An upper level low brought rain and snow to the Northern Rockies. Periods of heavy wet snow caused trees to bend over or break resulting in power outages over west-central Montana.
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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.
An upper level low brought rain and snow to the Northern Rockies. Periods of heavy wet snow caused trees to bend over or break resulting in power outages over west-central Montana.
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 May 19, 12:57 PM MST, 9 hours 3 minutes earlier.
Area Forecast Discussion
National Weather Service Missoula MT
157 PM MDT Wed May 19 2021
...LATE SPRING SNOW AND BENEFICIAL MOISTURE ARRIVING TO THE
NORTHERN ROCKIES TONIGHT...
.DISCUSSION...Widespread precipitation has started to stream
across the Northern Rockies this afternoon. Coverage and
precipitation intensity will continue to increase as the low
pressure intensifies and cuts off from the main flow overnight.
Expected snowfall for the mountains and higher valleys along the
Continental Divide remains on coarse. One change from the past
24hrs has been the increasing confidence for the lower valleys in
west-central Montana to see accumulating snowfall overnight. The
deformation zone (area of enhanced precipitation) that we have
been keeping an eye on, is now expected to develop in the area of
the Missoula/Bitterroot Valleys and the Seeley/Swan Region. This
area of enhancement will allow for snowlevels to lower to around
3000 feet. There is moderate confidence that 1 to 3 inches of snow
could fall in these areas, with low confidence, but still a
possibility that 4 inches or more could accumulate. Rain is
expected to change to snow as the deformation zone develops around
midnight tonight. The threat for valley snow begins to diminish
after sunrise tomorrow morning. Area roadways are not expected to
be impacted all that much, but snow is expected to accumulate on
area trees that now are almost fully leafed out. The snow loading
could bring down trees and tree limbs, creating power outages, and
possibly over some roadways. Due to these anticipated impacts,
the Missoula/Bitterroot Valleys have been added to the already
existing Winter Weather Advisories.
The mountains and mountain passes along the Continental Divide
(Marias and MacDonald Passes) are still expected to see the
highest snowfall amounts of up 12 inches possible. The mountains
will continue to see periods snowfall through the weekend, as the
low pressure system lingers just to the south of Montana. The low
will finally begin to move northeast out the area Monday, ending
the threat for precipitation for most locations. Glacier National
Park and possibly Marias Pass will keep snow in the forecast
through Wednesday as wrap around moisture from the low continues
to move over that area.
Another impact from this late Spring storm system, will be the
cold morning low temperatures. Many valley locations across the
Northern Rockies will likely see temperatures in the low to mid
30s through the weekend. Any sensitive outdoor plants will need to
be protect or brought inside.
An active weather pattern will likely remain over the region
through next week, though seasonal temperatures are expected to
return.
&&
.AVIATION...Rain and high-elevation snow will spread across the
Northern Rockies air space through Thursday, obscuring terrain.
North and northeast winds will become gusty at times, reaching
near 40 knots over the Continental Divide (particularly after
20/1200Z). Low ceilings and low visibility will be maximized as
snow levels drop near 3000 feet MSL 20/0600Z - 20/1500Z.
&&
.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Weather Advisory until 6 AM MDT Friday Butte/Blackfoot
Region...Potomac/Seeley Lake Region.
Winter Weather Advisory from 6 PM this evening to 6 AM MDT
Friday ABOVE 5000 FEET for the Lower Clark Fork Region.
Winter Weather Advisory until noon MDT Friday ABOVE 4000 FEET
for the West Glacier Region.
Winter Weather Advisory until 6 AM MDT Friday ABOVE 5000 FEET
for the Bitterroot/Sapphire Mountains.
Winter Weather Advisory from midnight tonight to 9 AM MDT
Thursday Missoula/Bitterroot Valleys.
ID...Winter Weather Advisory until 5 AM PDT Friday ABOVE 5000 FEET
for the Northern Clearwater Mountains...Southern Clearwater
Mountains.
Winter Weather Advisory from 6 PM this evening to 6 AM MDT
Friday ABOVE 5500 FEET for the Western Lemhi County.
Winter Weather Advisory from 6 PM this evening to 6 AM MDT
Friday ABOVE 6500 FEET for the Eastern Lemhi County.
&&The national radar mosaic for the moment the first event began.
5 winter weather. Local standard time, as filed.
Power outages occurred in the Sula area and points east due to snow loading on power lines. The COOP observer located 14 miles northeast of Sula reported 5.1 inches of total snowfall while the Skalkaho Summit SNOTEL reported 8 inches.
A sensor at Marias Pass reported 12.8 inches of snowfall from this event. Heavy snow occurred between 12 AM to 4 AM and also between 10 to 12 pm MST on the 20th. Gusty northeast winds between 20 and 35 mph were also observed. Snow amounts decreased substantially west of the pass where up to 1 inch of snow was estimated in Essex.
Heavy wet snow caused several power outages in the Potomac and Seeley lake regions. Snow totals included: 11 inches located 3 miles east-northeast of Clinton on Wallace Creek Road at five thousand feet, 6 inches near Potomac and 3.7 inches in Seeley Lake.
Heavy wet snow fell over the southern Bitterroot Valley. Total snow amounts ranged from 1 inch in Corvallis to 2.3 inches located 2 miles east-northeast of Grantsdale and up to 4 inches in the Conner area. A power outage was reported in the Conner area most likely due to snow loading to power lines.
A period of wet snow fell across portions of southwest Montana. Snow amounts ranged from 2 inches in Butte, to 3 inches in Anaconda and up to 4 inches, 7 miles south of Butte.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Montana in 2021Every state in 2021This day in weather historyWarnings in Montana, May 2021
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: 957993, 957988, 957996, 957994, 957997.