1 Year Anniversary of the 2 Tornadoes Hit Winnebago County/Derecho



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On August 10, 2020, a large and intense line of thunderstorms known as a “derecho” swept across much of the Midwest producing wind damage to an area more than 90,000 square miles. The derecho resulted in $11.5B worth of damage, making it the COSTLIEST known thunderstorm event in modern U.S. history. Peak wind gusts topped 140 mph in Cedar Rapids, Iowa, and at least 26 tornadoes were spawned by the storm. More than 20,000,000 people lived in the damage footprint of the derecho, with at least 1,000,000 customers losing power — some for more than 2 weeks. Numerous injuries were counted after the storm, and four people lost their lives.


In the NWS Chicago County Warning Area (CWA), over 160 reports of damaging winds were received and 15 tornadoes were confirmed, including an EF-1 tornado in the Rogers Park neighborhood of Chicago. The derecho stands as one of the most impactful weather events to ever impact the NWS Chicago County Warning Area. Additionally, as of 2021, August 10, 2020 is tied with June 22, 2016 as the single calendar day with the 2nd most tornadoes within the NWS Chicago County Warning Area behind the 6/30/2014 “Double Derecho” event.

Fast Facts:


  • Summary:  A well-organized and long-lived complex of storms produced widespread severe wind damage across Iowa, northern Illinois, and northern Indiana during the day on Monday, August 10. Much of this severe wind was significant (75+ mph winds) resulting in many downed trees, several toppled-over semi trucks, and many communities receiving at least some minor structural damage. Within the broader area of severe winds, 15 tornadoes were confirmed across northern Illinois and northwest Indiana.
    • ​Several tornadic circulations developed within the main line of thunderstorms, and these accounted for 13 tornadoes across northern Illinois and northwest Indiana. In addition, a lone supercell that was subsequently absorbed into the main line produced two tornadoes in and around the Rockford area, bringing the total number of tornadoes for this event to 15. As it stands at this time, this is tied for the 2nd highest number of tornadoes to occur on a single calendar day in NWS Chicago`s area of responsibility since the beginning of the National Weather Service’s official tornado records in 1950, and the most to occur on a single calendar day in the month of August.
  • August 10th by the numbers:
    • Over 700 reports of severe wind speeds (58+ mph) or wind damage from the Nebraska/Iowa border, across Iowa, northern Illinois, and northern Indiana. This was a high end, once-a-decade type event for the area.
    • Over 160 of the wind reports are from all of northern Illinois and northwest Indiana.
    • 12 injuries were reported in the NWS Chicago CWA, 5 being in Forreston, IL, 1 in Peru, IL, 3 in Matthiessen State Park near Utica, IL, 1 in Lake Villa, IL, 1 in Lake Station, IN, and 1 in Hammond, IN.
    • In the NWS Chicago County Warning Area, 15 tornadoes were confirmed, which includes portions of northern Illinois and northwest Indiana. As of this time, this count ties August 10, 2020 with June 22, 2016 as the single calendar day with the 2nd most tornadoes within our county warning area (behind the 6/30/2014 “Double Derecho” event).
    • In total, the derecho produced 26 confirmed tornadoes across Iowa, Illinois, Wisconsin, and Indiana.
    • The tornadoes include an EF-1 tornado in Rogers Park, Illinois (a north-side neighborhood of Chicago), that moved out over Lake Michigan and became a waterspout. This was the first tornado in the city of Chicago since September 3, 2018 (EF-0). This was the first F-1/EF-1+ tornado in the city of Chicago since March 12, 1976 (F-2) that clipped the far northwest side of the city, roughly near O’Hare Airport and Edison Park.
  • Derecho Facts:  This complex of storms was known as a derecho.
    • A derecho produces a swath of particularly damaging thunderstorm winds (specifically, wind gusts of at least 58 mph along most of its length with several well-separated 75 mph or greater gusts) over an area at least 250 miles long.
    • These are primarily classified as straight-line winds rather than tornadic.
    • Even so, wind speeds in a derecho can exceed 100 mph which is equivalent to that of an EF-1 tornado but over a vastly larger area than a tornado would impact.
    • Tornadoes can also be embedded within derechos and produce concentrated areas of even more intense damage.
    • Derechos develop in an environment with very warm and moist air at the surface, colder air aloft, and moderate to strong winds at upper levels of the atmosphere.
    • Northern Illinois and northern Indiana have long been known as a corridor of enhanced derecho activity, as shown in this graphic from a published paper by Guastini & Bosart (2016) titled “Analysis of a Progressive Derecho Climatology and Associated Formation Environments”.


  • Preparedness:  Despite the massive coverage and impressive intensity of the wind damage, there were a limited number of serious injuries. This speaks to the effort of our partners including fellow meteorologists in the weather enterprise effectively communicating the threats and action needed to be taken, as well as emergency management and law enforcement for their preparedness efforts beforehand and assistance efforts immediately after. Of course it also means that many public citizens heeded the threat and the warnings, as well as communicated them to their family and friends (being a “Force of Nature!”). For more on severe weather preparedness, see here.
  • The Forecast:  Forecast confidence increased markedly Monday morning that severe weather was likely across northern Illinois and northwest Indiana that afternoon. At NWS Chicago, our messaging accordingly increased, and we issued a Situation Report (briefing) for our partners at 9:15 a.m. and held a conference call with them near 12 p.m. The office also issued eight Severe Thunderstorm Warnings and six Tornado Warnings.

A total of 15 tornadoes were confirmed across northern Illinois and northwest Indiana in the NWS Chicago County Warning Area.

Tornado #1: Southwest Rockford, IL EF-0



A very brief tornado touched down near the intersection of Pierpont Ave and Montague Road and continued northeast into the southwest side of Rockford. The tornado ended near the intersection of Ferguson and Loomis Streets. All damage was to trees along the tornado path.


Tornado #2 – Rockford to Caledonia, IL EF-1


This tornado was associated with the same supercell thunderstorm that produced an initial brief EF-0 tornado immediately southwest of Rockford, but it has been determined these exist as two separate tracks. This tornado developed as the main line of thunderstorms caught up, and eventually merged with, the parent supercell with this path starting near Guilford Road just north of Rockford College before heading northeast. Significant tree damage, consistent with EF-1 wind speeds, occurred in a localized area immediately east of Rock Valley College west of North Perryville Road. The tornado continued northeast, crossing I-90/39, producing sporadic damage before lifting near Caledonia Road.

Source: https://www.weather.gov/lot/2020aug10

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