Thunderstorms formed during the afternoon and evening of May 21, some of them approaching severe strength. An approaching trough helped create lift for storms, and favorable CAPE and shear profiles led to the severe risk.
Every event with a place to put it. Tap one for its entry below.
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.
Thunderstorms formed during the afternoon and evening of May 21, some of them approaching severe strength. An approaching trough helped create lift for storms, and favorable CAPE and shear profiles led to the severe risk. A severe thunderstorm watch was issued for Idaho and Clearwater counties.
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 21, 10:45 AM PST, 7 hours 55 minutes earlier.
Area Forecast Discussion National Weather Service Missoula MT 1245 PM MDT Sun May 21 2023 ...STRONG THUNDERSTORMS THROUGH TONIGHT... .DISCUSSION... A trough moving onshore today is causing thunderstorms across the region. This evening cooling temperatures and falling pressure aloft will help make the atmosphere more unstable with 1,000 to 2,000 J/kg of CAPE widespread across central Idaho and far northwest Montana. This instability will allow thunderstorms to be triggered across the region early this afternoon. Precipitable water values will rise into the 3/4 to 1" range in northwest Montana and central Idaho this evening supporting moderate to heavy rain. The only ingredient missing for more severe thunderstorms is wind shear, 20-30 kts of wind shear is on the low to marginal side for severe thunderstorms. Some severe storms are still possible, but they will be isolated and marginal to low end for severe storms. A low end severe storm is still severe though, and likely to cause damage. The main threats from the isolated severe storms this evening will be heavy rain and winds near 60 mph. The upper level trough closes off into a closed low and lingers along the Canadian border and the Idaho panhandle on Monday. That sets up another day of thunderstorms, but it will be much cooler with better forcing east of the Continental Divide, so storms on Monday are expected to be rather typical and non-severe. The major limiting factor on Monday for storms will be more stable conditions due to much cooler temperatures. Tuesday most of the region will have a break from the thunderstorms as temperatures warm back into the 70s. The warming trend continues through Thursday or Friday, and then ensembles agree on another trough impacting the US West Coast. There is considerable disagreement on where along the coast that trough will come ashore and how fast it will be going, but there is general agreement that it will still be near average or warmer even under the influence of the next trough. This is a good set up for thunderstorms once again. && .AVIATION...A trough moving across the region today is causing thunderstorms across the region. We expect those thunderstorms to intensify this evening across central Idaho and northwest Montana. A lack of strong wind shear is the main factor preventing storms from becoming severe, but isolated severe thunderstorms are still possible, especially in central Idaho along the Oregon and southeastern Washington borders. Heavy rain, small hail, and winds 40-50 mph will be typical impacts from storms, and we expect them to continue well past 22/00Z. && .MSO WATCHES/WARNINGS/ADVISORIES... MT...None. ID...None. &&
Special Weather Statement
National Weather Service Missoula MT
605 PM MDT Sun May 21 2023
MTZ004>006-043-220030-
Bitterroot/Sapphire Mountains MT-Missoula/Bitterroot Valleys MT-
Lower Clark Fork Region MT-Potomac/Seeley Lake Region MT-
605 PM MDT Sun May 21 2023
...Strong thunderstorms will impact portions of south central
Missoula County through 630 PM MDT...
At 604 PM MDT, Doppler radar was tracking strong thunderstorms along
a line extending from near Frenchtown to near Wye to near Missoula.
Movement was northeast at 20 mph.
HAZARD...Winds up to 45 mph, pea size hail, and heavy rainfall.
SOURCE...Radar indicated.
IMPACT...Gusty winds could knock down tree limbs and blow around
unsecured objects. Minor damage to outdoor objects is
possible.
Locations impacted include...
Missoula, Lolo, Wye, East Missoula, Frenchtown and Evaro.
This includes the following highways...
Interstate 90 between mile markers 90 and 107.
Highway 93 N between mile markers 0 and 6.
Highway 93 S between mile markers 86 and 94.
PRECAUTIONARY/PREPAREDNESS ACTIONS...
These storms may intensify, so be certain to monitor local radio
stations and available television stations for additional information
and possible warnings from the National Weather Service.
&&
LAT...LON 4705 11403 4693 11396 4681 11391 4679 11415
4690 11417 4702 11422
TIME...MOT...LOC 0004Z 242DEG 18KT 4703 11417 4694 11409 4684 11406
MAX HAIL SIZE...0.25 IN
MAX WIND GUST...40 MPHSpecial Weather Statement
National Weather Service Missoula MT
617 PM MDT Sun May 21 2023
MTZ005-006-220045-
Bitterroot/Sapphire Mountains MT-Missoula/Bitterroot Valleys MT-
617 PM MDT Sun May 21 2023
...A strong thunderstorm will impact portions of north central
Ravalli County through 645 PM MDT...
At 616 PM MDT, Doppler radar was tracking a strong thunderstorm over
Victor, or 13 miles north of Hamilton, moving northeast at 20 mph.
HAZARD...Winds up to 45 mph, half-inch size hail, and heavy rainfall.
SOURCE...Radar indicated.
IMPACT...Gusty winds could knock down tree limbs and blow around
unsecured objects. Minor damage to outdoor objects is
possible.
Locations impacted include...
Stevensville, Victor and Bell Crossing.
This includes Highway 93 S between mile markers 56 and 66.
PRECAUTIONARY/PREPAREDNESS ACTIONS...
This storm may intensify, so be certain to monitor local radio
stations and available television stations for additional information
and possible warnings from the National Weather Service.
&&
LAT...LON 4634 11403 4639 11421 4655 11416 4648 11392
TIME...MOT...LOC 0016Z 204DEG 19KT 4642 11411
MAX HAIL SIZE...0.50 IN
MAX WIND GUST...40 MPHSpecial Weather Statement
National Weather Service Missoula MT
631 PM MDT Sun May 21 2023
MTZ003-004-220100-
Flathead/Mission Valleys MT-Lower Clark Fork Region MT-
631 PM MDT Sun May 21 2023
...A strong thunderstorm will impact portions of south central Lake
and southeastern Sanders Counties through 700 PM MDT...
At 629 PM MDT, Doppler radar was tracking a strong thunderstorm over
Old Agency, or 24 miles south of Polson, moving northeast at 25 mph.
HAZARD...Winds up to 45 mph, half-inch size hail, and heavy rainfall.
SOURCE...Radar indicated.
IMPACT...Gusty winds could knock down tree limbs and blow around
unsecured objects. Minor damage to outdoor objects is
possible.
Locations impacted include...
Ravalli, St. Ignatius, Old Agency, Dixon, Moiese and Charlo.
This includes the following highways...
Highway 93 N between mile markers 26 and 35.
Highway 200 W between mile markers 104 and 115.
PRECAUTIONARY/PREPAREDNESS ACTIONS...
This storm may intensify, so be certain to monitor local radio
stations and available television stations for additional information
and possible warnings from the National Weather Service.
&&
LAT...LON 4723 11435 4736 11443 4745 11415 4730 11405
TIME...MOT...LOC 0029Z 234DEG 22KT 4734 11426
MAX HAIL SIZE...0.50 IN
MAX WIND GUST...40 MPHSpecial Weather Statement
National Weather Service Missoula MT
656 PM MDT Sun May 21 2023
MTZ001>003-220145-
Flathead/Mission Valleys MT-West Glacier Region MT-
Kootenai/Cabinet Region MT-
656 PM MDT Sun May 21 2023
...A strong thunderstorm will impact portions of west central
Flathead County through 745 PM MDT...
At 656 PM MDT, Doppler radar was tracking a strong thunderstorm 8
miles southwest of Kalispell, moving northeast at 30 mph.
HAZARD...Winds in excess of 30 mph and pea size hail.
SOURCE...Radar indicated.
IMPACT...Gusty winds could knock down tree limbs and blow around
unsecured objects. Minor damage to outdoor objects is
possible.
Locations impacted include...
Kalispell, Evergreen, Kila, Creston, La Salle and Somers.
This includes the following highways...
Highway 2 between mile markers 105 and 129.
Highway 93 N between mile markers 103 and 122.
PRECAUTIONARY/PREPAREDNESS ACTIONS...
This storm may intensify, so be certain to monitor local radio
stations and available television stations for additional information
and possible warnings from the National Weather Service.
&&
LAT...LON 4798 11439 4807 11458 4837 11437 4823 11402
TIME...MOT...LOC 0056Z 220DEG 27KT 4811 11440
MAX HAIL SIZE...0.25 IN
MAX WIND GUST...30 MPHPRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE MISSOULA MT
718 PM MDT SUN MAY 21 2023
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0700 PM HAIL 2 S KILA 48.09N 114.46W
05/21/2023 M0.70 INCH FLATHEAD MT TRAINED SPOTTER
DIME SIZED HAIL, 35-39 MPH WIND.
&&
JMSpecial Weather Statement
National Weather Service Missoula MT
719 PM PDT Sun May 21 2023
IDZ006>008-220300-
Southern Clearwater Mountains ID-
Lower Hells Canyon/Salmon River Region ID-
Orofino/Grangeville Region ID-
719 PM PDT Sun May 21 2023
...A strong thunderstorm will impact portions of west central Idaho
County through 800 PM PDT...
At 718 PM PDT, Doppler radar was tracking a strong thunderstorm 13
miles west of Grangeville, moving northeast at 45 mph.
HAZARD...Winds in excess of 40 mph and half-inch size hail.
SOURCE...Radar indicated.
IMPACT...Gusty winds could knock down tree limbs and blow around
unsecured objects. Minor damage to outdoor objects is
possible.
Locations impacted include...
Grangeville, Cottonwood, Ferdinand, Canfield, Winona, Keuterville,
Harpster and Fenn.
PRECAUTIONARY/PREPAREDNESS ACTIONS...
This storm may intensify, so be certain to monitor local radio
stations and available television stations for additional information
and possible warnings from the National Weather Service.
A Severe Thunderstorm Watch remains in effect until 900 PM PDT for
central Idaho.
&&
LAT...LON 4603 11660 4600 11654 4600 11652 4613 11652
4612 11648 4620 11645 4618 11633 4617 11631
4617 11625 4615 11620 4618 11618 4622 11613
4621 11610 4588 11585 4572 11651 4592 11665
TIME...MOT...LOC 0218Z 242DEG 37KT 4594 11639
MAX HAIL SIZE...0.50 IN
MAX WIND GUST...40 MPHThe Missoula radar (KMSX), 206 km away, was scanning. The volume nearest the first event began at 02:38 UTC.
1 hail, 1 thunderstorm wind. Local standard time, as filed.
A strong thunderstorm formed and moved toward Grangeville. Radar estimated maximum size hail was a little over an inch. The public reported nickel size hail with the storm.
A thunderstorm went through Syringa and the Apgar Campground, northeast of Lowell. A weather spotter reported trees down along US Highway-12 from Apgar Campground to just downstream of Swan Creek. The spotter also mentioned roof damage in Syringa and a power outage from Syringa to the Selway River.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Idaho in 2023Every state in 2023This day in weather historyWarnings in Idaho, May 2023
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: 1093782, 1093785.