May Was Filled With Extreme Weather. Blame the Jet Stream.

If it seems as if the weather has been particularly extreme lately, there's a reason for that.

A "very persistent" weather pattern lingered over parts of the U.S. during the past month, says Russ Schumacher, a Colorado State University climatologist and director of the Colorado Climate Center.

The result is that millions of people in the U.S. have been affected by extreme weather, including tornadoes and flooding, in recent weeks.

Meteorologists say the conditions are due to the jet stream, an air current that has been lingering over the central and southern plains.

Patrick Marsh, a meteorologist with the Storm Prediction Center, describes the jet stream as a river of fast-moving air at the top of the atmosphere.

The jet stream tends to represent the dividing line between colder air and hotter air, Schumacher says. That's why the West has been having colder and wetter conditions than the South, which has seen hotter and drier conditions.

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At least eight tornadoes have been reported across 22 states for 13 consecutive days, which is the longest streak on record in the country, according to CNN meteorologists.

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One big unanswered question for those who research tornadoes is their relationship with climate change. A reason for that is a lack of data on the short-lived disasters.

"Tornadoes are very rare in space and time, and therefore you need to have a really large database over many years to begin to attribute observed changes to various causes," Marsh says in an email.

However, research has suggested that climate change could cause a higher frequency of the type of storms that produce tornadoes, according to Allen.

The picture is clearer for flooding, which has a more long-standing data set. The past 12 months have been the wettest on record, according to the National Oceanic and Atmospheric Administration.

"It's a bit easier to see a fingerprint of climate change on heavier rainfall," Schumacher says.