The Storm Prediction Center has issued a moderate (level 4 out of 5) risk for severe weather across west central Illinois Saturday. (NOAA/SPC)

A dangerous severe weather outbreak is likely across parts of the Midwest on Saturday, with tornadoes — a few large and intense — damaging hail, and destructive straight-line winds. Central Illinois is most at risk, but significant storms could develop from eastern Iowa and northeastern Missouri into the Chicago metro area.

The National Weather Service Storm Prediction Center has taken the somewhat rare step of issuing a moderate risk of severe weather (level 4 out of 5) more than a day in advance, highlighting the potential for a higher-end severe thunderstorm and tornado event.

“A significant severe weather outbreak is possible,” wrote the Storm Prediction Center, which also indicated the potential for “giant” hail. A string of rotating supercell thunderstorms is expected to march across Illinois Saturday afternoon, forming amid balmy temperatures in the 70s and an atmospheric powder keg of ample moisture.

The imminent storms also raise another concern — whether people should seek refuge in a community shelter at times when social distancing is vital. Experts we spoke with recommended prioritizing the most immediate threat — in this case, tornadoes — but acknowledge the challenging elements of mitigating both risks at once.

Areas affected

A broader look at areas encompassed by the Storm Prediction Center's severe weather outlook for Saturday. (NOAA/SPC)

Illinois is the state most at risk, particularly west of Interstate 39 and south of Interstate 88. That includes areas like Peoria, Galesburg, Moline and Macomb.

Along with those locations, Joliet, Decatur and Springfield, Ill., and Davenport, Iowa, are also at risk for “significant” tornadoes, according to the Storm Prediction Center.

Surrounding the moderate risk is an enhanced risk (level 3 out of 5) for severe weather, marking a region that could still see severe storms but is less likely to see the high-end severe potential. Meanwhile, Indianapolis, Nashville and Chicago are all under a slight risk (level 2 out of 5), where more scattered strong storms with wind and hail are probable. Still, an isolated tornado can’t be ruled out.

Timing and hazards

A look at the setup that will favor severe thunderstorms over Illinois and the Midwest on Saturday. Temperatures are modeled by the National Weather Service. (WeatherBell/Matthew Cappucci)

Key to the forecast is a warm front that will be draped west to east from near the Missouri-Iowa state line to Peoria and Bloomington around lunchtime Saturday. To the north, temperatures will hover in the lower 50s, but south of there should be considerably warmer — likely near 70 degrees.

Any storms that form south of the front in the “warm sector” will have a risk for tornadoes. Those that form to the north will be mainly large hail-producing storms, but could still rotate.

A look at where models highlight the greatest risk for significant tornadoes on Saturday. (WeatherBell)

The warm front will try to ride north during the afternoon, possibly approaching Chicago by early evening. Evaluating the severe weather risk there is quite difficult, as it largely depends on where the warm front ends up. Right now, large hail seems to be larger threat than tornadoes in the Windy City.

In the warm sector to the south, light rain will give way to partial clearing during the afternoon. Any sunshine that develops will be particularly concerning, heating the ground and elevating the severe risk.

The NAM model's timing showing a line of rotating supercell thunderstorms racing across Illinois on Saturday afternoon. (WeatherBell)

A line of individual supercell thunderstorms is likely to develop in eastern Iowa and northeastern Missouri by midafternoon, entering western Illinois by 4 or 5 p.m. That line of dangerous storms, including those with significant tornado potential where temperatures warm to near 70 degrees, will barrel across Illinois and probably reach Interstate 57 around dark.

The tornado threat should diminish some after dark, the storms potentially merging into a line with a damaging wind risk as they move into Indiana. Still, a couple isolated tornadoes are possible.

The storms will evolve “into a possible squall line as [they] move into the Ohio River Valley after dark Saturday evening,” wrote the National Weather Service in Chicago.

Farther south along the front, a few trailing storms are possible in parts of Kentucky and Tennessee. A slight tornado risk exists there as well.

Action items

With a rapidly evolving severe weather event looking likely, it’s imperative to have a severe weather plan in place before storms threaten. Communicating that plan with loved ones is also key.

Have multiple ways to get notified, especially if any warnings are issued for your location. A battery-powered NOAA weather radio is ideal, but remaining abreast of changing weather through local media outlets can help. Remember to disable “do not disturb” mode on your phone and turn the volume up to receive wireless emergency alerts.

Have a shelter already in mind before storms threaten, and know how long it would take you to reach safety. You should be able to relocate to your chosen shelter at a moment’s notice. Ensure vulnerable neighbors and relatives have a shelter as well.

Concerns have arisen about potentially seeking shelter in shared spaces given covid-19. Experts say there’s no perfect way to balance both risks, but when a warning is issued, seeking shelter should be your first priority. Once in shelter, do your best to practice social distancing.

Storms will end west to east Saturday evening in Illinois, and overnight for portions of the Ohio Valley.