F1 tornado in Knox County, Illinois

A tornado touched down 3.5 miles southwest of Williamsfield. It destroyed a steel corn crib, blew down a large tree which damaged a home. Also, several windows were blown out on the home. As the tornado travelled east it caused major tree damage for a mile. Then it blew a large tree onto another home and destroyed an empty silo. Next it destroyed a barn and two outbuildings, knocked yet another tree onto a home, as well as, breaking several windows and blowing several 2x4s into the home. Further down the road, the roof and east walls of two homes were destroyed. Then for the last half of a mile, ten power poles were blown down, as well as, numerous trees and a barn sustained moderate damage. One woman sustained minor injuries from flying glass. Damage is estimated around $2 million.

A large bow echo system developed over eastern Iowa and moved rapidly to the southeast into Illinois. It moved into Central Illinois's County Warning Area (CWA) around 4 pm in Knox County and exited the CWA (Lawrence County) around 830 pm. Damage was reported in all 35 counties with this system. The overall general area of wind damage was produced by the passage of the bow echo's "gust front" on the leading edge of the line of thunderstorms. Wind speeds were measured or estimated to be between 60 to 80 mph, blowing down or uprooting thousands of trees, tree limbs, power poles, and power lines. Hundreds of trees fell onto structures causing damage ranging from just torn guttering to major roof and structural damage. Also, hundreds of vehicles sustained damage from fallen trees and numerous outbuildings, sheds, and silos were either damaged or destroyed. Considerable crop damage was sustained in most areas. In some areas, more intense damage was observed, caused by stronger wind speeds. Speeds were measured or estimated in these areas at 100 to 110 mph. These areas of damage were apparently "microbursts" produced by a series of mesocyclones that formed on the forward edges of the bow echo. These microbursts, or swaths of more intense wind damage were generally about 1/2 a mile in width. In these areas significant structural damage occurred, such as peeling off roofs, blowing over freight railroad cars, bending steel power poles, and other structural damage. A third phenomena that occurred with this event were spin-up tornadoes along the leading edge of the bow echo structure. These tornadoes caused significant damage in narrow swaths along the bow echo's path and were often masked by the microburst damage occurring adjacent to them. Based on valid spotter observations and mesocyclone signatures on doppler radar, the existence of these tornadoes was validated. Overall, approximately twelve people sustained injuries and damage was estimated around $16 million.