Stalled jet stream has caused two weeks of tornadoes in the US

https://www.newscientist.com/article/2204892-stalled-jet-stream-has-caused-two-weeks-of-tornadoes-in-the-us/

May 29, 2019
By Chelsea Whyte

A stalled weather pattern over the middle of the US has resulted in tornadoes running through several states every day for nearly two weeks. Kansas, Oklahoma, Missouri, Ohio and northern Texas have been hit hardest, and millions of people in these states and neighbouring regions are under a state of emergency due to flooding and severe storms.

Much of the damage is in a region known as Tornado Alley, where tornadoes are most common in the US. But the frequency of twisters – at least 8 storms have been reported each day over 12 days – is so high even by Tornado Alley standards that it breaks a 40-year record. Tornado warnings have also been issued as far east as New York City, far to the east of Tornado Alley, which is unusual.

“A deep trough or southward dip in the jet stream has been in place over the western part of the country for a very long time,” says Jennifer Francis at Woods Hole Research Center in Massachusetts. The fact that it’s so persistent is due to another feature in the jet stream out in the Pacific – a ridge where the jet stream shoots way northward, has been in that configuration for most of the winter, she says.

She says that ridge is becoming more common and strengthening due to sea ice loss in the Arctic as the region warms.

These storms are part of months of unusual weather across the entire country, driven by the persistence of the jet stream pattern. “This is actually much broader than just the tornadoes. There are also the unusual rains in California, recent snow in Denver, and the heat wave in the southeast. These things are all connected,” she says.

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