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.

A pair of back-to-back storms were forecast to dump heavy snow and rain over parts of the Pacific Northwest, northern California and the Rockies this week, triggering winter storm and flood warnings.