A relatively moist and warm system moved over sub-freezing temperatures in the valleys of western Montana leading to freezing rain and icy roads.
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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 relatively moist and warm system moved over sub-freezing temperatures in the valleys of western Montana leading to freezing rain and icy roads.
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 Jan 5, 2:59 PM MST, 9 hours 1 minute earlier.
Area Forecast Discussion
National Weather Service Missoula MT
259 PM MST Fri Jan 5 2018
...Freezing rain causing slick roads, bridges, sidewalks tonight
into Saturday morning...
.DISCUSSION...A messy weather situation is still looking to shape
up for this evening and overnight as a moist and relatively warm
weather system rides over the top of cold air trapped in valleys
of western Montana and portions of north central Idaho. This will
lead to freezing rain for many locations, particularly for valleys
generally along and north of the Interstate 90 corridor as well
as in the vicinity of Salmon in Lemhi County, Idaho. The most
intense period of precipitation is expected from roughly midnight
tonight into roughly 6am Saturday morning, during which time a
light glaze of ice could cause roads, bridges and sidewalks to
become quite slick. In Idaho, temperatures have generally warmed
into the mid 30s to low 40s, which will help reduce the coverage
of freezing rain. However, some slick spots will still be
possible.
Precipitation should generally transition over to snow or a rain
snow mix by mid to late morning Saturday as a cold front moves
through the region. While freezing rain should come to an end,
roads will likely remain quite slick into Saturday afternoon, with
mountain passes seeing light snow accumulations.
Saturday night into Sunday, a weak ridge of high pressure is
expected to build over the region. This will largely lead to a
drying trend although snow showers will likely continue across far
northwest Montana.
Monday will see the redevelopment of high pressure across all of
the Intermountain West, with the usual inversion-like traits
resurfacing as well. This round of high pressure will be short-
lived, as the next wintry system will enter from the west during
the early half of Tuesday. Moisture will spread from west to east
through the day, bringing rain to central Idaho valleys and a mix
of rain and snow to western Montana valleys. At the same time,
arctic air will be moving south out of Alberta and banking up
against the eastern side of the Continental Divide. At this time,
the forecast models still do not depict much of this air making
its way over the Divide and into the Flathead valley, but
temperatures *will* drop quite a bit...enough to keep
precipitation as all snow throughout the day Tuesday. The rest of
western Montana and northern Idaho will follow suit, with
snowfall expected later on Tuesday and lasting through Wednesday
night.
The weather remains active going into the end of the week, with
more snow on the way Thursday night into Friday.
&&
.AVIATION...Periods of light freezing rain will continue through
this evening for most all valley locations across western Montana.
A period of more intense precipitation may lead to increased
freezing rain threat, generally after 06/0600z. KGPI, KMSO, and
KSMN have the highest likelihood of seeing freezing rain,
generally between 06/0600z and 06/1200z, though some light
freezing rain may linger until roughly 06/1500z. The best
potential for ice accumulations still appears to occur from KMSO
and northward to the Canadian border. Widespread obscurations of
the higher terrain are expected across all of western Montana and
central Idaho. In Idaho, precipitation will mainly fall as rain
due to the fact that both mountains and valley locations have
warmed into the upper 30s to even low 40s. Precipitation should
transition more to snow, or a rain/snow mix, by mid morning
Saturday as a cold front moves through the region. KBTM could see
a brief period of low visibility and ceiling due to snow between
06/1200z and 06/1500z. Winds will increase at ridge top level from
the west by Saturday with the cold frontal passage, with gusts up
to 35 kts possible. Some breezes will be possible down into
exposed valleys, though gust potential will be much lighter.
&&
.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Weather Advisory from 8 PM this evening to 11 AM MST
Saturday Butte/Blackfoot Region...Potomac/Seeley Lake
Region...West Glacier Region.
Winter Weather Advisory until 11 AM MST Saturday
Missoula/Bitterroot Valleys.
Winter Weather Advisory until 11 AM MST Saturday
Kootenai/Cabinet Region...Lower Clark Fork Region.
Winter Weather Advisory until 11 AM MST Saturday
Flathead/Mission Valleys.
ID...Winter Weather Advisory from 8 PM this evening to 11 AM MST
Saturday Eastern Lemhi County.
&&The national radar mosaic for the moment the first event began.
3 winter weather. Local standard time, as filed.
The Montana Department of Transportation reported severe driving conditions for Highway 56 from Noxon to Bull Lake at 6:36 am MST due to freezing rain and ice.
Severe driving conditions were issued for US-2 between West Glacier and Marias Pass and on Highway 83 from Condon to the Junction of S-209 between 5 and 9 am MST due to freezing rain and ice.
The Montana Department of Transportation reported severe driving conditions on Highway 200 between the Idaho state line to Plains between 6 and 8 am MST due to freezing rain and ice.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Montana in 2018Every state in 2018This day in weather historyWarnings in Montana, January 2018
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: 728264, 728265, 728266.