A shortwave trough over the Pacific Northwest began to compromise the ridge in place and introduced a more moist and unstable southwesterly flow to the Northern Rockies. This allowed daily rounds of afternoon and evening showers and thunderstorms to develop over Southwest and North-central MT. The period began with discrete supercell thunderstorms developing over the plains, producing severe wind gusts, large hail, and even a weak tornado. The strongest and most widespread activity occurred on July 7th as the Pacific trough and associated wind shear began to exert more of an influence on the region's weather and interacted with the moist and unstable environment already in place.

Thunderstorm Wind in FERGUS, MONTANA
From 07-JUL-21 14:08:00 to 07-JUL-21 14:08:00 (MST-7)
Location: LEWISTOWN
Magnitude: 52.00 EG
Reported by: Trained Spotter

Spotter reported winds snapping off large tree branches, also had pea-size hail.

Thunderstorm Wind in GALLATIN, MONTANA
From 07-JUL-21 15:34:00 to 07-JUL-21 15:34:00 (MST-7)
Location: BELGRADE
Magnitude: 52.00 MG
Reported by: Trained Spotter

Trained spotter measured a 52 kt (60 mph) thunderstorm wind gust.

Hail in GLACIER, MONTANA
From 06-JUL-21 22:30:00 to 06-JUL-21 22:30:00 (MST-7)
Location: SANTA RITA
Magnitude: 1.75 
Reported by: Trained Spotter

Trained spotter reported golf ball sized hail.

Thunderstorm Wind in FERGUS, MONTANA
From 07-JUL-21 13:55:00 to 07-JUL-21 13:55:00 (MST-7)
Location: (LWT)LEWISTON MUNI A
Magnitude: 63.00 MG
Reported by: ASOS

KLWT observed a 63 kt (73 mph) thunderstorm wind gust.

Hail in GLACIER, MONTANA
From 06-JUL-21 16:21:00 to 06-JUL-21 16:21:00 (MST-7)
Location: SANTA RITA
Magnitude: 1.00 
Reported by: Public

Damage to two vehicles from quarter size hail was reported by the public.

Hail in BLAINE, MONTANA
From 06-JUL-21 17:15:00 to 06-JUL-21 17:15:00 (MST-7)
Location: HAYS
Magnitude: 1.25 
Reported by: Public

A member of the public relayed a report of half dollar sized hail through social media.

Hail in BLAINE, MONTANA
From 06-JUL-21 16:46:00 to 06-JUL-21 16:46:00 (MST-7)
Location: HAYS
Magnitude: 1.75 
Reported by: Public

A member of the public relayed images of golf ball sized hail through social media.

Hail in GLACIER, MONTANA
From 06-JUL-21 16:30:00 to 06-JUL-21 16:30:00 (MST-7)
Location: SANTA RITA
Magnitude: 1.75 
Reported by: Trained Spotter

Two neighbors north of spotter reported golf ball size hail with storm.

Thunderstorm Wind in FERGUS, MONTANA
From 07-JUL-21 13:59:00 to 07-JUL-21 13:59:00 (MST-7)
Location: (LWT)LEWISTON MUNI A
Magnitude: 59.00 MG
Reported by: ASOS

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

Thunderstorm Wind in GALLATIN, MONTANA
From 07-JUL-21 15:25:00 to 07-JUL-21 15:25:00 (MST-7)
Location: (BZN)GALLATIN FLD BO
Magnitude: 50.00 MG
Reported by: ASOS

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

Hail in FERGUS, MONTANA
From 07-JUL-21 13:55:00 to 07-JUL-21 13:55:00 (MST-7)
Location: LEWISTOWN
Magnitude: 1.00 
Reported by: Trained Spotter

Trained spotter reported quarter sized hail.

Thunderstorm Wind in MADISON, MONTANA
From 07-JUL-21 14:55:00 to 07-JUL-21 14:55:00 (MST-7)
Location: ENNIS BIG SKY ARPT
Magnitude: 56.00 MG
Reported by: AWOS

KEKS measured a 56 kt (64 mph) thunderstorm wind gust.

Thunderstorm Wind in GALLATIN, MONTANA
From 07-JUL-21 15:30:00 to 07-JUL-21 15:30:00 (MST-7)
Location: BOZEMAN
Magnitude: 53.00 MG
Reported by: Trained Spotter

Trained spotter measured a 53 kt (61 mph) thunderstorm wind gust.

Thunderstorm Wind in GALLATIN, MONTANA
From 07-JUL-21 15:35:00 to 07-JUL-21 15:35:00 (MST-7)
Location: BOZEMAN
Magnitude: 50.00 MG
Reported by: Trained Spotter

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

Thunderstorm Wind in FERGUS, MONTANA
From 07-JUL-21 13:58:00 to 07-JUL-21 13:58:00 (MST-7)
Location: LEWISTOWN
Magnitude: 70.00 EG
Reported by: NWS Employee

A survey conducted by an NWS employee found damage consistent with straight line winds of 70 kts (80 mph). There was a downed power pole and a 12 foot section of roof damage on a building.

Tornado in BLAINE, MONTANA
From 06-JUL-21 17:11:00 to 06-JUL-21 17:20:00 (MST-7)
Location: HAYS
Tornado F-scale: EFU
Reported by: Broadcast Media

A supercell moved southeast across Blaine County early Tuesday
evening. This storm produced large hail of nearly two inches in
diameter along its path. As the severe storm approached Hays,
rotation strengthened slightly to where a brief tornado occurred.
The tornado occurred over open ranchland to the west of Montana
Highway 66 and, to our knowledge, no damage occurred. The
supercell then started to weaken as it move southeast between Hays
and Lodge Pole with the tornado likely lifting before it reached
Highway 66.