If you live in California, you’ve likely heard the term “atmospheric river” thrown around recently. The West Coast is slammed by an onslaught of these massive, fast-moving storm systems every winter, and the Bay Area and much of California are now bracing for another soaking over the next few days.

These storms can transport more than 25 times the moisture that flows through the mouth of the Mississippi River.

Atmospheric rivers are a normal winter weather pattern for California, which relies on them to replenish its water supply. Strong or extreme atmospheric rivers can trigger heavy rainfall and major flooding.