Ag Weather Forum

Irresistible Force and Then Some

Bryce Anderson
By  Bryce Anderson , DTN Senior Ag Meteorologist
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This mammoth grain bin in Slater, Iowa, was no match for tornado- or hurricane-strength winds in the Midwest derecho Monday, Aug. 10, 2020. (DTN photo by Matthew Wilde)

In these days following the Monday, Aug. 10, 2020, Midwest derecho, comprehension of the damage from this berserk windstorm is still a work in progress. Crops. Power grid. Grain storage. Outbuildings. Schools. Homes. Trees. These are more than identifier nouns. Collectively, they form the tapestry of Mid-America agriculture and society. The wild winds of August tore through the heart of America and tore many hearts in the heart of America as well.

Scientists and engineers have developed formulas for putting a mathematical value on the power unleashed in this event. After all, nature can be explained by three sciences: chemistry, biology and physics. But, for myself and maybe you also, a usable benchmark for putting some clarity on the power that was witnessed either firsthand or from afar staring at photos of destruction or at radar composite images would be a comparison with storms that have already been defined. A tornado is useful -- except, in this case, a tornado with a 770-mile-long and several-hundred-miles-wide path.

So, let's look at the EF-1 tornado. That level of storm has wind speeds of from 86 to 110 miles per hour. At this level, damage to mobile homes and other temporary structures becomes significant, and cars and other vehicles can be pushed off the road or flipped. Permanent structures can suffer major damage to their roofs.

But there's more. The upper end of that wind speed bracket matches the wind speed of a Category 2 hurricane. And, to follow on, a Category 2 hurricane produces the following damage features, according to the National Hurricane Center:

"Extremely dangerous winds will cause extensive damage: Well-constructed frame homes could sustain major roof and siding damage. Many shallowly rooted trees will be snapped or uprooted and block numerous roads. Near-total power loss is expected with outages that could last from several days to weeks."

Both those definitions offer details that match the impact of the Monday, Aug. 10, 2020, derecho. So, no matter which comparison one selects --tornado or hurricane -- the blockbuster August 2020 derecho etched a new entry in the extreme storm vocabulary to join the term "bomb cyclone" from March 2019. And as with the losses in that event, the impact of this week's derecho will be with us through at least the rest of this farming year.

Bryce Anderson

Twitter: @BAndersonDTN

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Bryce Anderson
8/17/2020 | 5:53 AM CDT
Some additional details from Justin Glisan, Iowa department of agriculture climatologist: Climatologically, Iowa expects one derecho every two years. In the past 10 years there were two derechos, but neither one was as widespread and powerful as the August 10 2020 event. Those storms happened July 11 2011 and June 24 2013.