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2020 Derecho Event Summary

A derecho swept across the states of South Dakota, Nebraska, Iowa, Wisconsin, Illinois, Indiana, Michigan, and Ohio on Monday, August 10, 2020, leaving behind widespread and utterly devastating damage in its wake, hitting central and eastern Iowa the hardest. Here is a comprehensive recap of what happened. This post will be updated with more information.

Saturday August 8, 2020

The Day 3 outlook from the Storm Prediction Center (SPC) on Saturday for Monday hinted at a stormy pattern to move across the Midwest, including Iowa with a marginal (1 out of 5) chance for severe weather.

In the forecast text, the forecasters suggested a wind threat:

Sunday August 9, 2020

The Day 2 outlook from the SPC on Sunday for Monday still showed a threat in the Midwest but further south of the actual track. In Iowa, they still called for a marginal (1 out of 5) risk for severe weather. Here is the general outlook and the damaging wind outlook:

Monday August 10, 2020 (Day of the Derecho)

Midnight Update

Here is the Midnight Day 1 outlook from the SPC on Monday, still calling for a slight chance of severe weather in Iowa, but more so to the south:

8 AM Update

As of the 8 AM outlook, it became more evident that a severe threat was likely over eastern Iowa with an upgrade to an enhanced risk (3 out of 5 chance). The wind threat is included:

11:30 AM Update

An upgrade to a moderate (4 out of 5 risk) was issued by the SPC. This included a 45% hatched threat for damaging winds:

Scale:
Probability of damaging thunderstorm winds or wind gusts of 50 knots (58 mph) or higher within 25 miles of a point.
Hatched Area: 10% of greater probability of wind gusts 65 knots (75 mph) or greater within 25 miles of a point

Here's how those outlooks verified in order of Day 1 midnight, day 1 8 AM, day 1 11:30 AM:

Mesoscale Discussions, Watches

Mesoscale discussions (MD) are issued by the SPC and are technical discussions on whether or not watches will be issued.

The first one was issued in South Dakota, Nebraska, Minnesota, and Iowa at 3:19 AM giving a 40% chance for a watch.

https://www.spc.noaa.gov/products/md/md1445.html

At 6:05 AM, SPC issued the first Severe Thunderstorm Watch in Iowa for the day.

Once the watch was issued, a new MD was issued at 7:46 AM discussing an extension of the watch farther south and east.

https://www.spc.noaa.gov/products/md/md1447.html

At 8:40 AM, it became likely that a new watch would be issued for western and central Iowa.

https://www.spc.noaa.gov/products/md/md1448.html

At 8:55 AM, the line of storms continues to move southeast and a new Severe Thunderstorm watch is issued for most of central Iowa. As far east as Bremer to Black Hawk to Tama Counties.

At 9:45 AM, the SPC became worried about the storm moving through the Des Moines Metro area.

https://www.spc.noaa.gov/products/md/md1449.html

The 10:42 AM MD contained strong language of a significant wind event in anticipation of the next Severe Thunderstorm Watch.

https://www.spc.noaa.gov/products/md/md1450.html

At 11:04 AM the Des Moines International Airport's automated station recorded a wind gust of 74.8 mph.

At 11:10 AM the first Severe Thunderstorm Warning for the KWWL viewing area came out from the National Weather Service in Des Moines. It was for portions of Hamilton and Hardin counties, and extreme western Grundy County for 60 mph wind and penny size hail (both were confirmed in Grundy Center).

At 11:19 AM a large Severe Thunderstorm Warning was issued for several counties, including Marshall, Tama and Poweshiek, for wind gusts up to 80 mph.

At 11:25 AM, another Severe Thunderstorm Watch was issued for the rest of eastern Iowa and areas east including northern Illinois and southern Wisconsin. This watch was a PDS (Particularly Dangerous Situation) Watch.

Here is the exact wording in the severe thunderstorm watch.

At 11:47 AM, a new Severe Thunderstorm Warning was issued for the northern portion of the derecho. This was for Hardin, Grundy and Black Hawk County. This same warning was extended to include the Waterloo metro at 12:13 p.m. The highest wind gust report in Waterloo was 40 mph.

One minute later at 11:48 AM, the National Weather Service (NWS) in the Quad Cities issued a Severe Thunderstorm Warning well in advance of the storms moving through.

At 11:52 AM, another Severe Thunderstorm Warning was issued. The storm was moving east at 60 mph with 90 mph wind gusts possible.

At 12:04, that tag was raised to 100+ mph possible.

Our team deployed our new storm chasing vehicle, Storm Chaser 7, to Tama County to get the aftermath of the storm. The storm chaser captured estimated 60 mph wind gusts near Traer. Here are a couple views from our coverage.

From there, we headed to Toledo and Tama to check out the damage. Here are some photos of the tree and power line damage.

At 12:09 PM, the update to the warning mentioned 90 mph winds.

The chart below shows when the warning was issued and how long before the report of severe weather occurred. The green shaded area shows the lead time (time from the when the warning is issued to when the storm occurred in that location). A lead time of 37-47 minutes is very good. The nation average is 10-15 minutes for a Tornado Warning.

The 12:14 PM MD concerning the watch continued to show forecasters' worry over the potential for a significant severe event.

At 12:34 PM, a Severe Thunderstorm Warning was issued and it extended to the Mississippi River.

At 1:36 PM, the last MD was issued for our area.

By 2:30 PM, the severe weather was now crossing the Mississippi River and moving east. The damage in eastern Iowa was done.


The National Weather Service (NWS) in the Quad Cities put together an informative timeline of events that shows the progression of events:

NWS Quad Cities

Storm progression and radar

Here is a hour-by-hour stitch of the composite reflectivity of the storm from its initiation in South Dakota through Indiana, courtesy of NWS Chicago.

Notice how the storm bows out like a bow and arrow. That is indicative of strong winds pushing in behind the line. You can read more about bow echoes here.

Here is a look of the infrared satellite of the storms as they began in South Dakota.

Here is an animation of the radar and with storm reports at the same time. As the storm progresses, watch all of the storm reports come. The storm intensifies in central and eastern Iowa.

All in all, the scope of the damage was massive. Covering 770 miles from South Dakota to the western side of Ohio. In Iowa, the width of the damage was roughly 3 counties or about 50 to 100 miles wide.

This storm qualified as a derecho which is defined as a damaging wind storm that is on the ground for 250 miles or more. Along that path you would find wind numerous reports of 58 mph or greater resulting in widespread damage. More information on derechos can be found here.

Wind strength

The derecho not only produced numerous wind gusts of 58 mph, but frequent gusts of 80+ mph, occasionally topping 100 mph. The highest reported wind gust was 130 mph 2 miles northeast of Clinton in Clinton County. Marshall, and Tama

The National Weather Service surveyed several areas and estimated that wind speeds could have been as high as 140 mph with some of the damage inspected. Click here for more information on this storm report.

The map shows the area with the estimated wind speeds could have exceeded 100 mph. The map is preliminary and can adjust as more information becomes available. The strongest winds, in eastern Iowa, came across the following counties (from West to East): Marshall..Tama...Benton...Linn...Jones...Cedar.

The storm took 14 hours to travel 770 miles. This equates to an average speed of 55 mph.

NWS Des Moines

Damage

Satellite images show the aftermath of winds. In the before and after below, look at the light-colored swaths, which are damage scars caused by downed trees and damaged crop. You'll see the swaths from central to eastern Iowa.

NWS Des Moines

This time lapse shows how the power grid and infrastructure were no match for the extreme winds. A swath of power outages left nearly 2 million people in the dark at one point. The power remained out in eastern Iowa days after the storm moved through.

There were more than 480,000 power outages reported in Iowa after the derecho. Below is a view of infrared satellite from NASA and NOAA. You can see August 10th in the top right, before the derecho, and the progression since. Read more here.

Here are the observations that were taken just before and after the storm moved through Cedar Rapids. The observations are taken at the Eastern Iowa Airport in Cedar Rapids (CID). The sensor went offline as the derecho moved through, so the 68 mph wind gust was likely peaked at 80-100 mph. The National Weather Service was not able to retrieve the peak wind gust from CID.

We will continue to update this page as more information becomes available.

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