A pair of potent storm systems known as atmospheric rivers are spreading rounds of rain and snow across the Pacific Northwest and Northern California over the next several days. The first storm is already pounding the region.

The next storm is expected to approach the Pacific Northwest Tuesday night, unleashing showers and thunderstorms along the coast and portions of the Coastal Ranges that have the potential to trigger flooding. As the storm moves east, it could bring more snow to the Cascades.

The storms pushing off the Pacific are the result of an atmospheric river, a corridor of air heavy with moisture that forms when warmer air from the tropics moves toward the poles. As the first of the two storms pushes inland on Monday,