The term ‘derecho’ was first used in 1878 by Gustavus Hinrichs. He used it to describe a storm that hit Iowa in 1877, but it wasn’t widely known by Iowans until 2020. But there have been multiple derechos in the state between 1877 and 2020, including one that hit Iowa 24 years ago today.
Don’t remember this derecho?
It is known to scientists as the Corn Belt derecho of 1998, but some people may remember it as something different. “The term ‘derecho’ in 1998 wasn’t really well known. Back then the term mesocyclone was thrown around, so some people probably remember it as the mesocyclone of ’98 versus the derecho,” said NWS Des Moines Lead Forecaster, Brad Small.
The storm’s track across Iowa
A cluster of storms formed in northern Nebraska before noon but expanded and gained strength as it moved into Iowa during the early afternoon on June 29th.
11:30 AM: Strong, but not yet severe, storms moved through Pocahontas, Kossuth, Humboldt, Webster, and Calhoun county
12:40 PM: Storms became severe in Carroll, Calhoun, and Webster county, and the first tornado warning was issued in Hamilton county.
1:30 PM: The line remained severe as it moved into Hardin, Greene, Boone, Story, Marshall, and Dallas counties with tornado warnings in both Boone and Dallas counties.
2:10-2:30 PM: The storm surged south into northern Polk county and lightning struck the radar which took it offline for the next 24 hours. Typically when this happens a neighboring NWS office will step in to help and assist with warnings and messaging.
3:00 PM: Several southcentral and southeastern Iowa counties are included in severe thunderstorm or tornado warnings.
3:30-4:00PM: The main line of storms exits Iowa and arrives in northeast Missouri and northwest Illinois. Another cluster of storms form in central Iowa producing tennis ball size hail.
Damaging wind, tornadoes, hail, and flooding were reported
Most wind gusts ranged from 70-90 mph, but there were a couple of areas where estimated wind speeds were as high as 120 mph. An unofficial instrument in Washington, IA measured a wind gust of 123 mph, which is the strongest wind gust ever recorded unofficially in Iowa.
In Crawford county in western Iowa, a tornado was on the ground for 11 miles. It caused damage to several grain bins and trees along with 30-50 residences.
One tornado cut a mile long path east of Marshalltown, but mainly damaged cornfields and trees. In Dallas county another tornado was reportedly on the ground for 2 miles in the open country.
Either one tornado or a family of tornadoes touched down in Boone county and moved with the line towards Pleasant Hill. Damage assessment showed that either one tornado touched down multiple times or several weak tornadoes touched down in these areas.
Tennis ball hail fell with another cluster of storms that formed near Des Moines
In Iowa, the strongest hit areas were from Granger and Johnston to northeast Des Moines. While most of the damage came from straight-line winds gusts, there were several tornadoes and hail as large as tennis balls reported in Des Moines.
Ray Wolf, the Science and Operations Officer at NWS Quad Cities said the derecho of 1998 finally gave meteorologists radar evidence that more than straight-line winds were being produced by these storm complexes. “That’s the storm where we kind of learned that these things produce tornadoes. We sort of knew that tornadoes did occur with derechos in the past, but now we had the Doppler radar so we had observations of it. So that was a big step with the one in 1998,” said Wolf.
Damage from the storms was reported from northeast Nebraska through northwest Kentucky, a swath of more than 600 miles. Storms formed in the early morning hours but started to become a derecho a few hours later as it moved into Iowa where the atmosphere allowed the storms to strengthen.
How the storm formed
In an extensive report by the NWS Storm Data team, meteorologists recount the events that led up to this derecho. “A complex weather situation was set up over the central U.S. as a mesoscale convective system (a small to moderate cluster of thunderstorms) passed to the south of Iowa during the early morning of the 29th,” the report said.
A warm front that was south of these storms surged north into northeast Nebraska by morning. The report added, “The air mass was very unstable to the south of the front with dew point temperatures will in the 70s.” This unstable airmass was located across almost all of Iowa.
Storms started forming along the warm front line in northeast Nebraska and northwest Iowa around 10:30 AM and continued to expand eastward until the line finally started to push south after noon.
The strongest part of the line exited Iowa by 5 PM but a few storms still lingered until about 7 PM.
Storms continued across the Midwest for several hours, tracking across parts of Illinois, Missouri, Indiana, and Kentucky.
In total there was $150 million in damage, 80 homes destroyed, 559 homes severely damaged, 125+ injuries, and zero deaths.