La Niña could supercharge this year’s tornado season, just as it did with lethal effect in 2011

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The last few months have seen the strongest La Niña signal since the winter of 2010-2011. The question then is whether this spring continues to reflect this year, which turned out to be the costliest ever for tornadoes and the deadliest in nearly 100 years.

“The extreme weather season is actually a collection of several short weather events, and anticipating individual events at long time frames is usually tricky,” said Sam Lillo, atmospheric researcher at the University of Colorado at Boulder.

“What we can say instead is whether the probability of the ingredients coming together for these events is above or below normal: this year it is above normal.”

The deadliest tornado season in modern history

The remarkable tornado season of 2011 was the deadliest in modern times, with more than 550 deaths – the second only after a total of 794 tornado deaths in 1925.

Almost all of the deaths in 2011 occurred during the extremely active months of April and May. This April alone saw 875 tornadoes confirmed, more than any month on record. The April 27 Super Outbreak recorded 226 tornadoes, the most tornadoes ever seen in a single day, including destructive tornadoes in Birmingham and Tuscaloosa, Alabama.
Volunteers search homes destroyed on April 30, 2011 in Tuscaloosa, Alabama.
Weeks later, Joplin, Missouri was struck by a large-scale EF5 tornado that killed more than 160 people. It was the deadliest tornado in over 60 years, as well as the costliest tornado on record, with nearly $ 3 billion in direct damage.

“Looking back on 2011, it was the scale of the number of events, the fact that so many hit populated areas and, of course, the incredibly high toll in terms of deaths, injuries and damage in dollars, ”said Bill Bunting, chief forecasting operations for the Storm Prediction Center (SPC) of the National Weather Service.

While localized and small-scale weather characteristics played a role in setting up these tragic two days, the large-scale global weather patterns that fueled the historic 2011 tornado season are worth examining to determine the risk. for similar days this year.

“Every year there is some potential (for tornado outbreaks); it’s just a matter of trying to accurately predict, with as much time as possible, where that area is likely to be, and then s ‘Make sure people are prepared and have a plan, “Bunting says.

A person observes the damage a day after a tornado swept through Joplin, Missouri, killing dozens of people on May 23, 2011.

Active forecast for this spring

To paint a picture of what weather conditions might look like in the coming weeks, forecasters look to La Niña and other global climate and weather models, such as the Arctic Oscillation (which is different from the polar vortex), to develop what are called sub-seasonal forecasts.
Lillo uses one of these models that “focuses on the slow, predictable parts of the atmosphere” to create forecasts several weeks in advance. Forecast models like this are important for things like seasonal temperature forecasts, which are used in energy trading markets, and hurricane season forecasts released annually by the National Oceanic and Atmospheric. Administration (NOAA).

Lillo’s model recently predicted – a month in advance – the Arctic epidemic that hit the central United States in February. Now, attention is turning to what these long-term patterns might reveal as we head into severe storm season in the spring.

“In general, the forecast shows peaks with above normal temperatures in the south, cooler in the north, and this temperature gradient improving the jet stream across the central United States,” Lillo said. (The jet stream is the main storm track through mid-latitudes of the northern hemisphere and divides cooler air in the north and warmer air in the south.)

During La Niña, larger temperature differences tend to develop between warm, humid air in the southern United States and cooler, drier air in the north. This sets up a faster jet stream which can cause severe weather epidemics.

“The faster jet stream holds all the potential for more powerful storm systems and extreme weather conditions,” Lillo said.

In March, the southern United States is historically the area where severe storms, including tornadoes, are most likely. Then, as the northern hemisphere begins to warm, the target of the tornadoes will move west into the central United States and eventually north into the northern plains next summer.

“The jet stream setup is not unfavorable for extreme weather conditions as we arrive a little later in March and certainly beyond,” Bunting said. “If this trend continues, very strong wind fields on the Gulf Coast near warm, humid air suggest that the Gulf Coast in the near term may be an area to watch closely.”

How La Niña relates to tornadoes

Like this year, a moderate La Niña was the main feature in 2011. La Niña, and its counterpart, El Niño, can play an important role in the position of the jet stream, temperatures and precipitation over the United States. , which all play a role in the formation of bad weather.

El Niño or La Niña conditions during the winter months can be used to help identify the frequency of tornadoes during the peak of the severe storm season in the spring, according to recent studies.

“The flow of warm, humid air from the Gulf of Mexico increases in force during the springs following La Niña, which produces the fuel needed to form storms,” said Jason Furtado, assistant professor of meteorology at the University of the ‘Oklahoma.

“The higher flow increases low-level wind shear which also promotes the formation of tornadoes and hailstorms.”

The past few months have featured the strongest La Niña since 2011, and this pattern is expected to continue to impact weather patterns over the next few months, in the midst of the inclement weather season, according to the NOAA Climate Prediction Center. .

The time to prepare is now

While severe storms occur year round in the United States, the peak time for severe storm outbreaks is in the weather spring, which includes March, April, and May.

So far this year there have been only 27 tornado reports, which is well below normal. Over the past 15 years, the United States has recorded an average of 130 tornado reports in the first days of March.

But it’s not just tornado reports that are on the decline. Reports of hail and damaging winds are also below average so far this year.

“There have been many seasons that started off quietly and did the exact opposite,” Bunting said.

The year 2011 also started off below average for tornado and hail reports for much of March, according to data compiled by the CPS, before accelerating rapidly through the rest of the spring.

The upcoming forecast depends on the destination of the jet stream in the coming weeks.

Now is the time to revisit your severe weather plans. Know where to find your daily local forecast. You have several ways to get severe storm and tornado warnings and alerts: via wireless emergency alerts, NOAA weather radio, local news stations and more. And know where to take shelter at home, at work or at school if a tornado hits.

Advised Bunting: “This is the time of year when we need to start thinking a bit more about the potential for severe storms and developing that pre-event planning.”

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