What's causing all this recent crazy weather?

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.

Tornadoes

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.

The spike in tornadoes has turned this year into a blast from the past: Most of history's deadliest tornadoes hit during the early part of the 20th century, when tornado science was in its infancy and warnings were poor. Tornado season isn't over yet, and the year could finish as the second deadliest on record, Mr. Brooks said. It currently ranks fifth.

"That has made us look like the kind of year my grandpa would have thought of as a normal year growing up," Brooks said.

La Niña's exit around three months ago precipitated the tornado-producing conditions that held sway this spring. Its departure allowed the jet stream to go rogue, driving winds into the heart of the country and violently mixing cool and warm air masses, creating the thunderstorms that spawned the deadly tornadoes. Had La Niña stayed strong, the jet stream would have been farther north during the first few months of tornado season.

But this season is not necessarily a portent of those to come. Brooks isn't ready to blame the extreme tornado season on climate change, and doesn't see any reason to call this year a major shift toward more tornadoes.

"These are the kinds of years that happen on rare occasion and hopefully won't happen again for quite some time," Brooks said.

• Natural Disasters: Top 10 U.S. Threats
• Infographic: Tornado! How, When & Where Twisters Form
• The World's Weirdest Weather

Email OurAmazingPlanet staff writer Brett Israel at bisrael@techmedianetwork.com. Follow him on Twitter @btisrael.

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