A ‘Pineapple Express’ Is Bringing Rain and Snow to California

Forecasters said the rain arriving in California on Wednesday is being caused by a “true Pineapple Express” — a specific example of a common atmospheric phenomenon that resembles a conveyor belt for moisture.

Over the past two weeks, California and other parts of the West Coast have been hit with a series of what meteorologists call atmospheric rivers — long, narrow regions in the atmosphere that transport most of the water vapor outside of the tropics.

“Like rivers in the sky,” according to the National Oceanic and Atmospheric Administration.

These rivers in the sky occur often on the West Coast but can happen in other locations, including the eastern United States, where they often channel moisture from the Caribbean. And they carry a lot of moisture — enough water vapor to equal or sometimes exceed the average flow of the Mississippi River at the point where it flows into the Gulf of Mexico.

When this moisture begins to interact with land, it can fall as rain or snow. Often, as we’ve seen lately in California, the precipitation is abundant because these atmospheric rivers provide a continuous flow of moisture.

Although atmospheric rivers come in different shapes and sizes, for one to be a “true Pineapple Express,” location matters. The tail end, where the moisture is pulled into the atmosphere, must start near Hawaii. Then the river must stretch continuously through the atmosphere to the West Coast of the United States.

Whether an atmospheric river is a Pineapple Express or not, the result is something like a conveyor belt for precipitation. That makes them essential to the livelihood of coastal states, which rely heavily on precipitation for their water supply: Between 30 percent and 50 percent of the annual precipitation on the West Coast occurs from just a few atmospheric river events, according to the NOAA.

But when the rivers are particularly strong — or come back-to-back in what are called “atmospheric river families” — the effects can be serious, like the extensive flooding California is experiencing now.