States in the Midwest, particularly Kansas, Oklahoma and Missouri, saw extensive severe weather in May including widespread flooding and multiple tornadoes.(KYLE RIVAS/GETTY IMAGES)

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.

Extreme weather can form when these cold and warm fronts meet in the jet stream.

"You kind of end up with all those hazards over the same spot," Schumacher says.


Kansas, Oklahoma and Missouri have been particularly damaged by the recent extreme weather.

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.

Marsh tweeted that more than 500 people reported witnessing a tornado between April 27 and May 27. Only four other time periods exceeded that number of eyewitnesses, all of which are in the past 16 years.

Still, over 500 reportings doesn't mean over 500 tornadoes, and the record sightings come with caveats, according to experts. These days, the U.S. has more people living in tornado-prone areas, and it is easier to report sightings with access to smartphones and the internet.

Meteorologist likely won't know the true number of tornadoes over the past month for about a year, according to John Allen, an assistant professor of meteorology at Central Michigan University.

Tornadoes also don't just pop up on their own. They are produced by storms known as "supercells."

"They're the rare beast of thunderstorms," Allen says.

These storms last a long time – sometimes half a day – and can travel hundreds of miles, Allen says. Supercells are rotating, which prevents them from collapsing onto themselves and helps them produce tornadoes.

That's one of the reasons why many of the areas experiencing tornadoes are also struggling with flooding, according to Allen.

The good news is that the affected areas should expect some relief starting this week as the weather pattern weakens, according to meteorologists.

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.

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