One or more clusters of severe thunderstorms are expected to plow from eastern Minnesota to southern Michigan late Wednesday, July 28. Tornadoes and large hail are possible, but the biggest concern is for a corridor of intense straight-line winds – perhaps with gusts above 75 mph – that could cause widespread havoc.

If conditions come together, Wednesday’s event could be powerful enough to qualify as a derecho, a type of thunderstorm-based windstorm that can persist for hours and cause serious destruction. The nation’s last major derecho, which ripped across 770 miles in 14 hours with winds topping 100 mph on August 10, 2020, ruined half of the entire tree canopy of Cedar Rapids, Iowa, and left $11.5 billion in damage.

The corridor at greater risk for damaging wind includes the heart of Wisconsin, extending southeast to Lake Michigan by late evening. “Really concerned from Appleton and south and west at this point. Bad timing coming in late evening,” tweeted Luke Sampe, a broadcast meteorologist at WFRV in Green Bay.

It’s much easier to identify the situations that look ripe for a derecho than to predict their outcome with certainty. Even just hours in advance, some derecho-favorable setups fail to produce.

However, Wednesday’s event appears to have the key ingredients needed for widespread damaging wind. The factors in play include very hot, moist air just south of a surface frontal system and overtopped by a strong jet stream. This setup allows for powerful upper-level winds to descend through a thunderstorm complex and propel it forward at close to highway speeds.

Before Wednesday’s storms congeal into the expected wind-producing complex, tornadic supercell thunderstorms may erupt. Underlying this threat is a dangerous blend of high instability (1500 to 2500 J/kg); strong vertical wind shear (45 to 55 knots); and strong helicity, which helps thunderstorms to rotate. High surface humidity means that any thunderstorms will tend to have a low cloud base, which would also support tornadoes. The highest chance of early-evening supercells will be in east central Minnesota (north of Minneapolis) into northwest Wisconsin.

The forecast for Wednesday

Mesoscale (regional) computer models were increasingly pointing toward the risk of a derecho at midday Wednesday, July 28. The 12Z run of the high-resolution 3-kilometer NAM model showed pockets of hurricane-force wind at 850 millibars (about a mile above the surface) in association with a storm complex tearing from northwest to southeast across Wisconsin between about 7 p.m. and midnight CDT.

Severe weather may still be possible as storms continue into northeast Illinois, northern Indiana, and southern Michigan well after midnight.

Among the events that could be affected by Wednesday evening’s severe weather are the Experimental Aircraft Association’s massive AirVenture Oshkosh Show. More than 7,000 aircraft are on hand, and more than 600,000 attendees are expected for the week-long event, with as many as 40,000 camping at or near the venue.

Gilbert Sebenste, an Illinois-based consulting meteorologist with AllisonHouse, expressed concern about the potential for severe weather hitting the air show. In a Facebook post, Sebenste stressed: “If you are there, you need to absolutely, positively pay attention to the weather, what the announcers are saying, and make sure that you have text messages and a weather radio to stay informed.”

Looking back at the 2020 Midwest derecho

The Midwest derecho of 2020, which hit eastern Iowa the hardest, was likely the costliest single thunderstorm complex in U.S. history – including tornadoes – based on data from NOAA’s billion-dollar disaster database. In fact, the derecho was more destructive than any of the nation’s record 10 landfalling hurricanes and tropical storms of 2020 except for Hurricane Laura ($14 billion in damage).

The highest observed wind gust of that 2020 derecho was 126 mph at Atkins, Iowa, but post-storm analysis based on damage to an apartment complex in Cedar Rapids found that the highest estimated gust was 140 mph. That’s roughly similar to the peak gusts one might experience in a Category 3 hurricane or an EF3 tornado.

Fierce winds persisted in parts of east central Iowa for the better part of an hour, substantially longer than a typical derecho, thus exacerbating the damage. See the graphic above, plus the embedded YouTube video below, in which burst after burst of damaging wind takes its toll on a once-leafy block in Cedar Rapids. Some of the most powerful segments are at around 7:00, 14:00, and 27:30 minutes.

Along with destroying tens of thousands of trees, the derecho wiped out more than a million acres of crops, including about 20% of the acreage in Iowa.

“You’d be hard pressed to draw a more efficient path across the Corn Belt to create a storm that has such a significant impact on agriculture at a sensitive time of the year,” said Iowa state climatologist Justin Glisan.

Thus far there is no conclusive research showing trends in derechos that might be linked to climate change. In theory, as a warming atmosphere tends to shift the jet stream poleward, the locations and times of year most favorable for derechoes could also shift northward, as noted by SPC in its FAQ page on derechoes.

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