National Climate Report - May 2011

« National Climate Report - May 2011


During the spring of 2011, persistent rainfall combined with melting snowpack caused historical flooding in some of the United States' major rivers, including the Ohio, the Mississippi, and the Missouri. A relentless storm track pattern provided precipitation amounts 150 percent of normal in the Ohio Valley. As the Ohio, Missouri, and Mississippi River experienced historical crests, dams and levees were breached. Those that were not breached were significantly tested throughout the enduring event. Smaller towns and farmland were intentionally flooded to save larger, more populated towns. While the slow-moving disaster provided extra lead time for the residents to prepare their homes and businesses, hundreds of thousands of acres of farmland from an area south of the Great Lakes to the Gulf Coast were swimming in 20 feet (6 m) of water for weeks.

The Lower Mississippi Basin begins at the confluence of the Ohio and Upper Mississippi Rivers in Cairo, Illinois. According to the U.S. Army Corps of Engineers, 4 million people reside within the 35,000 square mile Lower Mississippi watershed. After a massive flood overwhelmed the southernmost stretch the river in 1927, the Army Corps of Engineers built a 2,200-mile system of earthen levees, floodwalls and other controls along the Mississippi, Arkansas, and Red Rivers. The flood control system has been tested over the years, especially in 1973 and 1993 when extremely wet conditions swelled the rivers to comparable historical levels. In 2011, extreme amounts of precipitation led to unprecedented flooding in several locations along the major rivers.

The extreme amounts of precipitation was set up by a large-scale weather pattern during April which consisted of high pressure, which typically results in warm dry air, over the South and Southwest U.S. This high pressure was associated with a ridge which inhibited systems from entering the region deflecting storms into the Ohio River Valley region. The persistence of this pattern during the season exacerbated the magnitude of the precipitation amounts and subsequent flooding.

Precipitation Records

During the month of April, over 1,300 daily precipitation records were broken across the Midwest and South - 197 in Kentucky alone. For the month, 72 locations reported their wettest day in any April on record and 5 of these stations set a new all-time record for the wettest 24-hour period for any month. Rainfall totals for April exceeded 13 inches (330 mm) in cities along the Ohio River. At Louisville International Airport, the 13.97 inches (355 mm) surpassed the previous record of 11.10 inches (282 mm) set in 1970. A monthly record of 13.52 inches (343 mm) of precipitation was also a record in Cincinnati. Further inland, the 15.91 inches (404 mm) of precipitation in Paducah, Kentucky was also an April record. The 12.7 inches (329 mm) that was measured in Lexington surpassed the 1970 record of 9.3 inches (236 mm). Columbus, Ohio received 7.14 inches (181 mm) of precipitation which was also a record. The 6.89 inches (175 mm) that fell in Cleveland broke the 1961 record of 6.61 inches (168 mm). From March-May, departures were at least 150 percent of normal in an area that stretched from the Ohio Valley to the Middle Mississippi Valley.

Spring Record Precipitation Amounts
Location Record Amount Previous Record (year)
Northwest Climate Region Most March–May Precipitation 10.10" 9.39" (1993)
Washington Most March–May Precipitation 13.67" 12.85" (1997)
Wyoming Most March–May Precipitation 6.69" 6.41" (1906)
Indiana Most March–May Precipitation 19.38" 18.05" (1933)
Ohio Most March–May Precipitation 17.47" 16.22" (1964)
Kentucky Most March–May Precipitation 23.70" 22.08" (1935)
West Virginia Most March–May Precipitation 18.19" 17.11" (1967)
Pennsylvania Most March–May Precipitation 18.62" 15.48" (1983)
New York Most March–May Precipitation 16.14" 14.35" (1983)
Vermont Most March–May Precipitation 17.18" 16.94" (1983)


Mississippi River

In order to protect heavily populated cities from flooding, the Army Corps of Engineers opened several spillways along the Lower Mississippi River. On May 2 officials intentionally breached part of the Birds Point-New Madrid Levee near the confluence of the Ohio and Mississippi Rivers to protect the small Illinois town of Cairo, population of 2,800. The two-mile opening allowed water to pass through at a rate eight times that of Niagara Falls, flooding the Birds Point New-Madrid Floodway - which is 130,000 acres of Missouri farmland in addition to about 100 residences. The move was challenged in courts, but overturned, preventing devastating flooding in Cairo and elsewhere downstream. The Corps estimated it will take up to two months for the water to recede from the floodway and another month for the land to dry out.

One week later on May 9, the Bonnet Carre' Spillway was opened, allowing flood waters to flow into Lake Pontchartrain. When all 350 bays are opened in that spillway, it diverts 250,000 cubic feet (7,079 cubic meters) of water per second into Lake Pontchartrain and on into the Gulf. The last time all Bonnet Carre' bays were opened was in 1983. Farther upstream, a portion Morganza Spillway was opened on May 14. This move alleviated stress on the Old River Control Structure upstream and the levees which protect New Orleans downstream. The only prior time the Morganza Spillway had been opened was in 1973 and this marked the first time in history that all three spillways have been opened simultaneously.

Even with all the precautions taken, the populated cities and rich farmland along the riverside, which are normally protected by the system of levees, were flooded. The massive wall of water drifted slowly southward, overtopping its banks along the way. In Memphis, Tennessee on May 10, the Mississippi River crested at 47.9 feet (14.6 m), the highest level reached at Memphis since 1937 (48.7 feet or 14.8 m). In Greenville, Mississippi, the river crested on May 16, about one foot below the historical crest of 65.4 feet (19.9 m) set in 1927. The 2011 flood set a record in Vicksburg, Mississippi, cresting on May 18 at 57.1 feet (17.4 m) , besting the previous record of 56.2 feet (17.1 m) set in 1927. The flooded Mississippi also caused the Yazoo River to backfill, flooding out Yazoo City, Mississippi where some of the worst flooding occurred. Flood stage is 29 feet (8.9 m), but the crest reached 38.7 feet (11.8 m), just a few feet shy of the record set in 1927 of 43.4 feet (13.2 m).

Another stage height record was set farther downstream in Natchez, Mississippi on May 19. The crest was nearly 4 feet (1.2 m) higher than the previous record of 58.04 feet (17.7 m) set in 1937. To the south of Baton Rouge is the Atchafalaya Basin which, according to the U.S. Geological Survey, is the largest swamp in the United States. When the Morganza Spillway was opened as much as 1.2 million gallons of water per second flooded into the basin encroaching upon Morgan City which is perched on the banks of the Atchafalaya River. The river gauge at the riverside city, home to nearly 13,000, experienced its maximum peak at 10.35 feet (3.2 m) on May 29, just shy of the record of 10.53 feet (3.2 m) set in 1973.

Due to drastic steps of the Army Corps of Engineers that sacrificed farmland and less populated cities, major flooding was averted in the more populated cities along the southernmost sections of the Mississippi River. While the slow surge of water had dispersed some by the time it made it to Baton Rouge (flood stage is 35 feet or 10.7 m) and New Orleans (flood stage is 17 feet or 5.2 m), low-lying areas were still affected. The crest at Baton Rouge was more than 3 feet (0.9 m) below its record level and it was 5 feet (1.5 m) below the record level in New Orleans.

Missouri River

Additional flooding occurred later in the spring and into the summer along the Missouri River. The Missouri River is the longest river in North America and one of the largest tributaries of the Mississippi. Its headwaters begin in Montana and flow through several major cities including: Great Falls, Montana; Williston and Bismarck, North Dakota; Omaha, Nebraska and Kansas City, Missouri before joining with the Mississippi in St Louis, Missouri.

Record amounts of precipitation and melting snowpack contributed to historical flooding along the Missouri and its tributaries. The average rainfall across the state of Wyoming, which hosts several tributaries of the Missouri, was the most for any spring, based on records that data back to 1895. In eastern Montana, precipitation was 300 percent of normal for the month of May. The Missouri River basin experienced its fourth wettest spring and its third wettest May on record.

In Wyoming and Montana, for the month of May, a total 14 locations set precipitation records and seven locations set a new all-time record for the wettest 24-hour period for any month on record. In Glasgow, Montana, the monthly precipitation amount of 6.97 inches (177 mm) was a record in addition to the seasonal snowfall record of 108.6 inches (276 cm). This shattered the previous snowfall record of 70.7 inches (180 cm) set in 2006/2007. During the July 1 - June 30 snow season, Williston, North Dakota received a record 107.2 inches (272 cm) of snow. The previous record was 94.7 inches (241 cm) set during the 1895-1896 season. Record to near-record snowpack in the Northern Rockies and High Plains during the winter and spring contributed to high levels of runoff. The runoff quickly filled all six of the rivers reservoirs forcing the Corps of Engineers to release them. The swollen river breached levees, forcing mandatory evacuations downstream. Additional damaging flooding is expected to continue through early summer.

Lake Champlain

Flooding was not confined to just rivers and streams. Water levels at Lake Champlain, which straddles New York, Vermont, and Canada, also experienced historical crests. At Rouses Point, Vermont, water levels rose to 102.8 feet (31.3 m) on May 10 and remained at or near historical levels for several weeks. The previous record lake level was 102.1 feet (31.1 m) set in 1869 and the normal for this time of year is about 97 feet (29.6 m). Record amounts of rainfall and melting of winter snowpack in the state exacerbated the flooding. It was the wettest Spring on record in Vermont. In Burlington, it was the wettest May on record with 8.67 inches (220 mm) of precipitation - besting the previous record of 7.10 inches (180 mm) set in 2006. Warmer-than-normal spring temperatures also increased the melting and runoff of snowpack. In Burlington, 128.4 inches (236 cm) of snow fell this season, 45.3 inches (115 cm) more than normal.

Historical Perspective

The prolonged flooding during the spring of 2011 that affected the Ohio, Mississippi and Missouri River Valleys draws comparisons to the great floods during the early 20th century. During the fall of 1926, record precipitation amounts resulted in major flooding along the lower Mississippi in the spring of 1927. During the three-month period (March-April), Arkansas, Illinois, and Missouri each had their wettest such period in 117 years of record keeping. It was reported that the flood of 1927 submerged more than 165 million acres, drowning 246 people. Economic losses were estimated at 2.8 billion in 2010 dollars.

During the winter 1937, excessive precipitation during an 11-day period (January 13-24) contributed to flooding along the Ohio River. The state of Kentucky experienced a record 16.13 inches (410 mm) of precipitation in January. Individual locations had as much as 23 inches (584 mm) of precipitation during the month. From January 1-24 percent of normal precipitation in the area was approximately 600 percent of normal. Due to the copious amounts of precipitation, the Ohio River crested in Louisville at 85.4 feet (26 m). The Louisville flood stage is 51 feet (15.5 m). Further downstream in Paducah, the flood stage is 39 feet (11.9 m), but the river crested at 60.6 feet (18.5 m). It was reported that 3.3 billion in 2010 U.S. dollars worth of damage was done from the event.

As a result of a series of heavy snows in the Upper Midwest during the winter of 1972/1973 punctuated by heavy springtime rains in the South, the Mississippi swelled, overtopping its banks during the spring of 1973. Both Tennessee and Wisconsin had their wettest March-April period. Additionally, nearly every state east of the Rockies experienced above normal precipitation. Damages came to an estimated 256 million in 2010 U.S. dollars.

In the summer of 1993 frequent and excessive rainfall in the northern Plains southeastward into the central U.S. saturated soils, filling streams and rivers to capacity. For the summer period, record precipitation fell in Iowa, Illinois, Minnesota, Montana, Nebraska, and the Dakotas. Observed river crests in Iowa and Missouri easily surpassed previous record amounts by several feet. In St. Louis, at the confluence of the Missouri, Illinois and Mississippi rivers, the old record of 43.3 feet (13.2 m) set in 1973 was shattered on August 1, 1993 (49.7 feet or 15.1 m). The Missouri River also inundated towns. In Kansas City, the river set a new stage height of 48.9 feet (14.9 m), topping the old record of 46.2 feet (14.1 m) set in 1951. A total of 20 river gauges set all-time records. The devastating floods of 1993 are currently the costliest flooding disaster in the U.S. as damages neared 23 billion in 2010 U.S. dollars. In addition to the cost, more than 50 people were killed and at least 15 million acres of farmland were flooded. Other effects of the 1993 floods were: halted shipping on the Mississippi and Missouri for nearly two months, ten airports were flooded, all rail traffic in the Midwest was ceased, and both the 1993 and 1994 harvests were lost.


The impacts of the 2011 flooding are far reaching, affecting economic sectors such as: agriculture, fishing, shipping, insurance, refineries, and tourism. Economic losses will take years to recoup. Hundreds of thousands of acres of farmland were flooded, creating a nightmare at a time of year when the growing season is just beginning for many crops. In Tennessee, corn planting has been delayed and the winter wheat crop, which is typically harvested in June, was damaged. It was reported that nearly 900,000 acres of farmland in Mississippi was flooded - roughly 10 percent of all farmland in the state. In Arkansas, it is estimated that the flood waters wiped out 1 million acres of farmland - a staggering number when you consider that the agriculture industry generates 16 billion dollars annually in Arkansas. In all, the floods washed out more than 3.5 million acres of farmland within the Lower Mississippi River Valley.

The dangerous floodwaters shut down river commerce, which would have also caused additional stress on the levees. In addition to river commerce, in eastern Arkansas a 23-mile section of Interstate-40, a major east-west thoroughfare, was closed. Businesses and homes were closed or swept away, leaving many without a job or personal belongings. As the event continues to unfold, estimated economic losses are mounting. Overall, total insured losses currently amount to approximately 2-4 billion dollars.

Selected Estimated Economic Losses
Amount Type
$800 million Agriculture in Mississippi
$500 million Agriculture in Arkansas
$320 million Damage in Memphis, Tennessee
$317 million Agriculture and property in Missouri's Birds Point-New Madrid Spillway
$80 million First 30 days of flood fighting efforts in Louisiana

Farming was not the only economic sector in the South that was heavily damaged. The amount of fresh water flowing into the Gulf of Mexico caused an imbalance in the ecosystem wiping out the oyster beds which need the salt water to keep their metabolisms in check. The large amounts of fresh water saturated with fertilizers, pesticides and other farming chemicals is expected to flow into the gulf causing a "dead-zone" according to Lt. Col. Mark Jernigan, deputy commander of the New Orleans District of the U.S. Army Corps of Engineers. A dead zone is as area with low oxygen levels caused by the increase of fertilizers which fuels the growth of algae.

It is too early to be able to give an exact figure of the damages done and economic loss of the 2011 flooding. However, we do know that it will take years to recover from the 6.8 million acres that were flooded. The flooding that occurred across the United States in Spring of 2011 will be one of the worst flooding disasters in modern American history.

Citing This Report

NOAA National Centers for Environmental Information, State of the Climate: National Climate Report for May 2011, published online June 2011, retrieved on August 22, 2019 from