MID-MISSISSIPPI VALLEY — The Storm Prediction Center issued a level 4 of 5 risk for severe thunderstorms on Monday across southwest Illinois and southeast Missouri, including St. Louis. Severe storms are expected to sweep across the mid-Mississippi valley and midwest regions of the United States, placing nearly 50 million people at risk.

Forecasters warned of multiple strong to intense tornadoes, widespread severe or damaging wind gusts and scattered large to very large hail, with some hail potentially reaching baseball size. Supercell thunderstorms could spawn tornadoes rated EF3 or higher in parts of Missouri and Illinois, and the supercells are likely to persist into the overnight hours, bringing a threat of nighttime tornadoes.

Wind gusts could climb to 60 to 70 miles per hour, the National Weather Service said, with potential for considerable tree damage and damage to mobile homes, roofs and outbuildings. Hail could damage vehicles. A plume of very moist and unstable air was surging northward ahead of the front, setting the stage for several days of severe weather and heavy rainfall across the mid- to lower Mississippi valleys and the mid-south, the Weather Service said.

A severe thunderstorm watch was valid until 10 a.m. CDT for parts of eastern Kansas and west central Missouri, and a separate watch covered eastern Missouri and southwestern Illinois, including St. Louis, until the same time. A watch for central and southern Illinois, including Springfield, was valid until 4 p.m. CDT. Severe storms were impacting the Central Plains and mid-Mississippi River Valley, with the threat later spreading to the Mississippi Valley, lower Ohio Valley and parts of the Midwest. Kansas City, Little Rock, Nashville, Indianapolis, Chicago and Des Moines were under severe weather threat.

Several rounds of storms were possible Monday afternoon into the evening in the Chicago area, where most of the region was under a level 3 of 5 risk. The storms could produce damaging wind, hail, flooding and tornadoes, with flooding possible especially from rising river levels. The Weather Service urged residents to move to an interior room on the lowest floor of a building, seek shelter in a well-built structure and stay away from windows.