SIOUX FALLS, S.D. (KELO) — The derecho that pounded South Dakota on Tuesday, July 5, moved across a roughly 720-mile path about 90 miles tall, said KELOLAND meteorologist Scot Mundt.
The storm covered southeast Montana into northeast Iowa. The storms in northern Illinois were not connected to the derecho, Mundt said.
The storm moved through Iowa and was at its strongest in northern Iowa, said Craig Cogil, a meteorologist with the National Weather Service in Johnston, Iowa. Overall, the intensity, “was not to the extent it was in South Dakota.”
“The initial winds were due to the gust fronts ahead of the storms, 60-100 mph,” Mundt said.
“But we also had strong winds of 60 to 85 mph behind the system caused by a wake low. That’s a quick decrease in air pressure behind the line,” Mundt said. “This decrease in pressure sets up a wind field that can last longer that the initial gust fronts just ahead of the line.”
The storm earned a derecho designation because of long-lived wind and widespread impact. The NWS’s definition of derecho is a “widespread, long-lived wind storm that is associated with a band of rapidly moving showers or thunderstorms.”
Cogil said the storm must travel at least 400 miles and it must have wind gusts in excess of 75 mph.
This is the second derecho in South Dakota in three months. The state experienced a derecho on May 12.
Iowa had a devastating derecho in August of 2020.
“That was like nothing I’d ever seen. This never came close to being as severe as August (2020),” said Cogil.
NOAA estimated in October of 2020 that the August 2020 derecho caused an estimated damage of $7.5 billion.
Derechos are most common in late June through August, Cogil said.
Twenty two people died as a result of a derecho in mid-Atlantic states in 2012.
A 2020 ScienceNews story said derechos happened in the Midwest before the 2020 storm but the 2020 system was more intense than prior events. The story said it was difficult to determine if climate change would cause more derechos.
Meteorologists and other scientists are studying if more of the intense derechos will happen in the future and there are predictions that climate change will create more conditions ripe for intense derechos.
Cogil said the term derecho was invented by an Iowa scientist in the late 1800s. “But it wasn’t used (a lot) until 20 to 30 years ago,” he said.
Gustavus Hinrich was the “first scientist to identify the straight-line thunderstorms that can produce winds in excess of 100 mph, and gave it the name derecho”, according to a University of Iowa’s Iowa Now story by Tom Snee. Hinrich was on the university faculty at the time. He started the Iowa Weather Service in 1875, according to the university.
The weather warnings and special statements on the derecho started at least by around 1 p.m. in South Dakota on July 5 and continued at least until 11:45 p.m. when a flash flooding warning was issued for Albert Lea in Minnesota.
Into southern Minnesota, severe thunderstorm warnings covered Wilmont to Worthington, according to the NWS in Sioux Falls. Farther east in Minnesota, severe thunderstorm warnings were issued for Walnut Grove, St. James and other areas and flash flood warnings were issued at 11:45 p.m. for Albert Lea as the storm moved through the state, according to the NWS in the Twin Cities.
The coverage area for severe thunderstorm warnings in Iowa covered Larchwood to Sloan, according to the NWS in Sioux Falls. As the storm moved east across the state severe thunderstorm warnings were issued for Estherville, Buffalo Center, Fort Dodge and Iowa Falls.
NWS issued a special weather statement for wind and hail that included Des Moines and Ames to last until 8:45 p.m. A similar statement was issued later for cites farther east such as Ottumwa.