Hurricane-force wind gusts and hail the size of baseballs could accompany potent storms that also threaten to spin up tornadoes and drench the South with heavy rains, forecasters said Friday.

The national Storm Prediction Center said more than 18 million people in Louisiana, Arkansas, Texas and Oklahoma will be at an enhanced risk of storms Friday that could include strong tornadoes and flooding rains. The area includes several major Texas cities including Dallas, Houston and Austin.

In a briefing early Friday, the National Weather Service said the storms could bring wind gusts of up to 80 mph...