By Dan Klepal
The Cincinnati Enquirer

A weather system stalled over the Ohio River Valley has been triggering the thunderstorms that have drenched the region over the past week. More storms are expected today, which could produce localized flooding.

The system, or a "boundary," extends over the Ohio River Valley, Indiana and parts of Illinois. It helps form storms as moist air is heated by the sun or lifted into the atmosphere by spiraling air masses within the boundary. Once in the upper atmosphere, the moist air condenses and turns into rain.

Today is likely the last for the boundary, but it is also expected to be the worst.

Wednesday evening, thunderstorms and high winds caused widespread damage, downing wires and trees. Even heavier storms were expected overnight. About 9,000 Cinergy customers lost power because of the storms.

John Franks, a meteorologist with the National Weather Service in Wilmington, Ohio, said particularly strong lift and spin inside the boundary today could mean stronger thunderstorms. A flood watch is in effect today for Butler and Warren counties and other northern points.

"We can say with reasonable confidence that something not nice is going to happen," Franks said. "Since northern areas have been worked over more than (Cincinnati), they just need someone to dump a bucket of water and they get flash flooding. Some people in Butler and Warren counties may get 3 to 4 inches, and others may only get an inch."

It's not unusual for boundaries to form over the Midwest in the summer, but it is unusual for them to stall more than two or three days. This boundary has stayed put since July 4, said National Weather Service meteorologist Myron Padgett.

"The front moving through (today) will hopefully push all of this out," he said.

E-mail dklepal@enquirer.com