NWS wrapping up the July 15 historic tornado outbreak analysis…

At least 50 tornadoes across Iowa, Wisconsin, Illinois, and Indiana. In totality, 32 tornadoes have been confirmed in the NWS Chicago forecast area, which is the most from a single severe weather event on record, since reliable tornado records began in 1950.

From the NWS:

During the evening hours of July 15, a well-organized and long-lived complex of thunderstorms (officially classified as a derecho) rolled across Iowa, Illinois, and parts of Indiana. Widespread damaging winds of 60 to locally 100 mph were observed within the line, especially from eastern Iowa through northern and central Illinois. In addition, a total of 32 tornadoes were confirmed in the NWS Chicago County Warning Area.

With 32 tornadoes, this storm breaks the previous daily record for tornadoes in our forecast area.

This record was originally 22, set during the June 30th, 2014 “Double Derecho” event and later tied by March 31, 2023 Tornado Outbreak). See the Tornadoes tab for more information on these tornadoes.

The derecho was the fifth thunderstorm event to occur within the span of around 60 hours in northern Illinois and northwestern Indiana

It was the “finale” of an active “Ring of Fire” pattern. Ring of Fire patterns are characterized by episodic clusters of thunderstorms that parade around a dome of heat. They typically occur every summer but vary in severity and cadence with each year.

Northern Illinois and Indiana are within a region where derechoes are common, with an average of 1 derecho every year. Derechoes as strong as the one that impacted the area on July 15th have an occurrence interval of once every 5 to 10 years.

What is a Derecho?

Derecho development is tied to the formation of bow echoes. A bow echo usually arises from a cluster of thunderstorms but also may evolve from a single strong storm. As the rain-cooled downdraft of a thunderstorm reaches the earth’s surface, it spreads horizontally, most rapidly in the direction of the general atmospheric flow.

A s cool, dense air spreads outward, it forces the lighter, warm, and moist air surrounding the storm up along the leading edge of the outflow, or gust front. The rain produced by the newer storms reinforces the cold pool, strengthening the inflow of air from the back side of the developing storm complex and encouraging the downward transport of higher-momentum winds from aloft.

Wind speeds in derechoes often exceed 100 mph in the hardest-hit areas. Some of the strongest derechoes that have occurred have produced wind speeds estimated to exceed 120 mph.

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