BIRMINGHAM, Ala. >> A wave of storms pounded the Deep South today, leaving a trail of splintered trees and damaged buildings, and forecasters said still stronger ones were on the way with the potential for massive tornadoes, downpours and hail the size of baseballs.

While nearly 16 million people in the Southeast could see powerful storms, the Storm Prediction Center said, a region of about 3 million stretching from southeastern Arkansas and northeastern Louisiana across Mississippi into Alabama was at high risk for big twisters that stay on the ground for miles, straight winds up to 80 mph and destructive hail.

Possible tornadoes knocked down trees, toppled power lines and damaged homes in the Alabama communities of Burnsville and Moundville, where power was out and trees blocked a main highway.

“Downtown Moundville got it. Some roofs and stuff got taken off houses,” said Michael Brown, whose family owns Moundville Ace Hardware. and Building. “There’s a lot of trees down. I guess it had to be a tornado; it got out of here pretty fast.”

Additional damage was reported in Louisiana, Tennessee and Mississippi, where video showed an apparent tornado at Brookhaven. High winds blew down signs and and trees in northeast Texas, and hailstones the size of baseballs were reported near the Alabama-Mississippi line, the weather service said.

Storms were possible all the way from northern Texas in the west to northern Illinois and as far east as the Carolinas, the forecasters said, and the weather service issued multiple tornado warnings in Texas, Mississippi and Alabama. Tornado watches included parts of seven states.

Dozens of schools systems in Alabama, Louisiana and Mississippi canceled classes, switched to online learning or dismissed students early, and Mississippi State University moved to virtual teaching because of the potential for danger at its campuses in Starkville and Meridian.

Large vaccination clinics where hundreds of people an hour can get shots without leaving their vehicles were canceled in Alabama, Mississippi and Tennessee. In the Mississippi capital of Jackson, state employees were warned to head to stairwells if they hear weather sirens. Near Birmingham, labor organizers canceled an outdoors event at an Amazon facility where workers are voting on union representation.

At least two waves of storms were likely, forecasters said, and the worst might not hit until a cold front passes overnight.

“The biggest question is how strong to severe these storms are going to be and if they’re going to be tornadic right off the bat,” said Gary Goggins, a forecaster with the National Weather Service office for Birmingham.

Gov. Kay Ivey placed Alabama under a state of emergency, and communities across the South used social media to share the location of tornado shelters. Dozens of people gathered in a gymnasium that was opened as a shelter in Tuscaloosa, where more than 50 people died in a twister during a weather outbreak that occurred 10 years ago next month.

In Jackson, Tennessee, officials said a civic center and the regional airport would be open for residents seeking shelter.