The 2020 Midwest derecho was an unusually destructive and extremely powerful progressive derecho that devastated much of the Midwestern United States, primarily eastern Nebraska, Iowa, Illinois, Wisconsin, and Indiana. It caused high winds and spawned an outbreak of weak tornadoes. Some areas reported torrential rain and large hail. The event came with no prior warning as weather forecast did not show the possibility of a derecho occurring until it was already in progress.
Damage was moderate to severe across much of the affected area, as sustained wind speeds of 70 miles per hour (110 kilometers per hour; 31 meters per second) were prevalent. The greatest damage occurred in eastern Iowa, and northern Illinois, where multiple tornadoes touched down. The highest winds occurred in Iowa, measured at 126 mph (203 km/h; 56.3 m/s) and highest estimated from post-event damage surveys at 140 mph (225 km/h; 62.6 m/s).
Millions across the Midwest were affected by wide-scale utility disruptions, residential and commercial property damage, and severe damage to corn and soybean crops. Cedar Rapids, Iowa, was the most severely damaged, suffering a near-complete blackout that lasted for weeks in some areas, widespread and severe property damage, and an estimated loss of at least half of the city's tree canopy. The derecho caused over $11 billion (2020 USD) in damages and spawned a years-long cleanup effort.
Derechos of similar intensity to the August 2020 storm impact the Midwestern U.S. roughly once per decade, with similar derechos having occurred in 1998 and 2011. As with derechos in general, the continuous downwelling of high winds associated with the nearby jet stream and the expansion of dense rain-cooled air in the storm's wake induced the destructive winds and the storm's motion. However, the August 2020 derecho was unusual for the longevity of the damaging winds it produced; some areas were subjected to these winds for up to an hour compared to the 10–20 minutes of sustained damaging winds in a typical derecho, resulting in conditions similar to the passage of a hurricane's eyewall. Media outlets described the storm as an "inland hurricane". On average, the storm front traveled west-to-east at an average speed of 55 mph (88.5 km/h; 24.6 m/s). A combination of strong ambient winds and extreme convective instability facilitated the strength and unusual characteristics of the derecho. The latter factor resulted from the conducive overlapping of moist air drawn northward across the Mississippi Valley and warm and dry air aloft originating from the Southwestern U.S., resulting in an elevated mixed layer that allowed instability to escalate. These convectively unstable conditions permeated across Iowa and were characterized by steep lapse rates in the mid-levels of the troposphere as sampled via weather balloon launches from Omaha, Nebraska, on the morning of August 10, and from Davenport, Iowa, at noon that day; the atmospheric sounding retrieved from the Davenport balloon launch observed lapse rates of 8.5 °C/km. The warm front demarcating the northward push of the unstable air also focused the development of storms across the region.
Prior to August 10, the Storm Prediction Center (SPC) and the local offices of the National Weather Service (NWS) did not anticipate a storm of exceptional magnitude. Predictive weather models also yielded inaccurate projections of the storm and continued to do so even on the day of the event. On August 3, the SPC noted in a Convective Outlook that a series of shortwave troughs was forecast to move through northern portions of the U.S. in the coming days, becoming possible impetuses for thunderstorm development leading up to and on August 10. Three days before the event, the SPC assessed a Marginal Risk of severe weather for a swath of the central U.S. extending from northern Kansas to southern Michigan, including parts of southern Iowa, northern Missouri, and northern Illinois. The risk level was elevated to a Slight Risk at midnight on August 10 prior to the onset of the damaging storms. However, the slight risk area did not include any part of Iowa; it instead covered a narrow corridor from central Kansas northeastward into north-central Missouri. Parts of Iowa remained under either a Marginal risk or a general non-severe thunderstorm risk; northwestern Iowa was removed from all thunderstorm risk areas entirely. While the 0100 UTC outlook did include Marginal and Slight risk for this area overnight through the early morning hours of August 10, any storms that formed were expected to be mainly be elevated and produce large hail with the possibility of damaging winds being less certain. Temperatures and dew points were between 70–80 °F (21–27 °C) across the Midwestern U.S. around dawn on the morning of August 10. Winds blowing from the west were juxtaposed atop near-surface winds blowing from the southwest and south, resulting in strong wind shear over the region. The derecho began as a cluster of scattered thunderstorms that had formed during the previous night over south-central South Dakota. These storms tracked east along the South Dakota–Nebraska border and became better organized and coalesced, producing hail with diameters between 1–2 in (25–51 mm) and wind gusts between 60–70 mph (97–113 km/h) over a narrow swath. In response to these developments, the SPC issued a special Convective Outlook at 8:00 a.m., highlighting a now Enhanced Risk of severe weather for areas in the storms' path including Cedar Rapids, Iowa, the Quad Cities metropolitan area, and Peoria, Illinois. According to the SPC, there was a 30 percent likelihood of areas within the Enhanced Risk zone experiencing thunderstorm winds or wind gusts in excess of 50 kn (60 mph; 95 km/h). Meanwhile, essentially the rest of Iowa was put under a Slight Risk.
After 8:00 a.m., the storm moved through Sioux City, Iowa, crossing the Big Sioux River and entering western Iowa. Heating associated with the daytime hours made for warmer conditions near the ground, allowing strong winds produced by the storms to descend to and reach the surface. The storm's winds began to increase considerably after the storm reached west central Iowa at around 10: a.m. During this time, some of the derecho's winds spread outward from the storm beneath an atmospheric inversion aloft, spreading to the south and southeast and causing damage over 50 mi (80 km) away from the storm in areas with minimal rainfall. Over the next two hours, the storm traversed central Iowa with a rapid forward speed of up to 60 mph (95 km/h), impacting communities including Ames, Des Moines, and Marshalltown. Wind gusts approaching 120 mph (190 km/h) were produced by the storm during this period. Aloft, the storm was being supported by a rear-inflow jet sporting winds of 80–100 kn (90–115 mph; 150–185 km/h) with the downwind airmass exhibiting convective available potential energy (CAPE) values between 2000–2500 J/kg.
At 11:25 a.m., a severe thunderstorm watch tagged as denoting a particularly dangerous situation was issued by the SPC for areas ahead of the storm including central Iowa, southern Wisconsin, northern Illinois, and northwestern Indiana. Shortly after, the SPC introduced a Moderate Risk for severe weather in its categorical outlooks for similar areas. Around the same time, a counterclockwise vortex developed on the northern end of the storm, resulting in the storm attaining a bow-like structure with its strongest winds at the apex of this shape. University of Oklahoma meteorologist and tornado expert Stephen Corfidi remarked that the vortex was "one of the most distinctive ones of that size" he had ever seen. The core of stronger winds tracked east at speeds of 65–70 mph (110–130 km/h) and bore faster wind speeds. The derecho was at its strongest when it moved across the Cedar Rapids area of eastern Iowa. Based on the severity of damage observed, the NWS estimated that wind gusts of 130–140 mph (210–230 km/h) impacted parts of Benton and Linn counties in Iowa, including downtown Cedar Rapids and Marion. These winds diminished slightly as the derecho approached the Mississippi River, though gusts of 80–100 mph (130–160 km/h) remained widespread. The more extreme corridor of wind gusts transitioned into a broader swath of 60–70 mph (97–113 km/h) winds as the storm moved across northern Illinois and northwestern Indiana between 2–5 p.m. Despite the weakening of straight-line winds, the atmospheric environment became more conducive for tornadogenesis during this time. This resulted in several brief EF0 and EF1 tornadoes developing over northern Illinois and Indiana. The derecho's winds continued to lessen as the storm tracked farther east, weakening below damaging levels shortly after 7 p.m. as the storm was moving into Ohio and Michigan.