(CNN) The threat of heavy rain and flooding continues Thursday as a slow-moving storm makes its way across southern Texas through the Ohio River Valley.

More than 30 million people are under a flash flood watch or advisory from Texas to Illinois. The heaviest rainfall, 3 to 6 inches, is expected in Texas, according to CNN meteorologist Michael Guy.

Recent heavy rain and melting snow have already flooded the city of Davenport in Iowa, where the swollen Mississippi River pushed through a temporary barrier and swamped downtown streets.

Here is the #weather snapshot for #Thursday, May 2. An elongated front that stretches from #TX and the Mid to Lower MS Valley into the OH Valley and Northeast will produce strong to severe #thunderstorms and locally heavy #rainfall; heavy #rain will also be possible in #FL. pic.twitter.com/1knIwAkJVk — National Weather Service (@NWS) May 1, 2019

The chance for rain and storms will increase into the weekend from the Deep South to New England, Guy said.

River communities in several states are preparing for flooding conditions.

Read More