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. A new all-time daily precipitation record for Billings was set on May 24th. 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. The monthly precipitation for May resulted in the wettest May ever recorded in Billings.
Widespread, severe flooding occurred across the entire Billings Forecast Area. Many roads and bridges were damaged or washed out across the area, including closure of I-90 between Livingston and Springdale eastbound lanes. The Montana Department of Transportation alone estimated over $36 million in damage to state and federal highways across the state of Montana. Hundreds of homes and trailers and multiple businesses experienced flood damage. There was damage to flood control dams. On May 19th, Montana Governor Brian Schweitzer declared an emergency for the state. In addition, the Governor requested that the President declare a major disaster for the State of Montana on June 1st, as a result of substantial widespread flooding. Governor Schweitzer deployed the Montana National Guard in Roundup and Crow Agency to help with flood response efforts May 28th and 29th. Flood emergencies were declared by the Governor for all but three counties in the Billings Forecast area. Emergency declarations included the Crow Reservation in Big Horn County. There were two fatalities, an elderly woman in Boyd in Carbon County fell into a flooded ditch and was swept away, and a man from Huntley in Yellowstone County was killed on an excavator when flooding from Pryor Creek overtook him. Multiple Emergency Operation Centers and Incident Commands were set up across the area, including in Billings in Yellowstone County, and in Park County to coordinate flood relief and response efforts.
A persistant upslope northeast flow into the region resulted in another round of locally heavy rainfall again on May 29-30th. An additional 1-2 inches of rainfall were noted, especially for the Beartooth and Big Horn Mountain foothill areas. This resulted in prolonged or even renewed flooding in some locations through the end of the month.

Flood in CARBON, MONTANA
From 20-MAY-11 14:00:00 to 26-MAY-11 09:00:00 (MST-7)
Location: WASHOE
Flood cause: Heavy Rain
Property damage: 1.00M
Crop damage: 0.00K
Reported by: County Official

Significant flooding occurred on creeks and streams across Carbon County with numerous county roads flooded, closing roads and resulting in significant damage. Emergency Travel only was advised at one point due to the severity of the flooding. Specifically, Rock Creek flooded, resulting in adjacent lowland flooding and closure of Grape Vine Road near Fromberg. Creeks were running out of their banks from the Beartooth Foothills between McLeod and Red Lodge, as well as tributaries of the Clarks Fork of the Yellowstone River from Belfry to Bridger. Five Mile Creek flooded and washed out the bridge on East Pryor Road between Edgar and Pryor. Blue Water Creek east of Bridger flooded and washed out a county road as well. The heavy rainfall also resulted in water running over the spillway at Cooney Dam. This resulted in several homes flooded along Red Lodge Creek.
On May 25th, significant flooding was reported in the town of Joliet as debris backed up Rock Creek resulting in evacuations. At the peak of this flooding, Highway 212 was closed from Rockvale to Red Lodge as 18 inches of water was reported on Highway 212 through the town of Joliet. Fifty homes flooded in the area and seven people, mainly elderly, had to leave with the assistance of the fire and sheriffs offices. The Joliet Motel reported a foot of water in each of their rooms.
In addition, an 84-year-old woman drowned after she fell into a flooded ditch near her house. She was going to get her newspaper when she slipped and fell. The woman was swept a short distance downstream from her house near Boyd, where authorities found her body.
Damage estimates based on FEMA reviews and Individual Assistance approvals.

Flood in BIG HORN, MONTANA
From 21-MAY-11 00:00:00 to 31-MAY-11 23:59:00 (MST-7)
Location: BIG HORN WYE
Flood cause: Heavy Rain
Property damage: 4.00M
Crop damage: 0.00K
Reported by: Law Enforcement

From May 21st and even continuing into the month of June, extensive severe flooding occurred across Big Horn County from the Little Bighorn and Bighorn Rivers, as well as creeks and streams. An Incident Command Center was set up in Crow Agency where the Crow Tribal Chairman declared a disaster. At the height of the flooding, the Big Horn County Sheriffs office is quoted as saying, there is water everywhere.  Hundreds of people were evacuated as flooding affected every community on the Crow Reservation. Fifteen to twenty homes were heavily impacted by the flooding.  Interstate 90 south of Hardin to the Wyoming State line was closed for five days (May 21-26). Water rescues were required on the I-90 frontage road south of Crow Agency. Highway 212 was also closed for a period of time and at one point had water 3 feet deep overtopping the roadway. Railroad tracks and numerous rural roads were washed out in the area. In addition, flooding near Hardin brought down telephone equipment that handles 911 and long-distance calls for major cities and towns in eastern Montana.
The Bighorn River above Tullock Creek crested at a record 10.86 feet on May 23rd. The Little Bighorn River at Hardin rose above flood stage from May 22nd through May 27th and crested at a record 12.32 feet on May 23rd.
Damage estimates based on FEMA reviews and Individual Assistance approvals.

Flood in SWEET GRASS, MONTANA
From 20-MAY-11 14:00:00 to 27-MAY-11 08:00:00 (MST-7)
Location: MELVILLE
Flood cause: Heavy Rain
Property damage: 4.00M
Crop damage: 0.00K
Reported by: Emergency Manager

Significant flooding occurred on creeks and streams across Sweet Grass County resulting in widespread lowland flooding and numerous county roads flooded, closing roads and resulting in significant damage. Numerous bridges also washed out. Specifically, Upper Deer, Lower Deer, Bridger, Sweet Grass, Big Timber and Otter Creeks Flooded. Specific roads flooded include; Anderson, Lower Sweet Grass, Mendinhaul, and Windsor Loop. At the height of the flooding, water was overtopping Interstate 90 from Mile Post 376 to 377. Damage estimates based on FEMA reviews and Individual Assistance approvals.

Flood in ROSEBUD, MONTANA
From 21-MAY-11 13:00:00 to 29-MAY-11 00:00:00 (MST-7)
Location: NICHOLS
Flood cause: Heavy Rain
Property damage: 225.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Heavy rainfall caused rivers and streams to rise significantly resulting in some flooding. High water caused difficult passage on the road going into the town of Rosebud and water covered Highway 12 in the Hammond Valley. At the height of the flooding, Highway 12 was closed due to flooding from Ingomar to Forsyth.
The Yellowstone River at Forsyth went above flood stage with minor lowland flooding reported from May 22nd through May 29th. The river crested at 12.35 feet on the 24th and 12.18 feet on the 28th.
Damage estimates based on FEMA reviews and Individual Assistance approvals.

Flood in CUSTER, MONTANA
From 20-MAY-11 19:00:00 to 31-MAY-11 08:00:00 (MST-7)
Location: KNOWLTON
Flood cause: Heavy Rain
Property damage: 1.10M
Crop damage: 0.00K
Reported by: County Official

Significant flooding occurred on creeks and streams across Custer County with numerous county roads flooded, closing roads and resulting in significant damage. At the height of the flooding, emergency only travel was advised due to the magnitude of the event. The Powder River near Locate rose above flood stage from May 10th through May 23rd. The river crested four times at 11.29 feet, 11.70 feet, 11.27 feet and 9.28 feet on the 10th, 21st, 22nd and 23rd respectively. In addition, Pumpkin Creek was reported to be flowing over Highway 59 southeast of Miles City. Pumpkin Creek had a record crest of 13.98 feet on May 20th and 21st. The Yellowstone River at Miles City went above flood stage from May 22nd through May 29th. The river crested at 14.74 feet on the 24th and 14.53 feet on the 29th. The Tongue River at Miles City rose above flood stage from May 11th through May 23rd and crested at a record 13.99 feet on May 21st. Damage estimates based on FEMA reviews and Individual Assistance approvals.

Flood in POWDER RIVER, MONTANA
From 23-MAY-11 13:00:00 to 26-MAY-11 13:00:00 (MST-7)
Location: SONNETTE
Flood cause: Heavy Rain
Property damage: 120.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Rainfall amounts ranging from 4-6 inches resulted in significant flooding across Powder River County. Most tributaries feeding the Powder and Little Powder River reported flooding. The Little Powder River subsequently flooded with water reported as flooding Highway 59 in several spots. This flooding resulted in numerous roads being washed out. Flood damage estimates were based on FEMA reports and Individual Assistance approvals.

Flood in PARK, MONTANA
From 24-MAY-11 13:00:00 to 27-MAY-11 12:00:00 (MST-7)
Location: CLYDE PARK
Flood cause: Heavy Rain
Property damage: 130.00K
Crop damage: 0.00K
Reported by: County Official

County officials reported significant flooding across Park County, especially along the foothill locations and on the Shields River in the northern part of the county. The Shields River and its tributaries experienced significant flooding with adjacent low lying areas under water. Some of the creeks that were impacted by high water included Mission, Billman, and Fleshman Creeks. County officials also reported that several roads were closed, damaged and eventually washed out by high water. During the height of the event, the Department of Transportation closed the eastbound lanes of Interstate 90 at Springdale due to water over the road from Peterson Creek.
Damage estimates were based on FEMA reports and Individual Assistance approvals.

Flood in ROSEBUD, MONTANA
From 23-MAY-11 01:00:00 to 30-MAY-11 12:00:00 (MST-7)
Location: LAME DEER
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Highway 212 was closed on Lame Deer Divide due to the road sliding down a hill. Significant lowland flooding was also reported along Rosebud Creek. Sandbagging efforts were required on the Tongue River to avoid flooding of homes in the St Labre and Ashland areas.

Flood in FALLON, MONTANA
From 20-MAY-11 08:00:00 to 26-MAY-11 11:00:00 (MST-7)
Location: WEBSTER
Flood cause: Heavy Rain
Property damage: 375.00K
Crop damage: 0.00K
Reported by: County Official

Significant small stream flooding across much of Fallon County. Numerous roads closed and eventually washed out with additional damage reported to several bridges. Water breached a small dam and a couple private holding ponds in the region. Significant erosion threatened the Waste Water Treatment Plant in Baker. Significant flooding reported on O'Fallon Creek where one spotter reported the highest levels on the stream in at least 58 years with over a foot of water covering much of their farmland.  At the Boxelder and Horse Creek confluence, flood waters rushed over dikes and into hayfields. Floodwaters surrounded one house in that region. Damage estimates based on FEMA reviews and Individual Assistance approvals.

Flood in CARTER, MONTANA
From 20-MAY-11 08:00:00 to 27-MAY-11 11:00:00 (MST-7)
Location: ALBION
Flood cause: Heavy Rain
Property damage: 320.00K
Crop damage: 0.00K
Reported by: County Official

Record to near record flooding reported on rivers and streams across Carter County. A total of 10 homes were reported as damaged by floodwaters. Damage estimates were based on FEMA reports and Individual Assistance approvals. On Boxelder Creek, at it's highest levels, water was flowing across Highway 277 between Hammond and Ridgeway and across Highway 323 (3 miles north of Belltower) about a half mile wide in spots. On the Little Missouri River, record flooding was reported. Significant flooding noted in the Albion area where one individual had to be rescued as floodwaters surrounded their house. Two dams also breached due to the high waters. Widespread farmland flooding occurred along Thompson Creek in southern Carter County between Ridge and Alzada. Significant erosion and flooding occurred along numerous small stream tributaries all across the county due to the rapid rainfall rates, resulting in considerable damage to county roads. At one point, Hammond Road between Hammond and where it intersects Highway 323 just northeast of Ridgeway was flooded in 7 different places.

Flood in TREASURE, MONTANA
From 21-MAY-11 13:00:00 to 31-MAY-11 23:59:00 (MST-7)
Location: MYERS
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Heavy rainfall caused significant lowland flooding of rivers and streams across Treasure County, including some flooding and damage to area roads. The most concentrated flooding occurred along Muggins, Boxelder and Sarpy Creeks feeding into the Yellowstone River. Sarpy Creek flooding impacted travel on Highway 10. Across the entire county, an estimated half dozen roads were washed out. Additional heavy rainfall of 1-2 inches on May 29-30th resulted in prolonged flooding into the month of June. Specific damage estimates were not known.

Flood in STILLWATER, MONTANA
From 20-MAY-11 14:00:00 to 30-MAY-11 08:00:00 (MST-7)
Location: RAPELJE
Flood cause: Heavy Rain
Property damage: 2.00M
Crop damage: 0.00K
Reported by: County Official

Significant flooding occurred all across Stillwater County, including streams and creeks emerging from the Beartooth Foothills. This resulted in numerous road closures and significant damage in spots to rural roads. Specifically; Whitebird, Benbow, and the Stillwater River roads were closed. During the height of the flooding, a couple of families were stranded by the flood waters. A bridge was washed out on Whitebird Creek. Subsequent flooding on the Stillwater River was observed the May 24-25th. Damage estimates based on FEMA reviews and Individual Assistance approvals.

Flood in YELLOWSTONE, MONTANA
From 21-MAY-11 08:00:00 to 26-MAY-11 11:00:00 (MST-7)
Location: MOSSMAIN
Flood cause: Heavy Rain
Property damage: 300.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Widespread rainfall of 2 to 3 inches with locally higher amounts fell across the Yellowstone County area during a two day period. Billings had its all-time daily rainfall record broke on May 24th with 3.12 inches. This was just a part of a record setting precipitation month for Billings which totaled 9.54 inches. This rainfall on top of an already wet month resulted in some streams and creeks over Southern Yellowstone County to overflow their banks. Flooding occurred on Blue Creek in the Briarwood area. Several homes and structures were surrounded by water on Aviemore Court where some voluntary evacuations occurred. In addition, the Briarwood Golf Course was inundated with water.
Pryor Creek was out of its banks resulting in water spilling over a dike in a low lying area of Huntley. Fly Creek near Pompey was out of its banks and flooded a roadway leading to town. Flooding occurred along most of Pryor Creek between the towns of Pryor and Huntley. In addition, county officials reported that Pryor Creek Road Bridge between Interstates 90 and 94 was washed out.
Blue Creek Subdivision at Jene Helene Avenue flooded many homes and the Briarwood Golf Club May 20th-22nd. This resulted in the Billings Fire Department issuing voluntary evacuation orders. In addition, flash flooding occurred in Billings on May 24th forcing many road closures downtown and causing large boulders to fall off the Rimrocks. Flooding from Pryor Creek inundated a Huntley neighborhood and destroyed Pryor Creek Road on May 22nd.
The Yellowstone River at Billings went above flood stage on May 25th through May 26th. The river crested at 13.92 feet on May 26th.
In addition, a man operating a backhoe trying to save his house, drowned near Pryor Creek. The roadway had washed out from underneath the backhoe.
Damage estimates based on FEMA reviews and Individual Assistance approvals.