A Pacific trough approached and eventually moved through North-central and Southwest Montana on Saturday, August 10th and the morning of Sunday, August 11th. A belt of southwesterly mid- to upper-level winds up to about 50 knots preceded the axis of this trough and contributed to moderate to strong vertical wind shear, which favored the development of organized thunderstorms in the form of multicells and supercells. At the surface, winds exhibited an easterly component at times. These surface winds transported greater low-level moisture into the area from the Dakotas. Farther aloft, southwesterly winds near 10,000 feet MSL were present ahead of the Pacific trough axis and transported abundant amounts of Pacific moisture into the region. The resulting unusually-high humidity and daytime heating contributed to a weak to moderate instability especially over Southwest Montana. Also notable was a pronounced elevated mixed layer (EML), a term for an elevated dry layer of the lower to middle atmosphere, moving north from Utah and eastern Idaho. EMLs are much more common across the Great Plains of the United States and are associated with many of the classic spring severe weather outbreaks in the central part of the county.

Hail in JEFFERSON, MONTANA
From 10-AUG-19 16:26:00 to 10-AUG-19 16:26:00 (MST-7)
Location: CORBIN
Magnitude: 1.00 
Reported by: Public

A member of the public reported hail estimated to be the size of quarters (1 inch Diameter) near Park Lane. hail was covering the ground.

Hail in JEFFERSON, MONTANA
From 10-AUG-19 17:25:00 to 10-AUG-19 17:25:00 (MST-7)
Location: CARDWELL
Magnitude: 1.50 
Reported by: Trained Spotter

A trained spotter reports hail of 1.5 inches in diameter.

Hail in JEFFERSON, MONTANA
From 10-AUG-19 16:30:00 to 10-AUG-19 16:30:00 (MST-7)
Location: BASIN
Magnitude: 1.00 
Reported by: Trained Spotter

A trained spotter reported quarter size hail (1 inch diameter) for a short period but then mostly dime sized hail after that.

Hail in JEFFERSON, MONTANA
From 10-AUG-19 17:26:00 to 10-AUG-19 17:26:00 (MST-7)
Location: WHITEHALL
Magnitude: 1.50 
Reported by: Trained Spotter

A trained spotter reports sized 1.5 inches in diameter about 5 miles NNW of Whitehall.

Hail in JEFFERSON, MONTANA
From 10-AUG-19 17:28:00 to 10-AUG-19 17:28:00 (MST-7)
Location: WHITEHALL
Magnitude: 1.50 
Reported by: Trained Spotter

A trained spotter reports hail size of 1.5 inches in diameter.

Hail in BROADWATER, MONTANA
From 10-AUG-19 17:45:00 to 10-AUG-19 17:45:00 (MST-7)
Location: TOWNSEND
Magnitude: 1.75 
Reported by: Trained Spotter

A trained spotter reports hail size of 1.75 inches in diameter.

Heavy Rain in MEAGHER, MONTANA
From 10-AUG-19 18:05:00 to 10-AUG-19 18:05:00 (MST-7)
Location: WHITE SULPHUR SPGS
Reported by: Trained Spotter

A trained spotter reported 1 inch of rainfall in 20 minutes.

Hail in BROADWATER, MONTANA
From 10-AUG-19 18:20:00 to 10-AUG-19 18:20:00 (MST-7)
Location: TOWNSEND ARPT
Magnitude: 1.75 
Reported by: Trained Spotter

A trained spotter reports hail size of 1.75 inches in diameter. Some body damage to trucks and the spotter reported that a neighbor had broken windows.

Hail in MEAGHER, MONTANA
From 10-AUG-19 19:20:00 to 10-AUG-19 19:20:00 (MST-7)
Location: MARTINSDALE
Magnitude: 1.00 
Reported by: Trained Spotter

A trained spotter reported hail size of 1 inch in diameter. Also, hail cracked a windshield on a side by side and stripped most of the leaves from an aspen grove. the hail started as sporadic large stones but then smaller stones fell for over 20 minutes.

Heavy Rain in CASCADE, MONTANA
From 10-AUG-19 19:21:00 to 10-AUG-19 19:21:00 (MST-7)
Location: NEIHART
Reported by: Trained Spotter

A trained spotter reported 1 inch of rainfall in less than an hour.

Heavy Rain in MEAGHER, MONTANA
From 10-AUG-19 20:00:00 to 10-AUG-19 20:00:00 (MST-7)
Location: MARTINSDALE
Reported by: Trained Spotter

A trained spotter reported 1.3 inches of rain for a storm total.

Thunderstorm Wind in FERGUS, MONTANA
From 10-AUG-19 20:22:00 to 10-AUG-19 20:22:00 (MST-7)
Location: GARNEILL
Magnitude: 50.00 MG
Reported by: AWOS

An AWOS station at Lima (K1LM) measured a wind gust of 58 mph (50 kts).

Hail in LEWIS AND CLARK, MONTANA
From 11-AUG-19 07:15:00 to 11-AUG-19 07:15:00 (MST-7)
Location: EAST HELENA
Magnitude: 1.00 
Reported by: Trained Spotter

A trained spotter reported mixed nickle and quarter sized hail (.88 to 1 inch in diameter).

Heavy Rain in LEWIS AND CLARK, MONTANA
From 11-AUG-19 09:00:00 to 11-AUG-19 09:00:00 (MST-7)
Location: HELENA
Reported by: CoCoRaHS

COCORAHS reported a 24 hour rainfall total of 1.05 inches, with .72 inches falling within 10 minutes.

Hail in JEFFERSON, MONTANA
From 10-AUG-19 17:00:00 to 10-AUG-19 17:00:00 (MST-7)
Location: BOULDER
Magnitude: 1.50 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

A trained spotter reports hail up to 1.5 inches in diameter 5 miles NNW of Whitehall at 5:26 PM MST.

Hail in GALLATIN, MONTANA
From 11-AUG-19 12:18:00 to 11-AUG-19 12:18:00 (MST-7)
Location: BOZEMAN
Magnitude: 1.25 
Reported by: Public

A member of the public estimated hail size of 1.25 inches in diameter. Time was estimated by radar. Report was posted to NBC Montana's Facebook page.

Hail in BEAVERHEAD, MONTANA
From 11-AUG-19 12:30:00 to 11-AUG-19 12:30:00 (MST-7)
Location: DILLON
Magnitude: 1.75 
Reported by: Law Enforcement

A windshield was cracked by hail sized 1.75 inches in diameter.

Hail in BEAVERHEAD, MONTANA
From 11-AUG-19 12:31:00 to 11-AUG-19 12:31:00 (MST-7)
Location: GRANT
Magnitude: 1.50 
Reported by: Public

A COOP from Grant 5 SSE relayed a public report of hail sized 1.5 inches in diameter.

Hail in MADISON, MONTANA
From 11-AUG-19 13:28:00 to 11-AUG-19 13:28:00 (MST-7)
Location: SHERIDAN
Magnitude: 1.75 
Reported by: Public

A member of the public shared a photo of hail sized 1.75 inches in diameter.

Hail in MADISON, MONTANA
From 11-AUG-19 13:35:00 to 11-AUG-19 13:35:00 (MST-7)
Location: SHERIDAN
Magnitude: 1.00 
Reported by: Trained Spotter

A trained spotter reported a garage window blown out by hail sized 1 inch in diameter.

Thunderstorm Wind in MADISON, MONTANA
From 11-AUG-19 13:48:00 to 11-AUG-19 13:48:00 (MST-7)
Location: PONY
Magnitude: 87.00 EG
Reported by: Park/Forest Service

Beaverhead/Deerlodge National Forest Personnel estimated between 200 to 250 acres of trees were blown down from a severe thunderstorm that moved over the area. the blow down resulted in a trail leading back to Granite Lake in the Tobacco Root Mountains to become blocked. At least four people became stranded and had to be rescued by the joint effort of the Madison County Search and Rescue, Forest Service Fire Team, and US Airforce helicopter support. A storm survey conducted by National Weather Service Great Falls discovered that this event was a microburst.

Hail in BROADWATER, MONTANA
From 10-AUG-19 17:10:00 to 10-AUG-19 17:10:00 (MST-7)
Location: TOWNSEND
Magnitude: 1.75 
Reported by: Trained Spotter

A trained spotter reported hail up to the size of 1.75 inches in diameter.

Hail in MADISON, MONTANA
From 11-AUG-19 13:50:00 to 11-AUG-19 13:50:00 (MST-7)
Location: LAURIN
Magnitude: 1.00 
Reported by: Trained Spotter

A trained spotter reported hail size of 1 inch in diameter. The time was estimated by radar.

Hail in GALLATIN, MONTANA
From 11-AUG-19 14:35:00 to 11-AUG-19 14:35:00 (MST-7)
Location: BELGRADE
Magnitude: 1.50 
Reported by: Public

A member of the public posted images of quarter to golf ball sized hail (1 to 1.5 inches in diameter) on the KBZK Facebook page. The time was estimated by radar.

Thunderstorm Wind in BLAINE, MONTANA
From 11-AUG-19 17:40:00 to 11-AUG-19 17:40:00 (MST-7)
Location: FT BELKNAP
Magnitude: 50.00 MG
Reported by: Mesonet

The fort Belknap RAWS station (FBLM8) measured a thunderstorm wind gust of 58 mph (50 kts).

Lightning in CASCADE, MONTANA
From 11-AUG-19 08:51:00 to 11-AUG-19 08:51:00 (MST-7)
Location: GREAT FALLS
Property damage: 10.00K
Crop damage: 0.00K
Reported by: Fire Department/Rescue

Great Falls Fire department reported a structure fire caused by lightning striking a tree and adjacent house. The department received the report shortly after 10 AM. The timing of the lightning strike was determined by the lightning detection network. Also, the property damage cost was estimated.

Hail in BEAVERHEAD, MONTANA
From 11-AUG-19 13:00:00 to 11-AUG-19 13:00:00 (MST-7)
Location: DILLON
Magnitude: 1.25 
Reported by: Public

A member of the public posted an image of hail sized 1.25 inches in diameter to KXLF.

Hail in BEAVERHEAD, MONTANA
From 11-AUG-19 13:11:00 to 11-AUG-19 13:11:00 (MST-7)
Location: DILLON
Magnitude: 1.50 
Reported by: Trained Spotter

A trained spotter reports hail size of 1.5 inches in diameter.

Hail in BROADWATER, MONTANA
From 10-AUG-19 17:15:00 to 10-AUG-19 17:15:00 (MST-7)
Location: TOWNSEND
Magnitude: 2.00 
Reported by: Public

A member of the public estimated hail size up to 2 inches in diameter. There are reports of  house windows broken, car windshields cracked, and vehicles dented. the time of event was estimated.