An unusual stream of “rivers in the sky” are the reason for the daunting rains and snows over California in recent months.
The US National Centers for Environmental Predictions (NCEP) has reported that their global forecast systems tracked multiple atmospheric river (AR) events that flooded—or buried under snow—portions of California from October 2022 to early 2023.
According to the NCEP’s parent organization, the National Oceanic Atmospheric Administration (NOAA), “atmospheric rivers” act like long, narrow “rivers” in the sky. At some point, they will dump their water vapor back to Earth as rain or snow.
Though ARs vary in shape and size, those that come with large amounts of water vapor and strong winds can stall over watersheds and cause extreme rainfall, snowfall, and floods. And—like the recent, record-breaking AR events in California—they can be extremely dangerous, disrupting travel and taking a disastrous toll on life and property.
Unlike the extreme events seen in 2022-2023, most ARs are weak and bring beneficial rain or snow to crucial water supplies. According to NOAA, “ARs move with the weather and are present somewhere on the Earth at any given time.”
NOAA has learned a great deal about atmospheric rivers from more than a decade of scientific studies using new satellite and radar technology.
As much as half of annual precipitation in the Western US coastal states occurs—on average—in a few AR events.
ARs are 400-600 km (248-372 miles) wide on average.
They flow at the bottom of the atmosphere, about half a mile to a mile above the Earth.
Strong ARs transport water vapor amounts roughly equivalent to 7.5–15 times the average water flow at the mouth of the Mississippi River. ARs are, in fact, the largest “rivers” on Earth.
The US Geological Survey says that, “At any given time, 90% of the water vapor moving toward the poles is concentrated in about four to five atmospheric rivers across the globe.”
A well-known type of strong AR that can hit the US West Coast is called the "Pineapple Express." Its name is derived from its apparent ability to transport moisture from tropical areas near Hawaii to the US West Coast.
NOAA and partner organizations carry out targeted field campaigns that use “satellite measurements, offshore aircraft reconnaissance, and land-based AR observatories” to help develop forecasting models. NOAA’s US West Coast AR Landfall Tool, courtesy of Dr. Jason Cordeira of Plymouth State University, offers accessible graphic representations of AR events.