A strong thunderstorm produced excessive rainfall over Big Sky Lake during the afternoon of August 2nd.
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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 thunderstorm produced excessive rainfall over Big Sky Lake during the afternoon of August 2nd.
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 Aug 2, 1:44 AM MST, 11 hours 16 minutes earlier.
Area Forecast Discussion
National Weather Service Missoula MT
244 AM MDT Mon Aug 2 2021
...FLASH FLOODING POSSIBLE ACROSS LEMHI COUNTY, IDAHO AND
SOUTHWEST MONTANA TODAY...
.DISCUSSION...The axis of Monsoon moisture will slowly shift
eastward along the Continental Divide this afternoon before
moving into central Montana by the late evening. Scattered
thunderstorms will focus along the Continental Divide, Southwest
Montana, and Lemhi County this afternoon and evening. Slow moving
thunderstorms will once again be capable of producing heavy
rainfall; therefore, a flash flood watch remains in effect for
Lemhi County, Idaho and Silver Bow and Deer Lodge Counties in
Montana. The primary areas of concern include flood prone areas
along US Highway 93 in Lemhi County and the cities of Anaconda
and Butte, Montana. Regional wildfire smoke resting across
Washington and Oregon will move into the Northern Rockies this
afternoon with the return of westerly flow.
High confidence exists for a robust warming and drying trend
Tuesday into Wednesday. A ridge of high pressure centered across
the Great Basin will amplify across the Northern Rockies. High
temperatures will soar into the 100s across lower elevations of
north central Idaho and into the 90s across western Montana.
Active weather will return late Thursday into Friday. A pacific
weather disturbance will track across southern Oregon into the
Northern Rockies. Showers and thunderstorms will develop Thursday
afternoon and evening and will be capable of producing wetting
rains area-wide. A few strong thunderstorms are possible across
central Idaho and west-central Montana Thursday afternoon and
evening. Storms will organize as a upper level trough moves into
northeast Oregon. Model guidance continues to advertise the
potential for a quarter to half an inch of rainfall, particularly
across central Idaho and west-central Montana.
Looking ahead to next weekend, confidence is increasing for a
cool and wet weather pattern across the Pacific Northwest and
Northern Rockies. Below normal temperatures are possible during
this period along with opportunities for rain with shower and
thunderstorm activity. /Lukinbeal
&&
.AVIATION...Areas of valley fog and stratus will impact the
Northern Rockies airspace until 02/1400-02/1500. The axis of
Monsoon moisture will slowly shift eastward across the Continental
Divide this afternoon before moving into central Montana by the
late evening hours. Scattered thunderstorms will focus along the
Continental Divide, Southwest Montana, and Lemhi County from
02/1800Z to 03/0100Z. Modest confidence exists for thunderstorm
activity to shift east of the divide into central Montana after
03/0200Z. Thunderstorms will be capable of producing heavy
rainfall once again today, with visibility reductions below 5SM
expected. Winds will shift out of the west-northwest after
02/2100Z with gusts up to 20 knots across west-central and
southwest Montana. /Lukinbeal
&&
.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Flash Flood Watch from noon MDT today through this evening
Butte/Blackfoot Region.
ID...Flash Flood Watch from noon MDT today through this evening
Eastern Lemhi County...Western Lemhi County.
&&The Missoula radar (KMSX), 42 km away, was scanning. The volume nearest the first event began at 19:59 UTC.
1 flash flood. Local standard time, as filed.
A trained spotter reported 1.3 inches of rainfall during a 45 minute period in Big Sky Lake. Excessive runoff washed out a resident's driveway along Big Sky Lake.
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, August 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: 976613.