A tornado near La Crosse, Kan., on May 25, 2012. (Ian Livingston)

Not that we’re complaining, but it’s been a really slow start to this year’s severe weather season in the United States. More than really slow — record slow. Slow enough for the Storm Prediction Center to call it “uncharted territory.”

But just four years ago, in 2011, we were dealing with uncharted territory on the other end of the spectrum. “The 2011 tornado season wasn’t supposed to happen, not in our modern era of advanced technology and storm warnings,” said Climate Central’s Andrea Thompson. “But the 1,691 tornadoes in 2011 — the second most for any season going back to the 1950s — included outbreaks that killed hundreds, something not seen since the 1970s.”

If you’re wondering why this year has been so slow, when just a few years ago the central U.S. saw one of the most devastating tornado seasons in decades, then you’re not alone. A team of scientists led by John Allen at Columbia University has  been looking into how El Nino and La Nina are contributing to the severity of our tornado seasons, with the goal of producing a probability-based seasonal forecast for severe storms, similar to those issued by NOAA for things like hurricanes and the winter season.

[Silent Spring? No severe weather watches have been issued in March]

Needless to say, there’s a lot of natural variability at play in a severe weather season. To make matters more difficult, the period of record for tornado reports is short — going back to just the 1950s — and reporting methods have changed drastically over time. “Trying to tease out an ENSO signal from both the natural noise and the human noise becomes quite complicated,” said Michael Tippett, a co-author on the El Nino study from Columbia University. “You can’t get a robust correlation using the observational record alone.”

El Nino (left) tends to dampen tornado activity, illustrated in shades of blue, in the boxed region, while La Nina (right), which is shown in red, acts to increase it. (Allen et al., Nature Geoscience, 2015)

So the team turned instead to the ingredients that are necessary for tornadoes to form — things like humidity, winds  and temperature — and found that, in general, years in which El Nino conditions were present in the tropical Pacific were also years of suppressed tornado activity in the U.S. The opposite is also true for La Nina, which tends to increase tornado activity.

[Often-deadly March has had zero tornado reports so far]

The team took what they learned to create the first experimental seasonal forecast for severe storms, which Allen describes in this video:

The seasonal tornado forecast is really focused on an area where they were able to make the strongest connection between tornadoes and El Nino, shown in the box in the images above. Given the current weak state of El Nino, Allen is forecasting a 60 percent chance of an average tornado season, a 30 percent chance of a below-average season, and a 10 percent chance of an above average season.

Experimental seasonal forecast for tornadoes in the central U.S. in 2015. (Dr. John Allen, IRI, Columbia University) Experimental seasonal forecast for tornadoes in the central U.S. in 2015. (Dr. John Allen, IRI, Columbia University)

Given the team’s forecast model, if El Nino was stronger, their chances for a below normal season would be higher.

So does the quiet March so far sway the team on their forecast? Maybe a little, but the quiet start probably has more do with the unrelenting “cold east, warm west” pattern we’ve seen in the U.S. over the past few months. “This [pattern] in turn has reduced low pressure system frequency, which you need to bring the moisture up from the gulf, further reducing the energy available and potential for severe thunderstorms,” said Allen. “If we see a shift to this pattern then things could become active quite quickly.”

The team cautions, though, that an “average” or “below average” forecast doesn’t mean there’s a reason to let down your guard.

“Even the least active years produce 800 tornadoes or more, and so it only takes one outbreak or event to result in serious damage,” said Allen, “hence even in quiet years people should be paying attention to the watches and warnings issued by the Storm Prediction Center and the National Weather Service.”