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.

In the northern hemisphere, atmospheric rivers typically occur during winter and are often cited for strong storms in the West. According to NASA, they are the largest transport mechanisms of fresh water on Earth.

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,