The Chicago National Weather Service confirmed four more tornadoes Wednesday afternoon – all outside of the immediate viewing area – in the neighborhoods of Ottawa (La Salle County), Grant Park (Kankakee County), Park Forest (Cook County) and Plainfield (Kendall County). This brings the total so far to 11 tornadoes from Monday’s derecho event.
In addition to the confirmed tornadoes, it was also concluded that the damage done in Forreston (Ogle County) was consistent with 100 mph wind gusts from straight line winds. Five people were injured when several trailers were damaged/destroyed from the strong winds. Significant tree damage was noted, as well as damage to a metal building near the BP gas station with the associated debris blown toward the northeast. Altogether, this damage was consistent with straight line winds of nearly 100 mph – which can produce the same type of damage as an EF-1 tornado. Straight line winds of near 90 mph were responsible for tree and utility pole damage in Somonauk (DeKalb County).
The debris field left behind from straight line winds and tornadoes, while both very devastating, will look a little different. Damage done by straight line winds will often times be lying on the ground in straight lines, parallel to the outward movement of the wind flow from the thunderstorm. When looking at the debris field from straight line winds, most of the debris will be pointing in the same direction.
Debris from a tornado will often times be at angles (or in different directions) due to the curving of the wind flowing into the storm. Damage surveys conducted by the National Weather Service are important for understanding what exactly caused that damage. The survey teams will look for any evidence within the debris field for circulation (indicating a possible tornado) or indications of straight line winds. Based on the damage and damage indicators, the survey team will then assign a maximum wind speed and EF scale, if applicable, and note if damage was from a downburst or tornado. You can find more information on Monday’s derecho event here.