Severe rainfall, or heavy rain, generally refers to an amount of rain that substantially exceeds what is normal for a particular area. An atmospheric river is a long, narrow band of concentrated water vapour in the atmosphere — like a river in the sky. These weather systems travel from the tropics toward the poles and hit the west coast of North America.

Atmospheric rivers and slow-moving thunderstorms can result in flooding or flash flooding. Flash flooding can occur within a few minutes or hours of heavy rain.