Rivers in the Atmosphere Converge to Give California a Huge Downpour

All aboard the Pineapple Express

Strong atmospheric river events are driving record precipitation across the state of California. Bob Dass via Flickr (CC BY 2.0)

California has more water than it knows what to do with at the moment thanks to an atmospheric phenomenon known as the Pineapple Express.

Before you ask, the meteorological event has no relationship to the 2008 Seth Rogen comedy. Rather, it refers to a strong atmospheric river that carries moisture near Hawai’i to the West Coast. Pineapple Express is a colloquial term for just one type of atmospheric river, which are defined as narrow bands of water vapor in the atmosphere that can stretch for thousands of miles, from the tropics to higher latitudes.

In what was already a wet year for the state, the Pineapple Express has pushed precipitation to record levels in the past week. The NASA Earth Observatory reports that more than five inches of rain fell in parts of northern California and along the western foothills of the Sierra Nevada on February 19-20. Just days before that, an atmospheric river event set new rainfall records in southern California and caused a power outage for more than 100,000 people in Los Angeles.

Strong atmospheric rivers can transport up to 20 times the amount of water than flows through the mighty Mississippi River, writes Julia Rosen for Science Magazine. When the rivers make landfall, the water vapor condenses and unleashes a torrent of precipitation. Rosen reports that some places in California have received more than a three feet of precipitation since the start of the year.

Atmospheric rivers are not restricted to the Pacific Ocean, according to the National Oceanic and Atmospheric Administration (NOAA). In fact, the rivers move with the weather and are present somewhere on the planet at any given time. In the animation below, which shows the movement of water vapor around the globe, the rivers are clearly visible, including the one striking the West Coast.

Atmospheric river events are an important source of moisture for many mid-latitude regions. According to NOAA, these atmospheric waterways are responsible for nearly half the annual precipitation in West Coast states, on average. However, because these events strike land like an exploding water balloon, the burst of precipitation can trigger the flooding, landslides, and overflowing reservoirs like those that currently plagues California. Rosen discusses a study in the recent issue of Nature Geoscience that found that up to half of all extreme weather events in mid-latitude regions can be traced to atmospheric river events.

Because atmospheric rivers play such an integral role for water-starved regions, scientists are working to better forecast the events, Rosen writes. Forecasts of the rivers are currently good out to about five days, but scientists hope to lengthen the forecast time and predict the exact areas where the rivers will strike.

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