Derecho Watch for Upper Midwest

July 28, 2021

Flattened corn in Iowa following a strong Derecho storm

Make popcorn and get in the basement.

Bob Henson in Yale Climate Connections:

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.

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.

3 Responses to “Derecho Watch for Upper Midwest”

  1. dumboldguy Says:

    Good luck in MI tonight and tomorrow, Peter. We had a derecho roll through NO VA a few years back—-winds up to 85 at the local airport—-took off some roofs and siding, knocked over a few chimneys, knocked over two 40 foot trees in my backyard and broke another one off half way up as we watched. My wife and I did NOT make any popcorn as we huddled in the center of our basementless house. One of the scariest things I have ever experienced.

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