(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.
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.
A flash flood watch is in effect for northern Illinois, including Chicago, until Thursday afternoon, according to the National Weather Service in Chicago.
Many rivers across the northern part of the state have already flooded from recent heavy rains, the weather service said.
The Des Plaines River rose several inches above flood stage on Wednesday, according to CNN affiliate WGN.
The Des Plaines fire chief said there had not yet been any major flood-related emergencies in the area, but roads were partially submerged, according to the station.
The Illinois and Missouri rivers are expected to reach moderate to major flood stages at many points as rain continues throughout the week, according to CNN affiliate KTVI.
The town of West Alton, which sits between the Mississippi and Missouri rivers, has been issued a voluntary evacuation order, the station reported. The Mississippi River is expected to reach 35.5 feet — 13 feet over flood stage — over the weekend, according to the station.
If the Mississippi rises higher than 30 feet, the station reported, it is likely that the southbound lanes of US Highway 67 and Highway 94 will be submerged.
Gov. Kevin Stitt declared a state of emergency Wednesday in 52 counties in response to recent severe weather.
A 55-year-old man in Tulsa and a 58-year-old woman in Bokchito, have died in incidents the Oklahoma Office of the Chief Medical Examiner attributed to the storms.
Damages are still being assessed, according to the Oklahoma Department of Emergency Management.
-Madeline Holcombe reporting for CNN
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