A derecho storm system moved through the Midwest this week, starting in South Dakota and into northern Iowa. Dr. Justin Glisan is the state climatologist for Iowa. He says the storm systemâ€™s path began in Northwest South Dakota.
â€œSouth Dakota is an easy state because it’s a rectangle, so the top left corner of the state is where the line started, and then it transverses down to southeastern South Dakota through Sioux Falls, and then it entered Northwestern Iowa. That’s where we saw the comma head form. That’s where we see the derecho starting to pull in a rear inflow jet which makes it bow out. That’s where we see that backward z on the radar. We had some thunderstorms form in front of the main line, which looks like it took some energy out of the atmosphere, and the derecho started to somewhat fall apart. It did stay together across northern Iowa, but we had a branch down into central and southwestern Iowa that was not severe.â€
The stormâ€™s intensity amplified as it moved through South Dakota and into Iowa.
â€œWe had the initial wind and some hail reports in west-central South Dakota, and then we saw the line start to expand north and south. You get more wind reports as the derecho moves southeast, losing some power as it entered Iowa, but definitely, you see a long path. The definition of a derecho is a path length of 400 miles or more, 60 miles is the distance from, let’s say, north to south, and then, within that polygon that the derecho line makes, you have to have a substantial amount of 50 mile per hour wind gusts or above to classify it as a derecho. So, we definitely reached that peak.â€
The storm damaged structures and crops in both states.
â€œGrain bins that were not filled, the wind was punching those grain bins and denting them; not as severe as damage as we saw on August 10. But we’ve also seen flattened corn, some lodged corn, and then, we don’t think about this often, but these derechos also produced hail: tennis ball-sized hail in South Dakota but also that smaller hail, marble and pea-sized hail, that’s being blown by 80 or 90 miles per hour winds. That’s like machine-gun fire through those corn and soybean leaves, so we’ve seen some shredded crop out there. Looking across northern Iowa, we’ve had some lodged corn, some flattened corn. It’ll take a few days to figure out if this corn is going to stand back up, but thinking in my office as this derecho was forming, the damage could have been a lot worse.â€