U.S. President Barack Obama declared the western counties of Kentucky, Tennessee, and Mississippi federal disaster areas.[6] For the first time in 37 years, the Morganza Spillway was opened on May 14, deliberately flooding 4,600 square miles (12,000 km2) of rural Louisiana to save most of Baton Rouge and New Orleans.[7]
Fourteen people were killed in Arkansas,[8] with 348 killed across seven states in the preceding storms.[1][2][3] Thousands of homes were ordered evacuated, including over 1,300 in Memphis,[9] and more than 24,500 in Louisiana and Mississippi,[10][11] though some people disregarded mandatory evacuation orders.[12] The flood crested in Memphis on May 10 and artificially crested in southern Louisiana on May 15, a week earlier than it would have if spillways had not been opened.[13] The United States Army Corps of Engineers stated that an area in Louisiana between Simmesport and Baton Rouge was expected to be inundated with 20–30 feet (6.1–9.1 m) of water.[14] Baton Rouge, New Orleans, and many other river towns were threatened,[15][16] but officials stressed that they should be able to avoid catastrophic flooding.[17]
Rainfall totals within the United States for the week ending April 29.U.S. Weather Service map showing number of inches of precipitation above/below normal year to date at June 18, 2011. The map shows that significantly large amounts of precipitation in the Ohio River basin that resulted in the worst of the flooding being below confluence of the Ohio and Mississippi. Louisiana, which experienced the worst of the flood was actually in a major drought. Heavy precipitation in the head waters of the Missouri River in Montana resulted in the 2011 Missouri River Floods which began to affect the Mississippi in late June (although to a lesser degree than the April flood). Significantly lower than normal rain in eastern Arizona led to the Wallow Fire in May/June.
From April 14–16, the storm system responsible for one of the largest tornado outbreaks in U.S. history also produced large amounts of rainfall across the southern and midwestern United States. Two more storm systems, each with heavy rain and tornadoes, hit in the third week of April. In the fourth week of April, from April 25–28, another, even more extensive and deadly storm system passed through the Mississippi Valley dumping more rainfall resulting in deadly flash floods.[18] The unprecedented extensive rainfall from these four storms, combined with springtime snow melt from the Upper Midwest, created the perfect situation for a 500-year flood along the Mississippi.