June 21 (Bloomberg) -- A round of severe thunderstorms pushed through the U.S. Midwest and headed for the Mid-Atlantic and Northeast.

The storms, caused by a clash between a cold front and a warm and moist flow on the East Coast, will cross the Appalachians tomorrow, affecting the area from the St. Lawrence River Valley to Maryland. New York and Philadelphia are among the cities that may get drenching rain, strong winds and hail during the afternoon and evening.

The locally heavy downpours in the Midwest today had little impact on the Mississippi River floods that were spawned by as much as 15 inches of rain in Iowa this month. Breaches in levees will keep the river from cresting at the record highs set in 1993. Normally, the Mississippi easily handles the added flow of Illinois and Missouri rivers in St. Louis.

``Mother Nature sized the river for three rivers coming together,'' said Alan Dooley, a spokesman for the U.S. Army Corps of Engineers for St. Louis. ``Only the Mississippi is behaving badly at this time.''

The Mississippi River crested at St. Louis yesterday at 37.3 feet (11 meters), below the record 49.58 feet 15 years ago, Accuweather.com reported. The flooding won't dissipate quickly, Dooley said.

Tens of thousands of acres of farmland submerged in Iowa will swell the tributaries feeding the Mississippi, Dooley said. The river will fall at barely one-tenth of a foot per day. Even after the fields are no longer covered, the water beneath the surface has to find its way into waterways.

``There is a tremendous amount of water in the system,'' said Dooley, whose office overseas 300-miles of the Mississippi and portions of the Missouri and Illinois rivers. ``There was a lot of rain over a wide area north of St. Louis. If the water has to travel 20 miles at 3 miles per hour, it will take 8 hours for every drop of water to reach a stream or river.''

For related stories: Weather forecast: http://www.accuweather.com

To contact the reporter on this story: Nancy Kercheval in Washington at nkercheval@bloomberg.net.

Last Updated: June 21, 2008 17:57 EDT