A strong arctic front interacted with a deep trough and brought heavy snow and strong northerly winds with low visibility and drifting snow.
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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 strong arctic front interacted with a deep trough and brought heavy snow and strong northerly winds with low visibility and drifting snow.
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 Nov 7, 2:30 AM PST, 10 hours 30 minutes earlier.
Area Forecast Discussion
National Weather Service Missoula MT
330 AM MST Sat Nov 7 2020
.DISCUSSION...A mid-level circulation is still expected to slide
south out of British Columbia into eastern Washington during the
day today. As the low pressure system tracks to the southeast into
Oregon and Idaho, it is expected to intensify while at the same
time, an artic front will be moving south out of Alberta. The
interaction of these two airmasses will bring widespread winter
weather to the Northern Rockies, and a wide range of winter
impacts this evening through Sunday.
Most of the precipitation that starts to fall will be mostly
rain, as snow levels will be above mountain pass levels. Not until
later this afternoon and evening, when the arctic front begins to
move into western Montana, will most of the valleys begin to
transition to snow. One area that will likely see snow much
sooner, will be the Camas Prairie in the vicinity of Grangeville
and the higher elevations of Idaho. These locations will be closer
to the low pressure center, where dynamic cooling will aide in
snowlevels lowering with the more moderate to heavy precipitation.
While most of northwest Montana (with exception of the higher
terrain near Marias Pass) will see lesser snow amounts throughout
the duration of this event, gusty east-northeast winds with the
arctic front will create hazardous driving with reduced
visibility due to blowing snow and potential for icy conditions.
Highway 2 from Kila to Happys Inn and the southern Mission Valley,
will see the most snow of 2 to 4 inches. In these type of events
much of the Flathead Valley typically gets downsloped, reducing
snow accumulations. Strong gusty winds and moderate to heavy snow
is still expected along Highway 2 from Essex to Marias Pass.
Lemhi county, Idaho remains in a tricky area, as most of the
moisture will be north of your location, as well as the colder
arctic airmass. The mountains should see up to a foot of snow,
while the valleys could see around 2 to 5 inches of snow through
Sunday.
For north-central Idaho, west-central and southwest Montana, there
will be two rounds of precipitation. First, from the
aforementioned mid-level circulation interacting with the arctic
airmass, then a secondary low will be tracking northeast out of
southern Idaho into central Montana. Moisture wrapping around the
low will overrun the arctic air in place, potentially producing
widespread moderate to heavy snow Sunday morning. These locations
will have the chance for seeing the highest amounts through the
weekend. Area passes such as Lolo, Lost Trail, MacDonald and
Homestake, could see up to 1 foot of snow by the end of Sunday.
The valleys should see anywhere from 4 to 7 inches, depending on
where the heavier bands setup. Snowfall will begin to diminish
area wide by Sunday night as the low moves into eastern Montana
and the Dakotas. A large area of winter highlights are in place,
and further details on your specific locations can be found by
visiting www.weather.gov/missoula.
The Northern Rockies will remain under a cool and active weather
pattern through at least the first half of next week. A
disturbance on Tuesday/Wednesday, and another feature for the
latter part of the work week, will bring another round of
widespread snow for the Northern Rockies.
&&
.AVIATION...Patch valley fog will continue to impact visibility
across western-central Montana and north-central Idaho through the
early morning hours. Visibility at KMSO has periodically reduced
to one quarter mile this morning, and fluctuations up and down in
the visibility can be expected through 07/1400Z. A strong winter
storm will bring wide reaching impacts to the Northern Rockies
later this afternoon and evening, through Sunday. Strong east-
northeast winds gusting up to 30-35 kts along with snowfall is
expected to reduce visibility in the vicinity of KGPI around
08/0000Z. As the arctic front moves south through the day, KMSO
can expect similar deteriorating conditions by 08/0300Z, and KBTM
soon after. West-central and southwest Montana, as well as north-
central Idaho will likely continue to experience impacts from snow,
as wrap around moisture overruns the arctic airmass, producing
moderate to heavy snowfall intensities.
&&
.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Storm Warning from 8 PM this evening to 5 AM MST Monday
Butte/Blackfoot Region...Potomac/Seeley Lake Region.
Winter Storm Warning from 8 PM this evening to 5 AM MST Monday
Bitterroot/Sapphire Mountains.
Winter Storm Watch from this evening through Sunday evening
Missoula/Bitterroot Valleys.
Winter Weather Advisory from 5 PM this afternoon to 5 PM MST
Sunday Lower Clark Fork Region.
Winter Weather Advisory from 5 PM this afternoon to 5 PM MST
Sunday Flathead/Mission Valleys.
Winter Storm Watch from 11 AM MST this morning through Sunday
evening West Glacier Region.
Winter Weather Advisory from 5 PM this afternoon to 5 PM MST
Sunday Kootenai/Cabinet Region.
ID...Winter Storm Watch from this evening through Sunday evening
Eastern Lemhi County...Western Lemhi County.
Winter Weather Advisory from 4 PM this afternoon to 4 PM PST
Sunday Northern Clearwater Mountains...Southern Clearwater
Mountains.
Winter Weather Advisory from 1 PM this afternoon to 10 AM PST
Sunday Orofino/Grangeville Region.
&&The national radar mosaic for the moment the first event began.
1 winter storm, 1 winter weather. Local standard time, as filed.
A spotter west of Grangeville measured 10.9 inches of snow with drifts as deep as 3 feet. There were additional reports in the Camas Prairie area of 10 to 12 inches. Idaho County Light and Power also reported multiple outages due to lines down.
An Idaho Department of Transportation employee reported this event to be moderate to high impact due to staffing numbers this early in the season. At 5:40 pm, an Idaho Department of Transportation sensor at Whitebird Hill reported 'very slick' road conditions.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Idaho in 2020Every state in 2020This day in weather historyWarnings in Idaho, November 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: 927149, 928903.