By Mary Wisniewski
CHICAGO (Reuters) - The central United States braced for a severe storm system on Tuesday that could produce tornadoes, baseball-sized hail and a line of intense, dangerous winds known as a derecho, weather forecasters said.
Strong thunderstorms which started in South Dakota are expected to increase in number and severity throughout the day in central and eastern Nebraska, northern Missouri, southern Iowa and southwestern Illinois, said forecaster Bill Bunting of the National Storm Prediction Center in Norman, Oklahoma.
A derecho - a long-lived, wide line of thunderstorms that produces damaging winds - could start over portions of Iowa and Nebraska Tuesday night, then roll east into Wednesday morning, said AccuWeather.com senior meteorologist Alex Sosnowski.
"These storms can knock down trees, cause power outages and travel mayhem for the Wednesday morning commute," said Sosnowski.
Bunting said the strongest storms on Tuesday could produce tornadoes, with the possibility of a tornado rated EF-2 or higher, with winds peaking at 135 mph (217 kph) or more. Nebraska could see baseball-sized hail.
As of 2 p.m. CT (1900 GMT), a tornado watch was in effect for much of central and eastern Nebraska, southwest Iowa and northwest Missouri, Bunting said.
On Wednesday, the storms were expected to move into eastern Missouri, central Illinois, Indiana, Kentucky, southern Ohio, portions of Tennessee and West Virginia, forecasters said. AccuWeather.com said the storms could bring 2 to 4 inches (5-10 cm) of rain, with possible flash flooding in some areas.
Sosnowski said the region from the Great Plains to the Appalachians could see a stormy June, because of the temperature contrast between cool air around the Great Lakes and warmer air in the mid-south. "Thunderstorms like to move along this zone," Sosnowski said.
(Reporting by Mary Wisniewski; Editing by Doina Chiacu and Jim Loney)