A wave of monsoon moisture tracked south to north across western Montana on August 9, 2013.  Embedded multicellular thunderstorms produced very heavy rain with flash flooding reported near Drummond and gusty winds that brought tree damage to portions of the Bitterroot and Missoula Valleys.

Thunderstorm Wind in MISSOULA, MONTANA
From 09-AUG-13 18:17:00 to 09-AUG-13 18:17:00 (MST-7)
Location: (MSO)MISSOULA INTL A
Magnitude: 47.00 MG
Property damage: 7.00K
Crop damage: 0.00K
Reported by: ASOS

The Missoula International airport ASOS reported a gust to 54 mph from the south-southeast direction. An intense thunderstorm near Florence produced a strong outflow that traveled north into the Missoula Valley causing multiple trees to be uprooted, downed power lines, & damage to a tent at the Western Montana Fair. The Missoula 911 dispatch reported at least 13 calls related to either power outages or lines down.

Thunderstorm Wind in MISSOULA, MONTANA
From 09-AUG-13 18:00:00 to 09-AUG-13 18:00:00 (MST-7)
Location: LOLO
Magnitude: 52.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: NWS Employee

A NWS employee reported an estimated 60 mph wind gust just north of Lolo, Montana.

Thunderstorm Wind in RAVALLI, MONTANA
From 09-AUG-13 17:05:00 to 09-AUG-13 17:05:00 (MST-7)
Location: HAMILTON
Magnitude: 48.00 EG
Property damage: 3.00K
Crop damage: 0.00K
Reported by: Emergency Manager

The Ravalli County emergency manager reported that multiple power lines were down around Hamilton.

Heavy Rain in MISSOULA, MONTANA
From 09-AUG-13 18:52:00 to 09-AUG-13 19:22:00 (MST-7)
Location: CLINTON
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

A trained spotter two miles south of Clinton measured 1.05 of rainfall in 30 minutes.

Heavy Rain in MISSOULA, MONTANA
From 09-AUG-13 18:54:00 to 09-AUG-13 19:24:00 (MST-7)
Location: BONNER
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

A trained spotter in Bonner measured 0.85 of rainfall in 30 minutes.

Hail in GRANITE, MONTANA
From 09-AUG-13 18:30:00 to 09-AUG-13 18:30:00 (MST-7)
Location: (3DU)DRUMMOND
Magnitude: 0.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

A person from the public reported penny size hail in Drummond.

Flash Flood in GRANITE, MONTANA
From 09-AUG-13 18:40:00 to 09-AUG-13 19:30:00 (MST-7)
Location: (3DU)DRUMMOND
Flood cause: Heavy Rain
Property damage: 0.50K
Crop damage: 0.00K
Reported by: NWS Storm Survey

A slow moving thunderstorm produced over an inch of rain in less than one hour during the evening of August 9, 2013 over the terrain southeast of Drummond, Montana. This triggered a flash flood up to four feet high that moved from small hillside gulches located north of Interstate 90 over the highway. One vehicle slid off the road and I-90 was shut down for a period of time until the water levels subsided.

Flash Flood in GRANITE, MONTANA
From 09-AUG-13 18:40:00 to 09-AUG-13 19:30:00 (MST-7)
Location: NEW CHICAGO
Flood cause: Heavy Rain
Property damage: 5.00K
Crop damage: 20.00K
Reported by: Public

A slow moving wet thunderstorm dumped at least 1 of rainfall in one hour just southeast of Drummond. A rancher experienced a 5 foot wall of fast moving water that moved down their driveway. The fast flowing water laden with pea-sized hail and mud severely damaged grain & alfalfa crops, irrigation sprinklers and a fence. They said it was the worst flooding in at least 20 years.