This winter season has thrown some wild weather our way, from winter storm Elliott to the arctic airmass that consumed New York, and now a major snowstorm is simultaneously hitting the Northeast and California. Climate researchers generally agree this is not normal. So what’s causing it? Researchers are actively trying to answer that and it’s proving to be complicated.

Global warming may sound like the obvious answer, but it’s not that simple, since the climate naturally operates with a degree of random variability that can also produce strange weather patterns every so often. To get to the bottom of what could be influencing our current winter, I spoke to three climate researchers who boiled it down to three major factors:

First, the obvious: Climate change

There’s no question climate change is contributing to changes in weather patterns. But when looking at particular weather events, it’s hard to tease out what is climate change and what is simply the natural variability of atmospheric weather patterns. “You have climate change warming temperatures, maybe providing some heavier precipitation events, and then you just have the randomness of our atmosphere. And then sometimes these things will coincide into this climate soup,” said Tom Di Liberto, meteorologist for the National Oceanic and Atmospheric Administration’s climate prediction center.

While it’s difficult to disentangle climate change from other factors contributing to the winter storms, it’s clear that warmer temperatures add more moisture into the atmosphere. This creates the potential for heavier weather events, like the current snow storms the Northeast and West coast are experiencing.

Climate change also makes measuring unexpected weather challenging, both because it’s steadily becoming more intense every year and because it’s been happening over a relatively short period of time—the planet’s average surface temperature has risen 2 degrees Fahrenheit since the late 19th century. “We look at past seasons to inform the current season. But we’re looking at past seasons that may not have had the impact of climate change that the upcoming season will have,” said Andrea Lopez Lang, climate researcher at the University of Albany.

But here comes a curveball: An extended La Niña

Every three to seven years, the atmosphere’s ocean system goes through two opposing natural cycles; La Niña, a cool phase, and El Niño, a warm phase. They both have a strong impact on winter weather, and this year, the U.S. is returning to its third consecutive winter under La Niña.

That means surface winds across the entire tropical Pacific are stronger than usual, which makes the water in the Pacific Ocean—the largest and deepest in the world—a few degrees colder. Even a small fluctuation like that can affect the weather across the entire world, and, here in the U.S., it usually creates drier-than-average conditions in some regions of the country.

This year, however, La Niña hasn’t followed that usual pattern. For example, usually California would undergo a drought, but instead, the state is getting hammered with snow and rain, alleviating drought conditions, said Jeff Masters, meteorologist for Yale Climate Connections. La Niña also typically creates more rain over the Pacific Northwest, but this year it has deviated from its usual track. “We’ve seen an above average amount of rainfall across the Southwest and that’s not normally something we see with a La Niña,” said Di Liberto. “It tends to put most of that precipitation along the west coast or the Pacific Northwest.”

So why is this happening? Researchers are also stumped. “There must be something else going on, and a good candidate is climate change. Every weather event that happens now is impacted by climate change to some degree,” said Masters.

Putting these rare weather events into perspective can also be hard, since there’s a smaller sample size to make comparisons against. “It’s been an interesting year and a good example of how no two La Niñas are alike,” said Di Liberto. ”Sometimes there are other things that can override the normal, expected impact from a big climate phenomenon like La Niña … like the polar vortex, phases of other observations called the Arctic Oscillation, or even simply, and most frustratingly, the random chaos of our atmosphere.”

Plot twist! There’s also a disrupted polar vortex

The polar vortex is a band of strong winds that forms each winter in the stratosphere 10 to 30 miles above the North Pole. Every so often, including this year, the polar vortex gets thrown off its course and can sometimes cause extreme cold air outbreaks down on Earth—known as the troposphere. Those on the East Coast got a taste of this not too long ago.

This year’s disrupted polar vortex was the fourth such event in the last six years, which NOAA says is rare and hasn’t been seen “over more than six decades of stratospheric observations.” Typically these events take up to two months to impact weather down in the troposphere. “It’s common to see an increase of more impactful winter storms when we have a disrupted polar vortex…but where the colder outbreaks are and where those winter storms are varies,” said Lang.

This polar vortex phenomenon happening on top of La NIña conditions means most of the U.S. will likely experience colder-than-average conditions in the coming weeks. “We’re going to continue with this flow of cold air coming out of the Arctic,” said Masters. How wonderful.