A very strong low pressure system from the pacific northwest stalled over southern Montana and northern Wyoming with plentiful moisture. Significant amounts of moderate and heavy rain spread across many locations while enough cold air from the Canadian Rockies wrapped around the system to change the precipitation to a heavy, very wet snow for some higher elevations of central and northern Montana.

Flood in PHILLIPS, MONTANA
From 09-MAY-16 09:00:00 to 13-MAY-16 09:00:00 (MST-7)
Location: SACO
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Official NWS Observations

Two to four inches of rainfall as well as 2 to 3 feet of snow that eventually melted contributed to flooding in the southern portions of Phillips County.  Beaver Creek remained in it's banks south of Saco, but as it got closer to Saco the water left it's banks, and at least 100 yards of the Beaverton Road was underwater. Peoples Creek near Dodson was flooded out of it's banks in the Dodson area.

Blizzard in GARFIELD, MONTANA
From 10-MAY-16 16:00:00 to 11-MAY-16 08:00:00 (MST-7)
Property damage: 240.00K
Crop damage: 0.00K
Reported by: NWS Employee

NWS Employees driving along US Highway 200 through western Garfield County reported whiteout conditions with blowing and drifting snow during the evening hours on May 10th.  County emergency officials later reported 24 to 36 inches of snow and 60 power poles snapped.  Event times are estimated.

Blizzard in LITTLE ROCKY MOUNTAINS, MONTANA
From 09-MAY-16 21:00:00 to 11-MAY-16 05:00:00 (MST-7)
Property damage: 160.00K
Crop damage: 0.00K
Reported by: Other Federal Agency

The Little Rocky Mountains near Zortman received 24 inches of heavy wet snow and wind speeds of 35 to 50 mph for several hours on May 10th.  Numerous tree were down and dozens of power poles were snapped.  Snow drifts up to 14 feet high were reported.  Emergency evacuations were initiated at the Landusky Mine. Up tp 40 power poles were snapped due to the heavy, wet snow and the strong winds.  Two small communities were without power for at least 24 hours.

Flood in VALLEY, MONTANA
From 11-MAY-16 11:00:00 to 15-MAY-16 09:00:00 (MST-7)
Location: FT PECK
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Official NWS Observations

From 2 to 4.5 inches of rainfall occurred across the county from May 9th through the 12th, with Glasgow to Fort Peck Dam area seeing the highest totals.  Flood waters inundated a ranch on Antelope Creek.  Water went across the Tampico Highway west of Glasgow, and several gravel roads were under water west of Glasgow and south of Hinsdale from smaller creeks in the area.  One pickup truck was swept downstream a ways along the Billingsley Rd and the Tampico Highway when it encountered deep waters at the intersection.  A stock dam also broke on the Maxness Road near Fort Peck.  It caused no property damage according to law enforcement.
Beaver Creek near Hinsdale crested at 16.74 feet around 1 am on Friday May 13th.  Flood stage is 14 feet.  It hit 14 feet around 5 am Thursday May 12th.  It went below flood stage around 1 am Saturday May 16th.
The Milk River at Glasgow hit flood state around noon on Thursday May 12th and peaked around 15.62 feet on Friday May 13th.  It dropped below flood stage 24 hours later.

Blizzard in PETROLEUM, MONTANA
From 10-MAY-16 16:00:00 to 11-MAY-16 08:00:00 (MST-7)
Property damage: 50.00K
Crop damage: 0.00K
Reported by: NWS Employee

NWS Employees driving along US Highway 200 reported whiteout conditions with blowing and drifting snow during the evening hours on May 10th. A semi-truck slid off the road, temporarily closing it due to the icy conditions and strong winds.