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Derechos & Smoke Haze: An Explainer On Wisconsin's Recent Weather

Incoming Derecho approaching across corn fields.
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A derecho is a name for a particularly intense squall line storm system that has severe winds in excess 75 mph over a long swath of land that's generally around 150 miles or more.

One week ago, Wisconsin was hit with massive storms. They developed over the northern part of the state, formed a squall line and tracked down through southern Wisconsin.

The storms had many counties under severe thunderstorm warnings and tornado warnings, and straight-line winds caused a lot of damage to areas like Fond du Lac, Jefferson and Waukesha counties. Twelve short-lived tornadoes were also confirmed.

This was the first big summer storm we've had in Wisconsin, but it came much later in the season than we typically see, according to Clark Evans. He's a professor of atmospheric sciences at the University of Wisconsin-Milwaukee.

Evans says last week, southern Wisconsin, and really the whole state, saw some of the latest tornado warnings and tornadoes on record. "And that’s really emblematic of the large-scale weather pattern that we’ve had throughout much of this year," he notes.

The particularly intense squall line, like the one we experienced last week, is also known as a derecho, Evans says. "[A derecho] has to have severe winds — those in excess of 58 mph and, in particular, very severe winds — those in excess of hurricane force, 75 mph over a long swath of land," he explains.

"This particular derecho was in an environment that was conducive to having short-lived tornadoes in association with it," Evans notes.

Wisconsin usually experiences one to two derechos per year and Evans says we're on the northern fringes of where they most commonly occur.

But does such a late, yet intense start to the summer storm season indicate any impact from climate change? Evans says we just don't know enough right now.

"Unfortunately thunderstorms and their organization are one of the areas in which we don't really know a whole lot about how climate change is impacting them or may impact them," he says.

Evans says more research is needed to better understand trends in occurrence and severity of these storms and their relationship to climate change.

However, the one thing that is clear, he says, is that these storms will be found a bit further north over time as the overall jet stream pushes further northward as a result of climate change. "So that might put more of Wisconsin into the greatest corridor of these events," Evans explains

The impact of wildfires in the West can also be seen in Wisconsin, as a haze has settled. The greatest effects of this, he says, are reduced visibility and occasional air quality effects when more of the smoke gets closer to the surface.

But just like severe weather storms, Evans says we don't quite know the long-term impact haze will have. "It's really a growing area of research within the weather and climate communities as to what effects that those particulates up in the air may actually have on the weather," he notes.

It'll most likely take till autumn before the pattern changes or it gets cooler out in the West before we see a significant reduction in fires, smoke and haze, Evans says.

Audrey Nowakowski is a WUWM host and producer for Lake Effect.
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