The August 10, 2020 Midwest Derecho

Introduction

By The Numbers

The Meteorology

Upper Air Dynamics

Surface & Thermodynamics

A Challenge To The Forecast

Timeline

1:45 AM CDT August 8 - 12:30 AM CDT August 10, 2020: With a cold front moving into the Northern Plains, a shortwave trough approaching the Dakotas, and an upper level jet expected to be in close proximity, SPC forecasters anticipate at least some potential for severe thunderstorms over portions of the Mississippi Valley and western Great Lakes by the afternoon hours on August 10. Where exactly the best severe potential will materialize remains uncertain, and will depend heavily on the position of the advancing front and any prior thunderstorm activity across the region. Confidence in early day showers and thunderstorms becomes high by midnight on August 10, but their precise impact on the later-day forecast is unclear.

1:00 AM CDT - 5:00 AM CDT August 10, 2020: Scattered showers and thunderstorms develop, progressing east along the Nebraska and South Dakota border. The storms are elevated in nature, developing along a front in the 850-700 millibar (~5,000-10,000 feet above ground level) layer of the atmosphere. While passing largely north of the initiation zone, a glancing shortwave trough likely plays a role in storm formation as well. Whether or not these thunderstorms can organize moving deeper into the morning hours remains uncertain.

5:05 AM CDT - 9:00 AM CDT August 10, 2020: Organizing in the presence of favorable wind shear, initial elevated storm development consolidates into a bow echo along the Missouri River. Heightened severe weather potential -- namely in the form damaging winds and isolated tornadoes -- exists over portions of the Mississippi Valley and western Great Lakes through the morning and afternoon hours. SPC trends its forecast up accordingly.

9:05 AM CDT - 11:25 AM CDT August 10, 2020: A now fully-developed bow echo surges into central Iowa from the Missouri Valley. Numerous damaging wind reports are relayed to WFOs. Multiple significant (greater than or equal to 74 MPH) damaging wind reports occur shortly thereafter, particularly as storms approach the Des Moines metro. With sufficient instability and wind shear in place well to the east of the ongoing bow echo, it's apparent that a potential derecho may be unfolding. SPC continues amplifying its forecast messaging.

11:25 AM CDT - 2:00 PM CDT August 10, 2020: The most intense damaging wind gusts of the event occur as the derecho pushes east toward the Mississippi River. Many of these reach or exceed 90 MPH in magnitude, with additional triple digit gusts also occurring. The intense gusts overlap with decreasing CIN regionally, suggesting that winds are beginning to mix to the surface unimpeded. The Cedar Rapids and Quad Cities metro areas are directly in line with these winds.

2:05 PM CDT - 4:30 PM CDT August 10, 2020: A period of prolific tornado production occurs along the leading edge of the bow echo as it moves along the Illinois-Wisconsin state line and through the Chicago metro. Seventeen tornadoes in total develop over a two hour period. The tornadoes occur in an environment featuring an overlap of surface vorticity & increasing CAPE in the 0-3 km AGL layer. The organized bow echo intersects with this overlap, allowing for the vertical tilting of ambient vorticity into line-embedded circulations. Some of these circulations ultimately become tornadic. Numerous hurricane-force wind gusts continue, though the majority aren't as strong as those previously occurring in east-central Iowa and northwestern Illinois.

4:20 PM CDT - 10:00 PM CDT August 10, 2020: The bow echo continues to push into north-central Indiana, southern Illinois, far southwestern Lower Michigan, and western Ohio. Many more damaging wind reports occur, though most are below hurricane-force strength. A pair of tornadoes develop along the leading edge of the line of storms as well. Outrunning the surface warm sector & the favorable instability within it, the bow echo weakens below severe limits by the mid-evening hours.

Pictures

Further Reading