values
likely falling below -20F along the divide and in the Flathead
valley.

Model ensemble guidance remains in fairly good agreement with the
brunt of the cold impacting the area through at least Wednesday.
Widespread low temperatures below zero are expected across western
Montana, with valley locations in north central Idaho even
dropping into the single digits!

Looking ahead towards the end of next week, the arctic air mass
will begin to moderate, as a northwesterly flow pattern will open
the door for additional pacific storm systems to impact the
region. In fact, guidance has trended towards another shortwave
trough bringing a return of widespread snow shower activity
Thursday into Friday. The active weather pattern may continue into
the first week of January as well. The Madden-Julian Oscillation
looks to stay in Phase 7, which typically maintains below normal
temperatures and near to above normal precipitation across the
region. Furthermore, this lines up well with the latest 8-14
outlook from the Climate Prediction Center. /Lukinbeal

&&

.AVIATION...Widespread snow shower activity will continue across
northwest Montana and north-central Idaho through Christmas
morning and beyond. A pacific upper level trough will destabilize
the atmosphere this afternoon and evening, leading to convective
snow shower activity. Brief periods of moderate to heavy snow will
lead to variable conditions, particularly across northwest
Montana, confidence is higher for accumulating snow. Scattered
snow showers will also impact west-central and southwest Montana
after 25/0600z, with lesser confidence for widespread accumulating
snow. /Lukinbeal

&&

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

     Winter Weather Advisory until 5 PM MST this afternoon West
     Glacier Region.

     Winter Storm Watch from 5 pm this evening through Saturday morning
     West Glacier Region.

     Winter Weather Advisory until 5 AM MST early this morning
     Kootenai/Cabinet Region.

     Winter Weather Advisory from 2 PM this afternoon to 11 AM MST
     Saturday Kootenai/Cabinet Region.

ID...Winter Weather Advisory until 5 AM MST early this morning
     Eastern Lemhi County...Western Lemhi County.

&&