2021 tornadoes across the US below average
Forecasters predicted a more active season
There were fewer tornadoes than average in the U.S. this spring storm season, despite some predictions that it could be an active season based on La Nina ocean temperature conditions.
"Tornado activity — and, more broadly, severe thunderstorm activity — for 2021 has been lower than average after a somewhat late start," said John Allen, assistant professor of meteorology at Central Michigan University.
"La Nina was observed this past winter and was predicted to continue into the spring, leading to expectations and speculation in the media about a potentially active season," he said in a blog at climate.gov.
"La Nina’s influence is linked to a higher frequency of tornadoes in the spring. However, although La Nina conditions were present through April 2021, the year so far has recorded below-average tornado counts."
A preliminary total of 580 tornadoes were reported through May 31, less than the anual average of 676 over the past 20 years, he said.
In February, AccuWeather, a private forecasting company, published a story saying this year could "rival one of the most notorious severe weather seasons ever," based on a strong La Nina pattern in 2011. There were more than 800 tornadoes in April 2011.
That year also included several devastating tornadoes, including the Tuscaloosa–Birmingham storm in Alabama that killed 64 people and the Joplin, Missouri, tornado that killed 158.
"Now that we are through the peak of the season, discussion among forecasters has turned toward deliberating over the low tornado frequency in what many predicted to be an active season," Allen said in his blog post.
"La Nina certainly does not always increase tornado likelihood," he said.
"Like all climate influences on the weather scale, you can think of La Nina weighting the dice in favor of tornado events, while on the flipside, El Nino reduces the overall likelihood. In any one case, however, the specific atmospheric pattern on shorter timescales plays an important role," he said.
El Nino and La Nina are the warm and cool phases, respectively, of a recurring climate pattern across the tropical Pacific Ocean, known as ENSO — or the El Nino-Southern Oscillation.
So far this year there have been 25 tornadoes in Oklahoma, according to preliminary data from the National Weather Service. The state averages 57.2 tornadoes per year, with the most — 11.8, 24.3 and 7.3 on average — occurring in April, May and June, respectively.
The state averages 8.9 tornadoes between July and December.
"Open questions also remain about how long ENSO’s influence persists into the spring months and how robust its influence on the atmosphere is, factors that may have played a role in this season’s evolution," Allen said. "It also suggests that greater caution is needed to understand the other processes at play before suggesting any given La Nina year will be the next 2011."
Climate.gov is part of the National Atmospheric and Oceanic Administration, the parent agency of the National Weather Service and agencies related to ocean, climate and drought study.