Will La Niña Trigger a Dangerous Spring Severe Weather Season?

 Jonathan Belles

Published: February 19, 2021

Tornadoes may be more common in early spring this year due to the ongoing La Niña.

On average, tornado potential moves northwestward from the winter months to the Plains states where the potential maximizes in May as the juxtaposition of heat, humidity and wind shear overlap in Tornado Alley.

But ingredients that are known to produce tornadoes come together in a distinctly more northern location in La Niña springs according to research published in the Journal of Applied Meteorology and Climatology in 2017.

La Niña is the periodic cooling of waters in the eastern equatorial Pacific that is known to change seasonal weather patterns throughout the world. El Niño is the warming of these same waters, which leads to different effects on weather patterns.

Several trends in severe weather ingredients were examined during both El Niño and La Niña seasons: jet stream level winds, low-level winds at around 5,000 feet, and instability.

Severe weather is possible when large storm systems have these ingredients, among others, available to them.

The first of these ingredients – wind shear – is most frequently available as we transition from winter to spring and the jet stream begins its typical retreat northward but remains strong. The stronger the winds at 30,000+ feet, often, the more wind shear there is.

According to the study, the jet stream is often farther north and more amplified in La Niña conditions. This northward alignment encourages the development of thunderstorms farther north and west, which creates the opportunity for more tornado outbreaks.

Typical axes for the jet stream in La Niña (blue) and El Niño (red) during March and April

Closer to the ground, the amount of heat and turning of the atmosphere is important for tornadogenesis, the birth of tornadoes.

In La Niña conditions, the strength and placement of the strong winds at less than a mile high in the atmosphere are also more favorable. These winds, often called low-level jets, are instrumental in creating tornadoes by causing air to tumble or roll near the ground.

The study suggested that these winds were stronger over Tornado Alley in La Niña during April, while considerably weaker in El Niño and often much farther east, as seen by the arrows below.

The study also found that the energy that allows for the lift needed for a thunderstorm's updraft growth in La Niña is focused more in the Southeast - in so-called Dixie Alley - during March, and in the Plains states - what is typically known as Tornado Alley - in April. These alleys are locations that are climatologically-favored for severe weather.

The location of maximized lift, from northern Texas to southern Iowa (not shown), occurs near the low-level jet axis (blue arrow below) in a favorable spot to work with the low level winds. That location coincides with Tornado Alley and raises the chances of tornadogenesis in La Niña.

In El Niño conditions, however, the energy is more spread out and more minimal in March. This source of lift is bunted to the south in April.

Low Level Jet Stream Axes (arrows) and Areas of Lift (shaded) in April

These characteristics of a La Niña severe weather setup won’t occur in every La Niña year, nor do these setups tell us how much of these ingredients will be available.

Based on past decades, the chance for tornadoes typically increases from February into March in the South, and is highest from April through June across the Plains.

Could We Get An Active Severe Weather Spring This Year?

So far in 2021, the signature of a La Niña-amplified severe weather season has not shown up.

This is especially true in the first half of this February. The widespread arctic outbreak has pushed the storm track considerably further south, at times, into the Gulf of Mexico. This has also diminished the size of the warm sector and placed it over the northeastern Gulf of Mexico. This pattern is more in line with the El Niño pattern shaped by the study.

Two lower-end severe weather events in recent weeks have produced tornadoes in Florida, Alabama, Georgia and North Carolina.

Water temperatures remain on the cool side in the equatorial Pacific, especially in the Central Pacific. A moderate La Niña continues, as of Feb. 15, with water temperatures around a degree cooler than average.

This Central Pacific-based La Niña is atypical for a La Niña in location alone, but this displacement could also be a supporting actor in why the severe weather signatures found by the authors have not shown up.

According to outlooks provided by NOAA and IBM, a slow warming of the eastern Pacific Ocean should lead to more neutral-like oceanic conditions by late spring.

IBM also hints at a weather pattern, as opposed to a climate pattern like La Niña, that would be supportive of severe weather in March. In the latest outlook, released on Feb. 11, they suggest warmer than average conditions in the Central and Eastern U.S. with a trough possible over the western U.S. Intermittent bursts of colder air are possible in the Plains and South during this time, but the overall setup suggested by IBM is supportive of bouts of severe weather.

(MORE: IBM's Spring Outlook)

While we cannot pinpoint whether there might be an abnormal increase in severe weather this spring, we can say that, on average, severe weather is slightly more likely in a year with a similar oceanic pattern to this one.

Stay tuned as we head for the typical peak of severe weather season, which occurs in the late spring months.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.