High Precipitation and Strong Winds in the April 2020 USA Tornado Outbreak mostly strengthened by human-driven climate change

Press Summary (First published 2023/12/05)

• Low pressure systems similar to that producing the April 2020 USA Tornado Outbreak are 2-8 mm/day wetter and 2-5 km/h windier in the present than they have been in the past.

• April 2020 USA Tornado Outbreak was a somewhat uncommon event.

• We ascribe the strengthened large-scale winds and higher precipitation associated with April 2020 USA Tornado Outbreak to human-driven climate change, and natural climate variability likely played a modest role. 

Event Description

On Easter Sunday and Monday, April 12–13, 2020, a severe tornado outbreak struck the Southeastern United States, marking a significant meteorological event with 141 confirmed tornadoes, ranking as the fourth most prolific tornado outbreak within a 24-hour period on record. It brought widespread and locally catastrophic damage across 10 states, and caused 32 fatalities, making it the deadliest tornado outbreak since 2014. The outbreak was preceded by an unusually persistent high pressure over the the Southeast, leading to record-setting warmth along the Gulf coasts and increased instability from warm, moist air sourced from the Gulf of Mexico, which provided the fuel for severe storm development.

The outbreak was driven by a deep trough approaching from the West and inducing the formation of a surface low pressure system over the Southwestern states, which moved across the South of the US ahead of the main trough. This is a typical set-up for severe weather outbreaks, and it was well foreseen by the Storm Prediction Center starting on April 8, when the risk for severe weather was first outlined. On April 12, severe thunderstorms were initially organized in quasi-linear mesoscale convective system, but as they entered the favorable environment characterized by high energy and strong wind shear over the Southeast, storm rotation became dominant. Significant to violent (EF3-EF4) tornadoes were reported in Lousiana and Mississippi, with estimated winds up to 270-330 km/h, causing several casualties. The outbreak continued on April 13, shifting into Georgia and the Carolinas, with well-organized supercells bringing severe impacts, including an EF4 tornado in South Carolina. This deadly outbreak prompted governors to declare a state of emergency in five states, and relief efforts were further complicated by the ongoing COVID-19 pandemic. In summary, the 2020 Easter tornado outbreak left a lasting impact, underscoring the challenges posed by extreme weather events, especially when compounded by societal factors like the pandemic.

The pressure pattern associated with the 2020 USA "Easter" Tornado Outbreak displays clear negative Surface Pressure Anomalies over the analyzed domain, while Temperature Anomalies show a gradient from negative to positive values from West to East of the region. Precipitation data indicate large amounts of rain over Tennessee, Kentuky and Virginia, mainly to the North of the area affected by the tornado outbreak and likely associated with larger convective systems with different features compared to the supercells that produced the violent tornadoes over the South. Windspeed data show sustained winds associated with the large-scale low pressure system, although they cannot capture the impacts associated with tornadic storms, too small and short-lived to be represented in the dataset. 

Climate and Data Background for the Analysis

The IPCC AR6 WGI FR - Pages 1611,1614 states that in a warming world, the average and maximum rain rates associated with severe convective storms increase in some regions, including the USA, with high confidence. Climate models consistently project environmental changes that would support an increase in the frequency and intensity of severe thunderstorms that combine tornadoes, hail, and winds in the USA, with high confidence. However, there is significant uncertainty about projected regional changes in tornadoes, hail, and lightning due to limited analysis of simulations using convection-permitting models, with high confidence. The frequency of severe convective storms in spring is projected to increase in the USA, leading to a lengthening of the severe convective storm season with medium confidence. Overall, while there is high confidence in the increase of severe convective storms in the USA, there is low confidence in the details of the projected increase of tornadoes and hail .

Our analysis approach rests on looking for weather situations similar to those of the event of interest having been observed in the past. For the April 2020 USA Tornado outbreak we have medium-high confidence in the robustness of our approach given the available climate data, as the event is similar to other past events in the data record.

ClimaMeter Analysis

We analyse here (see Methodology for more details) how events similar to the April 2020 USA Tornado Outbreak have changed in the present (2001–2022) compared to what they would have looked like if they had occurred in the past (1979–2000) in the region [-100°E -80°E 30°N 43°N]. The Surface Pressure Changes show that the pressure over the storm area has not changed significantly, except for some limited areas at the edge of the considered region. Temperature Changes show no significant changes. Instead, Precipitation changes and Windspeed Changes show that similar events produce higher precipitation (0-8mm/day) and stronger large-scale winds (0-5 km/h) in the present than in the past. This suggests that equally deep low pressure systems in the area produce more intense phenomena, which could also reflect on a stronger severe weather activity associated with convective hazards including tornadoes, although our analysis cannot diagnose directly an increased occurence of these phenomena.  We also note that Similar Past Events have become more common in the month of April, while they previously occurred more in March and May. The Changes in Urban Areas show that present events are 2-12 mm/day wetter and 0-4 km/h windier in Jackson and Augusta while no changes are detected for Atlanta urban area. 

Finally, we find that sources of natural climate variability did not influence the event. This means that the changes we see in the event compared to the past may be primarily due to human driven climate change.

Conclusion

Based on the above, we conclude that low pressure patterns similar to that causing the 2020 USA "Easter" Tornado Outbreak are 2-8 mm/day wetter and 2-5 km/h windier in the present than they have been in the past. We interpret the 2020 USA "Easter" Tornado Outbreak as an event whose characteristics can be ascribed to human driven climate change.

Contact Author

Davide Faranda, IPSL-CNRS, France  📨davide.faranda@lsce.ipsl.fr  🗣️French, Italian, English

Flavio Pons, IPSL-CNRS, France  📨flavio.pons@lsce.ipsl.fr  🗣️Italian, English, French