WASHINGTON The deadliest tornadoes in decades. Severe flooding on the Mississippi River. What's up with the weather?

Some scientists say there are connections between some of the severe weather events of the past month and global warming.

"Basically, as we warm the world up, the atmosphere can hold more moisture in it," said Anne Jefferson, assistant professor in the geography and Earth science department at the University of North Carolina at Charlotte. "Weather patterns that used to be limited to the South move farther north now. Both of those things together will increase the frequency with which we see these big rainstorms, and those are likely to increase flooding in the future."

Flooding on the Mississippi has become more frequent and more extensive since about 1950, Jefferson said.

The study of long-term statistical trends shows an increase in heat waves and heavy rainfalls, Katharine Hayhoe, a climate scientist at Texas Tech University, said.

But for tornadoes, the jury is still out, because the historical record is too patchy. Climate change may enhance one precursor of tornadoes while diminishing another one, she said.

Kevin Trenberth, a senior scientist in the climate analysis section at the National Center for Atmospheric Research, said that the main driver of supercell thunderstorms - those with deep rotating updrafts - is warm moist air near the surface.

Tornadoes also depend on wind shear, a weather component that normally occurs at this time of year, he added.

Jeff Masters, a meteorologist and co-founder of the website Weather Underground, writing on his WunderBlog on Tuesday, said that this year's violent tornado season is "either a fluke, the start of a new trend, or an early warning symptom that the climate is growing unstable and is transitioning to a new, higher energy state with the potential to create unprecedented weather and climate events. All are reasonable explanations, but we don't have a long enough history of good tornado data to judge which is most likely to be correct."