A warning of flash floods is in effect until 11 tonight in the Cascades from Stevens Pass to the Canadian border.

Slow-moving thunderstorms are expected to pass through the mountains tonight, potentially dumping several inches of rain, according to the National Weather Service

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Flash floods are defined as lasting six hours or less, as opposed to the longer periods of flooding seen in the winter, said Gary Schneider, a meteorologist with the weather service in Seattle. Flash floods also usually affect only small streams rather than large rivers, he said.

The rainfall is expected to be at its heaviest in the northern part of the Cascades closer to the border, Schneider said.

Most of the rain is expected to bypass the lowlands and fall in the mountains, he said.