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Looking Back to the August 2020 Midwest Derecho

It has only been a few weeks since the notorious August 2020 Midwest Derecho that struck parts of the midwest and led to millions of power outages and widespread damage in the midwest. Before this event took place, forecasters had predicted that this was not going to be the case. However, on a hot and steamy Monday, August 10th, they would be proven wrong.

What is a derecho?

Before getting into the event, we will be looking into what a derecho is. A derecho is known as a fast-moving, straight line, long-lived wind storm that can travel up to 240 miles or more. They are also known to have speeds of 58 miles per hour or greater for most of its duration. Showers and storms are also known for occupying derechos. Derechos can happen at least once every one to two years in the midwest. They are most commonly known to form during the summer months.

A derecho rumbling through in the Great Plains on June 6, 2020. (Photo: NWS)


Now that we have taken a look into what a derecho is, we will analyze the event that took place last month.

The Origin of the August 2020 Midwest Derecho

Early in the day on August 10th, at around 2am EST (6Z), the National Weather Service issued the first Day 1 outlook for the day. The outlook, at the time, demonstrated that the majority of the midwest was under risk for severe weather. The highest threat was focused on parts of Kansas, Missouri, Illinois, and the southern edge of Iowa. This was nowhere near where most of the action would take place for the day. There were no indications that a derecho was likely to form at the time as well.

This map shows the 2am outlook (600 UTC time) of the risk for severe weather. (Source: NWS)


However, later on in the morning, there were some indications that it would change due to some strengthening storms that initiated in southern South Dakota and Nebraska. As the storms entered Iowa, the storms grew more intense and became severe.

This map shows the elements that led to sparking of the August 2020 Midwest Derecho. (Source: NWS)

The models at the time were beginning to note that storms were entering unstable air. Not only that, there were models indicating a short wave and associated 50-70 knot speed maximum moving forward through the area. This would lead to the eventual spark of the event that would be known as the imfamous August 2020 Midwest Derecho. Later in the morning, the outlooks for the day of August 10 were upgraded and by 12:30pm EST (1630Z), the outlook had put much of the midwest under a heightened risk of severe weather. The worst of the severe weather was centered in eastern Iowa, to northern Illinois, to southern Wisconsin, into the northwest corner of Indiana.

This map indicates the 12:30pm EST (1630Z) issuing of the severe weather risk for the afternoon of August 10th. (Source: NWS)

The Derecho’s Fury

Throughout the rest of the morning on August 10, the developing derecho swept through Iowa with winds up at 140 miles per hour in some locations. There was a lot of crop damage, downed trees, and loss of power throughout Iowa. In fact, Iowa was the hardest hit by the derecho when it was at its peak plowing its way through the state. After the derecho, Iowa would remain without power for days to come as the result of many downed power lines due to falling trees.

The damage left behind at the University of Iowa after the derecho swept through. (Photo: Tim Schoon)

Afterwards, the derecho continued its way through Illinois and produced some tornadoes in the northern parts of the state. A few of them were near Chicago as well. The derecho produced up to 100 miles per winds near Chicago. After battering Illinois, the derecho continued into Indiana. By then, the derecho was starting to weaken a little bit but it still produced straight line winds and some sporadic outages in the state. By the time the derecho reached Ohio and dropped into portions of Kentucky, it had fallen apart.

Lessons to be learned from the derecho

There are some lessons to be learned from the derecho event that took place on August 10th. Although the event was unprecedented and was not expected to happen, the National Weather Service is working on tools that would help forecasters make a prediction of close range of possibilities that may indicate the possibility of a derecho being possible with the presence of storms. The new predictions could help better prepare for when another event like that of August 10, 2020 comes along.

-Blog by Forecaster Eric Segbor

Sources
https://www.weather.gov/dmx/2020derecho
https://www.weather.gov/dvn/summary_081020
https://www.weather.gov/lot/2020aug10
https://www.weather.gov/unr/2020-06-06
https://now.uiowa.edu/2020/08/ui-campus-derecho-storm
https://www.weather.gov/lmk/derecho
https://www.mprnews.org/story/2020/08/15/forecasters-look-to-learn-from-deadly-midwest-derecho
https://www.spc.noaa.gov/cgi-bin-spc/getacrange.pl?date0=20200810&date1=20200810

https://www.weather.gov/lot/2020aug10


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