California’s terrain is so diverse, spanning almost the entire West Coast, that the Golden State gets it all, from earthquakes to snow and drought. In some areas, tepid year round temperatures and few rainy days are welcome staples, even tourist attractions.

So imagine everyone’s surprise when the state, which was in a drought for the last five years, saw weeks of precipitation and even some flooding during the winter months.

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This year California broke records, as it saw incredible rates of precipitation — so far in 2017, the state has gotten an average 22.45 inches of precipitation, 9.5 inches above the 20th century average. Enough to bring California officially out of drought status, as California Governor announced last month. In addition to the precipitation, California saw a considerable increase in its snowpack, which will help keep the state out of drought conditions this spring and summer when it begins to melt.

But with such a quick turnaround and such weather extremes, it’s easy to wonder whether such a pattern is normal.

The three month period of “winter” as measured by meteorologists includes December, January and February. The state of California got an average of 21.67 inches, 9.94 inches above average during the meteorological winter of 2016-17, Jake Crouch, a climate scientist at the NOAA National Centers for Environmental Information, told International Business Times.

“It is a little bit outside of ordinary,” he said, mostly because it’s a La Niña year. Usually La Niña years are drier and El Niño years are wetter. “One of the major modes of climate variability is El Niño and when we’re in El Niño there’s a large area of warm sea surface temps in the Pacific,” this leads to more precipitation on the West Coast, Crouch said.

This winter was the second wettest ever in California, second only to 1969, an El Niño year. “California, if you look at records going back to 1895, they have one of the most variable climates of all the states in the U.S. You can have one year with almost no precipitation, then next year have a record year,” said Dan McEvoy, an assistant research professor at the Climate Research Institute.

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While all this precipitation broke records and sounds like quite a lot, Crouch called it “a little bit outside of ordinary,” and McEvoy said, it’s “not abnormal. It’s slightly wetter than the previous wettest year, 1969, but I wouldn’t say it’s abnormal.” However, Crouch noted this wetter winter is “out of the ordinary in terms of what we would expect with a La Niña present.”

So if El Niño isn’t to blame for all the rain, then what is? An increase in atmospheric river storms.

What is an atmospheric river?

An atmospheric river is a channel in the sky that helps transport large amounts of water, almost like a normal river does on land. These narrow channels usually sit about a half mile to a mile above the ocean. They move with the weather and eventually bring all the water vapor with them to land where it ends up coming down as snow or rain. The storms caused by atmospheric rivers can last days or weeks and bring inches of rain, historically leaving floods and mudslides in their wake.

atmospheric river This chart helps show how atmospheric rivers are formed and end up causing precipitation on land. Photo: NOAA

Extreme floods likely caused by these kinds of storms happen every 100 to 200 years. With the climate warming, extreme precipitation events may start happening more frequently, a paper published by the American Water Resources Association said.

“You have more evaporation, more energy, more heat and that’s driving more moisture from the tropics which is where these atmospheric rivers originate,” Lynn Ingram, a professor of Earth and Planetary Science at the University of California, Berkeley, told IBT. Ingram studies California and past climate and environmental change. “All climate models show that as the climate warms, we should expect more frequent atmospheric river storms, which isn’t good in California because it’s almost like too much rain at one time,” she said.

It wasn’t one or two particularly heavy atmospheric river storms that pulled California out of a drought, increased snowpack and caused flooding. The state saw far more atmospheric river storms than usual.

From Oct. 1 to March 31, 45 atmospheric river storms made landfall along the West Coast, the Center for Western Weather and Water Extremes reported. Additionally, many atmospheric river storms move down the coast over time, so those that made landfall above California frequently moved down to the state as well. McEvoy called this level of atmospheric rivers “way above normal.”

But so far nobody really knows why there are more atmospheric river storms hitting the West Coast and causing so much rain in California.

“That’s an active area of research right now,” McEvoy said. “As far as precipitation and storms, we can’t attribute a season or storm to climate change. You have to look at the long-term trends and with precipitation, there really is no long-term trend,” he explained. In short, it’s too early to tell whether or not the increase in storms and precipitation is due to climate change or not.

In February, the Oroville Dam north of Sacramento reached full capacity and broke open, flooding the surrounding area and causing more than $100 million in damages. Flooding across the state as a whole during the winter season prompted state officials to request $50 billion to help control the flooding. The state plans to beef up infrastructure and make sure it’s in good enough condition to handle rain like this in coming seasons.

The reservoir system in the state helps distribute water throughout the year and plays a vital role during the typically hot and dry summer months. Thanks to the atmospheric river storms this year, the state’s water supply and snowpack is in good shape going into the summer. But the unpredictability of what the next wet season may bring has officials and researchers working to predict and prepare.