ATLANTA (AP) — Storms dumped as much as 6 inches of rain on Mississippi and Alabama on Tuesday ahead of threatening weather that forecasters said could include tornadoes across a large part of the South from Texas to Georgia.

With isolated flooding already being reported in western Alabama, the Storm Prediction Center said hail as large as tennis balls and intense twisters were possible across the region Wednesday. More than 6 million people live in an area stretching from eastern Arkansas to eastern Alabama that forecasters said was most at risk.

More than a dozen Alabama school systems canceled classes, planned online sessions or announced early dismissals because of the threat. More could be added to the list. Storms likely will intensify in waves during the afternoon, forecasters said, and the worst wasn't expected until overnight.

“This is going to be a long-haul type of event,” said Gary Goggins in the National Weather Service office for central Alabama, where Gov. Kay Ivey issued a state of emergency for all 67 counties that would make it easier to provide state aid if needed.

Forecasters issued tornado warnings in southwest Alabama as storms moved through early Tuesday, but no damage was reported immediately. Schools in Cullman County, north of Birmingham, delayed opening because of downpours. Rainfall was particularly intense around the Mississippi-Alabama state line, where as much as 6 inches (15 centimeters) fell.

“’We did get a strong band of pretty heavy precipitation,” said Ashlyn Jackson, a forecaster with the weather service in Jackson, Mississippi. "We’ve been getting hit pretty hard.”

On Wednesday, strong twisters that stay on the ground for an extended period are possible in a region that includes large parts of Arkansas, Mississippi and Alabama as well as parts of Louisiana and Tennessee, forecasters said. Residents need to pay attention to alerts in cities including Memphis, Tennessee; Birmingham, Alabama; and Jackson forecasters said.

Large, outdoor sites for administering COVID-19 vaccinations around Birmingham and Memphis canceled appointments because of the threat of severe weather. With people getting shots in their cars, officials were concerned that strong winds, rain, hail and the threat of tornadoes could make the operations unsafe.