As the historic storms on July 31 showed, windstorms are no stranger to the Midwest. However, they typically do not pack as much power as that one did. While the July 31st storm in Omaha was relatively localized to the Omaha/Lincoln metro areas, the damage it caused may have brought back some memories to what occurred in Iowa four years ago.
On August 10, 2020, a wicked windstorm swept from South Dakota into Illinois, hitting Iowa the hardest. Winds between 100-140 mph from Denison to Des Moines to Davenport caused substantial damage. In particular, the region around Cedar Rapids was hit hardest. In this installment of This Week in Weather History, we look at the conditions that caused this historic storm and its impact on the Midwest.
THE SET-UP
Overall, the 2020 severe weather season was relatively quiet, thankfully for the world as we underwent a global pandemic. On the morning of August 10, nothing seemed out of the ordinary. Temperatures across Nebraska and Iowa were in the mid-70s with some humidity. Although a few storms were expected, the severity of the storms was not forecast well by the morning of August 10. The Storm Prediction Center had Omaha not even in a risk for severe weather, and areas south of I-80 at a level 1/5 risk at their update at 1 am.
Overnight from August 9-10, thunderstorms formed over southern South Dakota. These storms were originally hail-producers and were not quite severe. This was because these storms formed behind the cold front draped over eastern Nebraska into northern Iowa. Forecasters believed this cluster of storms would die as they headed southeast into Nebraska. By sunrise, the storms failed to collapse, producing damaging winds as they headed into the Cornhusker State.
As the storms failed to die, the forecast started to change. A Severe Thunderstorm Watch was issued around 6 am for the Siouxland area. In the 8 am update, the Storm Prediction Center upgraded western Iowa from a 0/5 to a level 2/5, with a level 3/5 introduced for eastern Iowa.
HIGH WINDS IN OMAHA
As the storms approached Sioux City, it was believed that those storms would remain north of the Omaha metro. In effect, they did, Omaha did not see a drop of rain from those storms. However, the city was still rocked with 60-70 mph winds that knocked power out to 55k customers that morning. How?
The answer comes from an outflow boundary. Thunderstorms are large machines carrying currents of warm and cold air, it sucks up warm air and spits out cold air. This area of rain-cooled air rushes to the ground from the rain. As wind cannot go into the ground, it spreads outward ahead of the storm, known as an outflow boundary. Outflow boundaries are typically denoted by a thin green line on the radar.
Sometimes, outflow boundaries can produce a breeze, but rarely produce damaging winds. This outflow boundary formed in unique circumstances as the area of rain-cooled air was trapped below a layer of warmer air right above the surface, known as an inversion. Inversions are common in the morning as the air near the ground cools faster than the air above it. In this circumstance, the warmer air above helped to trap the cooler air below it, making the wind go faster. To explain it further, blow air into your hand, then blow with that same force through a straw. The air from the straw is much faster because you are trying to fit the same amount of air through a smaller hole. This happened on August 10, 2020, in the Omaha metro around 9 am.
Eppley Airfield recorded a wind gust of 67 mph, and other reports near 70 were reported in Bennington and Papillion. Across the Omaha metro, many trees were damaged, some coming down on power lines. Overall, 55k customers in OPPD's area were knocked out of power, and it took several days to restore power for everyone.
While this would've been the main story in Omaha, the actual line of storms near Sioux City was nowhere near done, it hadn't even begun. What would transpire in Iowa from the mid-morning into the afternoon would be some of the most violent weather experienced in the Hawkeye state in recent memory.
THE IOWA DERECHO
It began near Denison and Carroll after 10 am when the storms finally caught up with the cold front. It now entered an untapped airmass ripe for severe weather, and in turn, the storms exploded in intensity. Winds as high as 80 mph ripped through Crawford and Carroll Counties, causing significant tree and crop damage in both counties.
The storms moved into the Des Moines area closer to 11 am, where 70-80 mph winds caused damage within the metro. On the north side of the metro, winds reached upward of 100+ mph between Des Moines and Ames. Entire cornfields were flattened, and buildings sustained structural damage. Another area of 100+ mph winds developed east of Ames and raced down US-30 into the Marshalltown area, and eventually Cedar Rapids.
As the storms plowed through Des Moines, the 11 am update from the Storm Prediction Center upgraded central and eastern Iowa to a level 4/5 severe weather risk.
The worst damage occurred in the Cedar Rapids area, where winds between 120-140 mph swept through, and the city experienced winds greater than 60 mph for almost an hour. Not a tree or building was left untouched in the city. Cedar Rapids lost half their trees from the storm, and many more were damaged. In some parts of town, entire buildings were destroyed. A storm chaser for local CBS affiliate KGAN-TV had a back window blown out due to the high winds, and later the KGAN studios had to evacuate on live TV as the entire studio shook due to the intense winds. Nearly the whole city lost power, and some took two weeks to see their power restored.
As the winds continued east, they continued to cause major damage over eastern Iowa as widespread 80-100 mph winds enveloped the area. The storms gradually lost intensity as they crossed into Illinois, but still produced isolated winds upward of 80-100 mph. The line reached Chicago around 3:30 pm producing 60-80 mph winds and a couple of tornadoes. As it crossed into Indiana and Lake Michigan, the complex finally began to weaken. The storms dissipated in western Ohio.
THE AFTERMATH
This storm was classified as a "derecho", a technical definition for a long-lived windstorm. The criteria for a derecho is a consistent swath of damaging winds over 400 miles long and 60 miles wide. This derecho easily met that. The swath of winds occurred over 90,000 sq mi and affected 20 million people. At least 2,000 square miles had winds greater than 100 mph. The storms also produced 26 tornadoes in Iowa, Illinois, and Indiana, but these were mostly weak and indistinguishable from the wind damage.
In terms of cost, this was the costliest thunderstorm event in United States history (not counting tornadoes), totaling 11 billion dollars. It was the second costliest natural disaster in the United States, behind Hurricane Laura.
Over 1 million people were without power by the storms. Agricultural damage was extensive, particularly to the corn and soybean crops. Crop damage was so widespread, it was visible in satellite imagery taken in the days following the storm. According to the USDA, 14 million acres of corn were within the path of the derecho in Iowa in 2020, and 3.57 million acres were destroyed. For soybeans, the numbers were similar. The corn harvest for 2020 in Iowa was effectively halved.
Iowa declared a federal disaster area in the days following the storm. The response following the storm from the national news media and politicians, some found it lackluster and criticized those institutions. Regardless, Iowa would slowly recover, and some aspects of the recovery process are ongoing today.
