A Pacific cold front brought a wintry mix of precipitation across northwest Montana. Freezing rain was observed as valley cold air pools kept ground temperatures at or below freezing.
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Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.
The episode as NWS MSO wrote it up.
A Pacific cold front brought a wintry mix of precipitation across northwest Montana. Freezing rain was observed as valley cold air pools kept ground temperatures at or below freezing.
The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.
NWS Missoula, MT wrote its last area forecast discussion before the first event at Dec 9, 3:02 AM MST, 2 hours 58 minutes earlier.
Area Forecast Discussion
National Weather Service Missoula MT
302 AM MST Wed Dec 9 2020
.DISCUSSION...Radar returns have been increasing overnight, but a
layer of dry air is preventing most of the moisture from reaching
the surface. Drizzle with isolated slick spots cannot be ruled
out before sunrise but recent trends suggest that the threat for
freezing precipitation is low. Model guidance agrees that moisture
will be limited in most places until the cold frontal passage
when most of the showers are expected. Snow levels are starting
off high and will lower with the cold front. Light snow
accumulations are possible in the mountains while lower elevations
will have rain and snow showers. The Glacier Region remains the
main area with the highest likelihood for accumulating snow
through this evening so a winter weather advisory remains in
effect.
High pressure briefly returns for Thursday with dry conditions
expected.
Showers may return on Friday, but there is still a lot of
question on how the next weather system will transition the
Northern Rockies. It is likely that a trough will dig into the
region Friday night and Saturday with increased shower chances. A
low pressure center may also develop but the evolution and
placement of the low are very uncertain.
Another trough is forecast to move into the Pacific Northwest on
Sunday bringing the potential for a wet southwesterly flow into
north-central Idaho and western Montana. This weather system will
favor mountain snow and valley snow is also possible.
&&
.AVIATION...Moisture is increasing throughout the region which
has mostly resulted in increased cloud cover. Radar returns are
showing up in northwest Montana but a dry layer is keeping most of
the moisture from reaching the ground. A cold front is moving
through Washington and is on track to move through central Idaho
and western Montana this morning. Radar projections show weakening
showers as the front moves into drier air, so northwest Montana
is expected to have the most showers and southwest Montana will
likely only have a few showers or stay dry. The Glacier Region
will have the majority of the shower activity today with help from
a reinforcing front moving out of Canada. Cloud cover will linger
overnight and showers may linger along the Divide as the
disturbance exits the region and high pressure briefly returns.
&&
.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Weather Advisory until 11 PM MST this evening West
Glacier Region.
ID...None.
&&The national radar mosaic for the moment the first event began.
1 winter weather, 1 ice storm. Local standard time, as filed.
A light glaze of freezing rain was reported in Libby and vicinity. Lincoln County dispatch reported icy and slick roads.
A glaze of ice due to freezing rain led to the closure of I-90 from Lookout Pass to St. Regis. I-90 was closed for approximately three hours. Cold ground temperatures allowed for ice accumulation to occur.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Montana in 2020Every state in 2020This day in weather historyWarnings in Montana, December 2020
NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 927360, 928823.