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Unit 13: Derechos

What is a Derecho?

Since last August, many questions have come up about what a Derecho is and what qualifies a storm as one. A Derecho, by definition, is a widespread, long-lived wind storm associated with thunderstorms. For a line of severe thunderstorms to qualify as a Derecho, the damage must extend for more than 240 miles and include wind gusts of 58 m.p.h or greater during the length of the storm.

NOAA SciJinks Lesson on Derechos

According to climate data, most Derechos occur during the months of May to August in the United States. The National Weather Service reported that these types of events are not uncommon and the eastern half of the United States can see this type of event every 2-4 years. But with any type of weather event, the strength and damage associated form each Derecho can vary by storm.

For Derechos to form, they require large amounts of energy (also known as instability), wind shear, and a disturbance to initiate the thunderstorms. In reference to the August 10 Derecho, Iowa had plenty of energy on Monday due to the several hours of sunshine in the morning, coupled with the very high dew points. As a Derecho becomes fully organized and mature, the structure at the mid and upper levels can allow for higher wind speeds that are found at those layers to move to the surface. When looking for the area for the most intense wind gusts, look for the bow-shaped radar signature and find the furthest forward part of the storm, which we saw with the Derecho that impacted the state of Iowa.

August 10, 2020 Midwest Derecho

The August 10th Derecho that impacted the state of Iowa traveled 770 miles from South Dakota to Ohio in just 14 hours bringing a large area of damage in its path.

Maximum estimated wind speeds were up to 140 m.p.h. and the maximum recorded wind speed was 126 m.p.h. in Atkins, Iowa.

Here are the latest wind speed estimates from the National Weather Service Derecho Storm Survey. Top wind gusts estimated at 140 mph. -Joe

Posted by KCRG-TV9 First Alert Weather on Thursday, August 20, 2020

What made this storm different than Derechos in recent years, is the duration of the high wind speeds. Areas across eastern Iowa saw extremely high winds and wind gusts for upwards of 30-60 minutes, compared to the normal 10 to 20 minutes that can normally happen with Derechos.

One tornado was reported within the system as well. The National Weather Service found damage, on satellite imagery, north of the Eastern Iowa Airport that would indicate a path of a tornado weeks after the storm hit. It moved through cropland, and no direct damage was measured, so that tornado rated it as an EF-U.

In Iowa though, a storm of this intensity is a one in a decade event, according to the National Weather Service in the Quad Cities. There were two other Derechos in recent years in eastern Iowa, in 1998 and 2011.

There were over 500 wind reports associated with the derecho as it moved through multiple states. Maximum customers without power at one time were 1.4 million people across multiple states.

The USDA’s Risk Management Agency determined that the storm touched and impacted 57 counties across Iowa. It is estimated that 3.57 million acres of corn and 2.5 million acres of soybeans were severely damaged as of October 10, 2020. The U.S. Department of Agriculture said that the number of crop acres that Iowa farmers were unable to harvest grew to 850,000 from original estimates right after the August Derecho.

Damage Seen From Space

The impact after the Derecho could even be seen on satellites in space days after the storm. Imagery from NASA’s MODIS satellites showed scars on the landscape of Iowa’s cropland.

A comparison of images with relatively clear skies, on July 24 and August 11 below, shows the change in color of Iowa’s otherwise typical green land cover. These changes line up with where the storms tracked through the center of the state.

Satellite imagery shows the change in Iowa's land cover from the derecho on Monday, August 10,...
Satellite imagery shows the change in Iowa's land cover from the derecho on Monday, August 10, 2020.(KCRG / Imagery: NASA Worldview / MODIS satellies)

Another view of the storm’s impact was seen using the VIIRS nighttime imagery, which usually shows bright city lights along with clouds, particularly when the moon is bright. It showed a distinct darkening for cities along the Derecho’s path on an otherwise mostly clear night. The comparison below is on the same dates: July 24 and August 11.

Comparison of Day/Night Band imagery from the Suomi NPP/VIIRS satellite.
Comparison of Day/Night Band imagery from the Suomi NPP/VIIRS satellite.(KCRG / Imagery: NASA Worldview/Suomi NPP/VIIRS)

Timeline of Midwest Derecho

7:16 a.m. – Enhanced Risk was issued by Storm Prediction Center (SPC)

10:57 a.m. – Upgraded to Moderate Risk by SPC

11:25 a.m. – Potentially Dangerous Situation (PDS) Thunderstorm Watch was issued

11:48 a.m – Severe Thunderstorm Warning issued for Keokuk, Johnson, Benton, Linn, Iowa, & Washington Counties

12:15 a.m. – Spotter report of winds 70+ mph from Belle Plaine to Luzerne

12:30 p.m. – Storm hits Downtown Cedar Rapids with winds between 90-110 mph

12:34 p.m. – Second Severe Thunderstorm Warning issued

12:45 p.m. – Storm hits Downtown Iowa City with winds between 80-100 mph

3:32 p.m. – Severe Thunderstorm Watch canceled