The National Weather Service has issued a tornado watch for a large section of the Washington area as the remnants of Hurricane Katrina move north. The storm could bring extremely heavy rainfall and localized tornadoes, the weather service warned.

The tornado watch, which covers the District, Northern Virginia and Frederick, Montgomery and Prince George's counties as well as Southern Maryland, is in effect until midnight, the weather service reported.

Katrina, which has now been downgraded to a tropical depression, is expected to move across the Ohio River valley tonight as it races north.

In an advisory issued this afternoon, the Weather Service said thunderstorms are expected especially in the areas along the Blue Ridge Mountains. It said the Baltimore-Washington area could expect showers and thunderstorms until about midnight that may produce heavy rainfall. Although the metropolitan area is not the most likely to see particularly violent weather, the forecasters warned that threat of isolated tornadoes and damaging wind gusts does remain.

"Any thunderstorms that do develop have the potential for extremely heavy rainfall," meteorologist Andy Woodcock of the Weather Service said this afternoon. "Katrina is a monstrous storm that could produce incredible amounts of rainfall in short periods of time."

Woodcock warned of possible flooding and ponding of water on local roadways.

"There's a lot of spin in the atmosphere with these types of storms," Woodcock said. "Because of the spin, there's a potential of tornadoes."

For Wednesday, when Katrina is expected to be in Quebec, Canada, showers and thunderstorms are also predicted for the Washington metropolitan area.

But as the storm moves on, the Labor Day weekend is expected to be sunny and nice, Woodcock said.

"After we suffer through a bit of pain today and tomorrow, we'll be rewarded with a good weekend," Woodcock predicted.