A Pacific low moved into the four corners region on the 17th and migrated northeast into the Dakotas through the 22nd, bringing another round of heavy precipitation aided by a tap of Gulf of Mexico moisture. Another system passed to the south on the 23rd and 24th, continuing the precipitation across the area. These two systems brought a total of 4 to 7 inches of precipitation to the area through the 25th. From May 29th through May 31st, another Pacific storm system moved northeast through the Billings Forecast Area, producing a half inch to 2 inches of precipitation across the area. A few areas received close to 3 inches of precipitation. The culmination of precipitation events resulted in historic flooding across the Billings Forecast Area.
Flooding along the Musselshell River began on May 24th and continued through the end of the month. There was extensive flooding in Harlowton, Ryegate, Lavina and Roundup. Highways 87,12 and 3 into these towns were inundated and closed. Evacuations occurred in Wheatland and Golden Valley Counties, and about 30 homes were evacuated in Roundup in Musselshell County. About eight feet of standing water surrounded the Busy Bee Cafe, a local landmark in Roundup. Houses and businesses in Roundup continued under several feet of water through May 30th.

Flood in MUSSELSHELL, MONTANA
From 23-MAY-11 09:00:00 to 31-MAY-11 23:59:00 (MST-7)
Location: ROUNDUP ARPT
Flood cause: Heavy Rain / Snow Melt
Property damage: 6.00M
Crop damage: 0.00K
Reported by: Emergency Manager

Persistent heavy rainfall combined with snowmelt in the mountains resulted in extensive and significant flooding along the Musselshell River and its tributaries, including the Roundup area where record flows were observed. The Musselshell River at the gauge at Roundup crested at a record of 14.78 feet on May 26th. As the water on the Musselshell rose quickly on the 26th, floodwaters quickly inundated the town of Roundup. Portions of Highways 12 and 87 were closed in and around Roundup with up to 8 feet of water covering the roads in spots. This completely cut off the town from surrounding areas for several days. Homes and businesses in Roundup were inundated by flood waters with water up to rooftop levels in some cases. 39 homes experienced major damage and were complete losses. Another 110 homes and businesses experienced at least minor flood damage. Four teenagers were rescued from the high waters after they disobeyed orders to stay out of the area and then became stranded. Further downstream on the Musselshell River, another 14 homes were damaged by the flood in the town of Musselshell. In all, hundreds of families were isolated due to the floodwaters and severely damaged roadways. Railroad tracks were reported as washed out in spots. Musselshell County was reported as one of the worst hit counties in the state of Montana.

Flood in WHEATLAND, MONTANA
From 23-MAY-11 09:00:00 to 31-MAY-11 23:59:00 (MST-7)
Location: TWODOT
Flood cause: Heavy Rain / Snow Melt
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Persistent rainfall combined with snowmelt in the mountains resulted in significant flooding along the Musselshell River. The Musselshell River at Harlowton rose above flood stage from May 22nd through May 30th. The river crested at 7.47 feet on the 22nd and again on the 29th at 10.25 feet which was a record stage. In addition, the Musselshell River at Shawmut crested at a record 9.27 feet on May 26th. Extensive lowland flooding resulted along the Musselshell River as well as its tributaries. Numerous roads were closed by floodwaters with many of those eventually washing out.  Flooding persisted into the month of June. Damage estimates were not available.

Flood in GOLDEN VALLEY, MONTANA
From 23-MAY-11 09:00:00 to 31-MAY-11 23:59:00 (MST-7)
Location: SAHARA
Flood cause: Heavy Rain / Snow Melt
Property damage: 50.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Persistent rainfall combined with snowmelt in the mountains resulted in significant flooding along the Musselshell River and its tributaries, including the Ryegate area.  The Musselshell River at the gauge at Lavina crested at a record 14.04 feet on May 25th. Extensive lowland flooding resulted along the Musselshell River as well as its tributaries. As the water on the Musselshell rose quickly on the 24th, floodwaters quickly inundated Highway 12 east of Ryegate (Slayton Underpass) and at the height of the event, water was over Highway 12 in 3 different locations. Approximately 10 homes required evacuations due to the rapidly rising flood waters. One resident canoed in/out of their home for 2 weeks straight. Extensive flooding was reported in both Ryegate and Lavina where numerous homes and businesses were under water. A grocery store in Ryegate reported 3 feet of water both in and outside their business. Numerous roads were closed by floodwaters with many washing out. A bridge was also washed out east of Lavina.  Flooding persisted into the month of June. Damage estimates were based only on individual assistance approvals through FEMA. No dollar amount was available for infrastructure losses.