A powerful cross-country storm system that will bring what forecasters call “a multiday severe weather episode” has prompted a rare highest-risk threat alert for parts of the South.

The storm system is expected to bring damaging wind gusts from 60 to 100 miles per hour, hail the size of baseballs, tornadoes and strong thunderstorms, forecasters said.

The system is expected to unleash widespread severe weather across the Midwest on Friday before heading into the South on Saturday and then passing through the East Coast on Sunday.

The Storm Prediction Center on Friday issued a rare level-five threat rating for central Alabama and Mississippi, the highest risk. Other areas of the Gulf Coast states and Georgia face a level-four threat.

While the effects are expected to be significant, Scott Kleebauer, a meteorologist with the center, said a storm of this strength is not unusual for this time of year.

“It is a very textbook early spring disturbance,” Mr. Kleebauer said.

Winds high in the atmosphere are pulling warm, humid air from the Gulf of Mexico northward, where it will collide with colder air to the north.

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