The name comes from their long, narrow shape and the prodigious amount of water they carry.
Read more about these types of storms — and how climate change is shaping them — in this piece by Raymond Zhong, a New York Times climate reporter.
Not all atmospheric rivers are harmful.
Not all atmospheric rivers lead to dire situations. The weaker events benefit the water supply in the West — in fact, these atmospheric river events deliver half of California’s water supply and nearly all heavy precipitation events.
They can become hazardous when too much rain falls at once, as happened this week in San Diego, where the edge of a weaker atmospheric river went over ocean water that had been made warmer than average by the effects of El Niño. That increased the amount of rain that fell over a short period, leading to flooding in the city. They also are more likely to be a problem when back-to-back atmospheric rivers fall over the same location.
Last winter, California experienced the longest stretch of continuous atmospheric river conditions in the 70 years that records have been collected on these events. With nine back-to-back atmospheric rivers from December through January, farmlands turned into lakes and snow piled up well above homes in the Sierra.
This season has been less dramatic, with seasonable precipitation numbers falling below average. That could change next week as an atmospheric river flows south toward California.