An outbreak of severe thunderstorms is expected Saturday afternoon and evening across portions of the Midwest and especially central Illinois, with the possibility of tornadoes, very large hail and damaging winds. A few of the tornadoes could be large, long-lived and destructive, while hail may grow larger than baseballs in some areas. Chicago could be in line for dangerous storms too, but the forecast there is uncertain.

The responsible storm system already unleashed severe thunderstorms on Friday, dropping large hail across parts of the Southern Plains and Missouri Valley. Golf ball- to baseball-sized hail fell from near Oklahoma City to Springfield, Mo. — a path roughly 300 miles long. Another hailstorm slammed central Missouri, narrowly avoiding St. Louis.

That’s just a taste of what’s in store for Saturday, with the potential for widespread rotating supercell thunderstorms over much of the Midwest, with the risk greatest in the zone between roughly St. Louis and Chicago. A more scattered to isolated severe threat could extend down the Mississippi Valley, with a few storms even possible in eastern Texas.

“A significant severe-weather outbreak is expected,” the National Weather Service Storm Prediction Center wrote. It’s a day where remaining vigilant and keeping abreast of changing weather conditions is key.

Areas at risk

The epicenter of severe weather is expected in central and northern Illinois, where the Storm Prediction Center has determined the risk of severe thunderstorm is a Level 4 out of 5. Threats in this area include tornadoes, including some which are large and remain on the ground for an extended distance, large and very large hail, and severe straight-line winds. The center is even considering an upgrade to a Level 5 “high risk” in its Saturday afternoon update if placement of the worst weather becomes more readily apparent.

Joliet, Aurora, Springfield and Peoria, Ill., are all in the Level 4 “moderate” risk zone.

Chicago and St. Louis sit in the Level 3 (or “enhanced”) storm risk zone along with much of Illinois, eastern Iowa, and western Indiana west of about Indianapolis.

Farther south, storms will be more scattered in nature, with a lesser tornado risk. While an isolated twister can’t be ruled out, it’s mainly hail that has prompted the “slight” severe thunderstorm risk (Level 2 out of 5) along the Mississippi Valley.

A few bouts of strong to damaging wind can’t be ruled out in the Ohio Valley, which is also placed in the Level 2, slight risk zone.

Timing and hazards

Rain and nonsevere thunderstorms were ongoing Saturday morning across northern Illinois and Indiana associated with a warm front lifting northward. An additional slug of heavier rain and thunder was trekking across northeastern Missouri. Those marked the warm front, south of which an unseasonably mild and humid air mass will fuel severe storms Saturday afternoon and into the evening.

After some late morning clearing and early afternoon heating, rotating supercell thunderstorms are forecast to erupt northwest to southeast from east central Iowa down the Mississippi River into western Illinois. Storms in the “warm sector” south of the warm front could drop large hail, produce tornadoes — some significant — and bring about damaging winds. It’s likely that strong non-thunderstorm winds will also develop behind storms.

“Given the fast storm motions, any tornadoes that do form may persist for nearly as many miles as minutes of time,” wrote the Storm Prediction Center, indicating storms may be racing northeastward at highway speeds — 60 to 70 mph. That makes it all the more important to seek shelter when a warning is issued.

The warm front itself will feature an extremely sharp temperature difference over distance. Just north of the front, temperatures will remain in the 50s. South of it, near 70 degrees or greater.

Storms that form north of the warm front won’t have much of a tornado risk, but will still be capable of dropping large, damaging hail larger than the size of limes.

It’s unclear if the warm front clears Chicago to the north. If it does, the risk of tornadoes around Chicago would increase. But we won’t know how far the warm front will travel until hours before, making it a challenging and very high stakes forecast in the Windy City. Even if the warm front doesn’t lift north, large hail is still a possibility.

“All modes of severe threats are in play for later today, with supercells capable of producing very large hail, damaging wind gusts, and tornadoes,” the Weather Service office serving the region wrote.

Eventually, storms will merge into a line Saturday evening and night as they move into Indiana, with a risk of damaging winds. Those will march into the Ohio Valley.

South of Illinois in the remainder of the Level 2 slight risk area, more scattered storms capable of producing isolated tornadoes, large hail and damaging winds are possible.

Action items

It is important to have a way to get notified of any severe weather warnings for your area. “Make sure you have multiple ways to receive warnings and have a plan if/when severe weather strikes!,” the National Weather Service has advised.

Given the extremely rapid storm motions expected, you may have less advanced notice than typical when storms approach. That means you’ll have to take action more quickly.

Have a shelter in mind before storms even develop, and ensure it is a location you can access in only a minute or two. Communicate your plan with loved ones. If you have vulnerable neighbors, friends, or family without access to a basement, consider calling them in advance and making sure they have arrangements.

There has been some concern regarding community sheltering in an era of social distancing imposed by covid-19 concerns. Experts in several states have recommended prioritizing the more immediate threat — in Saturday’s case, tornadoes. Find a safe place to shelter and then practice social distancing to the extent possible.