An anonymous reader shared this report from the Washington Post:
An outbreak of severe storms, including deadly tornadoes, hail bigger than DVDs and life-threatening flooding, has ravaged the South, coming amid a month of wild weather across North America. Texas is baking beneath heat indexes as high as 120 degrees, the coasts are cool and mostly calm and Canadian wildfire smoke is suffocating much of the northern U.S.

If it seems the weather has been a little bit “off” since the calendar flipped to June, you’re not imagining it — things have been downright weird. It’s all linked to a bizarre jet stream pattern, which is displacing air masses from their typical positions and disrupting the movement of weather systems across the continent.
Among other things, the jet stream created a sprawling heat dome in Canada which “helped sap the landscape of moisture, leaving it ripe to burn,” the article points out.

“Meanwhile in the southern U.S., the roaring southern branch of the jet stream has been energizing storms. That’s brewed back-to-back rounds of severe weather, complete with strong winds, tornadoes and ‘gargantuan’ hail — and the pattern doesn’t look to budge soon.”
[El Niño] historically, has been linked to split-flow jet stream patterns like the one driving wild weather across parts of the Lower 48. Natural variability, a.k.a. randomness, is also a big player, but it stands to reason that the two factors, overlapping together, are in large part culpable for what we’ve been facing.

Some scientific research also suggests human-caused climate change may increase the chances of slow, wonky jet stream patterns such as the one being observed this summer. The idea is that the disproportionate warming of the high latitudes is reducing the temperature contrast between the north and south, weakening the jet stream and thus causing it to take bigger dips and meander more. It remains a controversial idea.