An unstable southwesterly flow developed ahead of an approaching Pacific trough and brought strong to severe thunderstorms over a three day period.  Surface based CAPE routinely exceeded 1000 j/kg as steep mid-level lapse rates above 8C/km were common. Dry sub-cloud layers helped produce strong thunderstorm wind gusts and effective shear well above 30 kts was enough for quarter size hail and larger. The peak in severe weather occurred on Wednesday July 21st as the main energy associated with the trough moved through the region.

Thunderstorm Wind in GALLATIN, MONTANA
From 22-JUL-21 13:16:00 to 22-JUL-21 13:16:00 (MST-7)
Location: BELGRADE
Magnitude: 50.00 EG
Reported by: Public

A medium sized out building was flipped over by thunderstorm wind gusts, some tree damage was reported as well.

Hail in MADISON, MONTANA
From 22-JUL-21 15:03:00 to 22-JUL-21 15:03:00 (MST-7)
Location: ENNIS BIG SKY ARPT
Magnitude: 0.75 
Reported by: Trained Spotter

Hail up to 3/4 inch in diameter reported by a trained spotter visiting the area.

Thunderstorm Wind in TETON, MONTANA
From 20-JUL-21 17:36:00 to 20-JUL-21 17:36:00 (MST-7)
Location: CHOTEAU
Magnitude: 50.00 EG
Reported by: Newspaper

The Choteau Ancantha Newspaper posted images of large healthy tree limbs and power lines taken down by thunderstorm winds. Power outages were also reported.

Thunderstorm Wind in TETON, MONTANA
From 20-JUL-21 17:40:00 to 20-JUL-21 17:40:00 (MST-7)
Location: CHOTEAU ARPT
Magnitude: 51.00 MG
Reported by: Mesonet

KCII Choteau Airport measured a 51 kt (59 mph) thunderstorm wind gust.

Thunderstorm Wind in BROADWATER, MONTANA
From 21-JUL-21 16:26:00 to 21-JUL-21 16:26:00 (MST-7)
Location: TOWNSEND
Magnitude: 56.00 MG
Reported by: Mesonet

EKHM8 station measured a 56 kt (65 mph) thunderstorm wind gust.

Thunderstorm Wind in BROADWATER, MONTANA
From 21-JUL-21 16:28:00 to 21-JUL-21 16:28:00 (MST-7)
Location: TOWNSEND
Magnitude: 53.00 MG
Reported by: Mesonet

Ginger RAWS measured a 53 kt (61 mph) thunderstorm wind gust.

Hail in LEWIS AND CLARK, MONTANA
From 21-JUL-21 19:27:00 to 21-JUL-21 19:27:00 (MST-7)
Location: AUGUSTA
Magnitude: 1.00 
Reported by: Park/Forest Service

Forest service tower staff reported quarter sized hail (1 inch in diameter).

Hail in LEWIS AND CLARK, MONTANA
From 21-JUL-21 20:00:00 to 21-JUL-21 20:00:00 (MST-7)
Location: AUGUSTA
Magnitude: 1.00 
Reported by: COOP Observer

Co-Op observer reported 1 inch hail.

Thunderstorm Wind in GALLATIN, MONTANA
From 22-JUL-21 13:12:00 to 22-JUL-21 13:12:00 (MST-7)
Location: (BZN)GALLATIN FLD BO
Magnitude: 59.00 MG
Reported by: ASOS

KBZN measured a 59 kt (68 mph) thunderstorm wind gust.

Flash Flood in TETON, MONTANA
From 20-JUL-21 17:50:00 to 20-JUL-21 17:50:00 (MST-7)
Location: CHOTEAU
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Newspaper

Flooding with debris on roadways near 4th Avenue SW and 1st St, with additional flooding at 7th Avenue NW and Division St. Both areas had impassable roads. Up to 1.82 inches of rain were reported with the thunderstorms. Pictures provided by Choteau Acantha Newspaper.

Thunderstorm Wind in HILL, MONTANA
From 21-JUL-21 20:53:00 to 21-JUL-21 20:53:00 (MST-7)
Location: HAVRE CITY CO ARPT
Magnitude: 50.00 MG
Reported by: ASOS

KHVR measured a 50 kt (58 mph) thunderstorm wind gust.