This week, parts of the West Coast are going to be inundated with a storm system that is projected to bring anywhere from 3 to 7 inches of rain and up to 8 feet of snow in the mountains. This may sound like an astronomical amount of precipitation, but it’s actually more common than you may think for our friends on the West Coast. Here’s why.

California’s wet season is essentially the wintertime. For instance, Los Angeles and San Francisco both receive over half of their years’ worth of annual precipitation in the months of December through February. This happens because this time of year, the jet stream (the current of air high in the atmosphere that weather systems ride on) is typically situated in a position that channels one storm system after another directly into the Golden State.

Every now and then, however, an “atmospheric river” will ride along the jet stream and release a tremendous amount of precipitation. Think of these rivers as exactly that, a river. But instead of liquid water moving along the surface of the earth, they transfer water vapor in narrow channels in the sky.

Water vapor imagery of atmospheric river in California

Now, what is water vapor? When water evaporates from the oceans, it transitions from the liquid phase to the gaseous phase and picked up by the jet stream. Evaporation happens in remarkably large quantities in the tropics, where water is warm due to its proximity to the equator.

When this atmospheric river makes landfall, the warm, moist air is forced upward into the atmosphere. In doing so, the air condenses into large clouds where eventually rain and snow fall. Think of this as moving a wet sponge across a table at the same height that also has little mountains. The farther you move the sponge into the mountain, the water will be squeezed out as you force the sponge up and over the mountain.

North America from space. Elements of this image furnished by NASA

In California’s case, this “sponge” has the liquid water equivalent to the average flow of water found at the mouth of the Mississippi River! And when this sponge moves into the mountainous terrain of California, the rain and snow that is squeezed out can translate to inches of rain and feet of snow.

This is typically great news since much of the rest of the year, California is dry and arid. For instance, San Francisco’s average July rainfall is 0.00 inches! The bad news is that these atmospheric rivers can bring flash flooding, mudslides, blizzard conditions and hurricane force winds.