Chicago, Ill. – Thousands of residents across southern Cook County and eastern Will County were left asking the same question after the destructive July 27 thunderstorms: Was it a tornado or straight-line winds? The National Weather Service says the answer is both, but the overwhelming damage was caused by an intense swath of non-tornadic winds reaching 80 to 100 mph.
According to the National Weather Service Chicago office, radar detected a broad corridor of destructive straight-line winds sweeping across Monee, Beecher, Crete, Flossmoor and surrounding communities. While brief tornadoes developed within the storm, meteorologists determined that the widespread tree, power line and structural damage resulted primarily from the larger area of extreme thunderstorm winds.
The agency explained that tornadoes typically leave narrow, concentrated damage paths and often produce radar-indicated rotation along with debris lofted into the air. Straight-line winds, by contrast, generally push trees and debris in a similar direction across a much broader area.
To distinguish between the two, National Weather Service survey teams examine several factors, including photos or video of a tornado, radar signatures showing rotation, debris detected by radar, whether damage follows a narrow or widespread path, and whether debris appears to have been lifted and scattered rather than simply blown over.
Despite the lack of a long-track tornado, winds of 80 to 100 mph are capable of producing destruction that closely resembles tornado damage. The National Weather Service emphasized that severe thunderstorm warnings are issued whenever winds are expected to exceed 80 mph, and those warnings should be treated with the same urgency as tornado threats. Residents are urged to move immediately to an interior room on the lowest floor of a sturdy building whenever either warning is issued.
Additional storm surveys and damage assessments may continue as the National Weather Service finalizes documentation from one of the most powerful severe thunderstorm events to impact the Chicago metropolitan area this summer.
