The floods were caused by heavy winter snows and torrential spring rains, driven by La Niña and the North Atlantic Oscillation. All that water ran into the two rivers, and the US Army Corps of Engineers has been forced to release record amounts of water from the Missouri's reservoirs to cope with the flooding. As the Midwest continues to be soaked by rainstorms and parts of the snowpack have yet to melt, the Corps may find itself releasing reservoir water through mid-August.
Extreme floods could be a common sight in a warmer world, Mr. Karl said. The hotter the Earth's atmosphere gets, the more water it can hold; this could cause more intense rain and snow in certain parts of the world.
A warmer world is also expected to bring on more severe droughts, like those the Southwest has experienced this year, Karl said. More moisture in the air doesn't automatically mean extreme rain will fall everywhere.
"You need an atmospheric disturbance to wring that out of the air," Karl said.
An atmospheric disturbance – a storm – would be welcome in the Southwest today, because the region is in the middle of one of the driest years since record keeping began in the 1930s.
The 2011 tornado season is already one of the most active ever, due to a steep rise in twisters in April, the most tornado-filled April on record. It may end up as the biggest tornado month of all time after the counts are finished, said Harold Brooks of the National Severe Storms Laboratory.