EF1 tornado in Williamson County, Illinois

An EF-1 tornado entered into Williamson County from Jackson County at S. County Line Rd. Damage was primarily to tree limbs along a path continuing eastward. A large tree was uprooted on Skyhawk Road. Moving east, a barn suffered roof damage on New Hope Road. The tornado lifted after crossing Spillway Road on the southwest side of Crab Orchard National Wildlife Refuge. Video of this tornado was shared on social media. Peak winds for the Williamson County portion of this tornado, and for the tornado as a whole, were estimated at 90 mph.

A major outbreak of severe weather occurred on the 8th for the Quad State region. On the synoptic scale, a longwave trough was centered across the Rockies with deep-layer southwesterly flow from the Southern Plains to the Ohio Valley. A 110 kt upper jet extended from northern KS to IA while a 60 kt mid-level jet was positioned across northern MZ into IA. Surface low pressure near the MZ/KS border lifted to St. Louis around 12am CST on the 9th. A warm frontal boundary stretched across the Quad State, with a cold front extending southward from the surface low.

The environment became unstable by early afternoon, especially in far Southern Illinois, with MLCAPE of 2000-3000 J/kg. Deep-layer shear was on the order of 55-65 kts, though low-level shear was 15 kts, increasing to 20 kts later in the day. SRH was around 100-150 m2/s2 with higher values north towards St. Louis. STP values were 2-4 due to deep-layer shear and instability. Supercell activity was more prominent during the afternoon in Southern Illinois with lapse rates of 6.5-7 C/km aiding with hail production.

For Southern Illinois, storms moved through with the cold front late evening on the 7th through the overnight hours with tree and power line damage in Joppa. The front reversed and lifted north during the morning, with storms entering Southwest Illinois late morning. A brief EF-1 tornado in Cora damaged trees and a few homes while injuring one person due to a falling tree limb. The afternoon brought a significant ramping up of supercell development and the arrival of a line of storms from the west. An EF-0 tornado crossed from Jackson County into Williamson County. An EF-0 tornado went through central subdivisions of the Lake of Egypt community, while an EF-1 tornado progressed eastward in Saline County with a broader area of straight-line wind damage around it. An initial evening round of storms swept through the northern half of Southern Illinois with wind damage most prominent in northern Wayne County with a swath of 75 to 85 mph straight-line wind damage and an embedded downburst.

Training heavy rainfall during the afternoon produced flash flooding from Jackson County eastward with rainfall totals commonly in the 2.5-4 inch range for northern Jackson, Williamson, Saline, Gallatin, and northern Union County. Hardest hit by flooding were Eldorado and Harrisburg in Saline County which received 1.5-2 inches within an hour. In Eldorado, Ferrell Hospital had floodwaters intrude on 55,000 sq ft of hospital space, necessitating transfer of patients. The high school had water in the entrance areas, and numerous roads flooded. Harrisburg avoided some of the higher end impacts of Eldorado but had many roads flooded for much of the afternoon and evening.