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(COLORADO) — You might’ve heard posts about it or meteorologists and forecasters talk about an “atmospheric river” when explaining forecasts and things to expect in the future, but what exactly does that mean?

In simple terms, an atmospheric river is a long band of moisture very high in the atmosphere that carries large amounts of water vapor from the tropics toward the mid-latitude areas, such as here in Colorado.

It’s often described as a “river in the sky,” because it transports moisture much like a river moves water on the ground; except these moisture streams can stretch thousands of miles across the Pacific Ocean while only being a few hundred miles wide.

The jet stream plays a big role in steering these systems. Acting like a conveyor belt, the jet stream helps guide and sometimes strengthen the plume of moisture as it travels toward the West Coast. When that moisture reaches land and is lifted over mountain ranges, it cools and condenses, leading to heavy rain, mountain snow, and occasional flooding. 

A fun fact: a single strong atmospheric river can carry more water than the Mississippi River at its mouth. Not all of them are destructive, as many bring much-needed rain and snowpack to help ease drought conditions. But when they stall or hit the same region repeatedly, they can lead to flash flooding and other damaging modes of severe weather.

In a gist, the tropics provide the moisture, the jet stream carries it, and the mountains hash it out, creating a powerful event and moisture supply within our weather patterns.