California is in a unique predicament, and it’s one of the reasons the state continues to suffer from long-term exceptional drought. Nearly all of its precipitation comes during winter, and small diversions in storm tracks can make or break annual rain or snowfall. If a rainy season is particularly dry one winter, it’s extremely difficult to make up that deficit during the traditionally dry summer months.

But researchers at Stanford University found that during the wet months, the pattern in the atmosphere that leads to extremely dry and warm conditions is happening more often. It’s the kind of pattern that has exacerbated the ongoing California drought — nicknamed the “ridiculously resilient ridge” — by both lack of precipitation as well as warmth, which amplifies the drying effect.

But they also found that the cool and wet patterns aren’t really changing that much, either.

How is that possible?

It means that over long periods, California may no obvious trend in precipitation — up or down — but will instead get wild swings from a heavy rain years to drought years. The variability is increasing.

“While there are more years that are like the extreme ridging years, there’s no reason that those should have to come at the expense of the cool and wet patterns,” said Noah Diffenbaugh, professor of Earth System Science at Stanford and one of the authors on the study, which was published in Science Advances. “We’re seeing fewer neutral years and more extreme warm and dry years.”

Interestingly, the increase in extreme events on both ends of the spectrum is exactly what other research has suggested would happen as Earth continues to warm. “Although we don’t make any predictions about the future, it is actually very consistent with the studies that say the extremes will become more frequent,” said Daniel Swain, Stanford graduate student and lead author on the study.

This area of research is critical because recently, California has seen a lot of the “hot and dry” and not nearly enough of the “cool or wet” — even during strong El Nino winters. It’s not that this was a dry year, it just didn’t pan out like previous very strong El Ninos, like that of 1997-1998 and 1982-1983 before that.

This year’s El Nino was well-predicted, Swain says. But what wasn’t expected was the impact in California.

Instead of above-average rainfall, most of California ended near-normal in terms of precipitation. The California snowpack survey noted that snow depths were below average for this time of year. Southern California got the shortest end of the stick, seeing no drought recovery at all.

Swain says it’s because El Nino is just one of the things that can have an effect on West Coast rain, while others could be having the opposite impact. “If someone is pushing a rock uphill and another is pushing downhill then the net effect is that the rock isn’t going to get to the top of the hill,” Swain said. “We’re still going to be on pins and needles because if next winter is another dry one then we’re back to square one.”

This study is the tip of the iceberg for the research group, which plans to dig deeper into why it’s happening, and — possibly even more important — if it’s predictable.

“There’s so much changing in the climate system right now,” says Diffenbaugh. “We had a really strong El Nino and yet the precipitation has been quite different than we saw in previous El Ninos like 1997-1998 and 1982-1983. We want to know if we can predict that in advance.”

“This year will be an interesting case study because we didn’t see a complete obliteration of the drought, despite [the wet winter],” Swain added. “It points to how California is so uniquely susceptible to small shifts in the pattern.”