High precipitation in USA January 2024 winter storm mostly strengthened by human-driven climate change

Press Summary (First published 2024/01/23)

• Winter storms akin to those in January 2024 in the Western USA are encountering 6 to 12 mm/day higher precipitation amounts at present compared to the past, spanning much of the Eastern USA and parts of the West Coast. 

• USA January 2024 winter storm was a largely unique event.

• We mostly ascribe the increase of precipitation in the USA January 2024 winter storm to human-driven climate change and natural climate variability likely played a modest role.

Event Description

On January 12, 2024 a depression located over the Rocky Mountains rapidly crossed most of the US, first moving across the Midwest and the South, then surging to the North and affecting the eastern states and Canada. As it moved across the Midwest, the cyclone underwent a rapid intensification, making it a particularly intense storm.  A second depression crossed the US from west to east during the two following days. 

This sequence of cyclones produced a variety of impacts, including extreme winter conditions with blizzards and exceptionally low temperatures from the Northwest to the Midwest, prompting weather alerts for winter severe weather over 17 states, concerning about 70 million people. Road transportation was disrupted in several areas, and thousands of flights were canceled or delayed, while hundreds of thousands of households were left without power during the event. 

During the Western USA January 2024 snowstorm we observe a distinct pattern of Surface Pressure Anomalies with respect to the climatology in correspondence to the path of the cyclones. Negative anomalies stretch along the Rocky Mountains across the South, the Midwest and the Great Lakes, reaching into eastern Canada,  while positive anomalies concern western Canada. Temperature Anomalies up to 20 ºC lower than the climatology are observed from western Canada across the central US. The storm developed in the context of a January of active winter in the US, just a few days after other storms affected several states on January 8-9, and at the peak of a strong positive phase of the El Niño–Southern Oscillation. Precipitation Data show large precipitation totals on the west Coast, and widespread precipitation from the Midwest to the northeast. Windspeed data show windy conditions over most of the central and eastern US, and high winds over the Rockies.

Climate and Data Background for the Analysis

The IPCC AR6 WG1 report discusses the significant impact of climate change on the frequency and intensity of cold outbreaks in North America. According to the IPCC, as the region has experienced an overall increase in average temperatures, a notable shift has occurred in the occurrence of temperature extremes. Specifically, extreme high-temperature records are now being set more frequently than extreme cold records. This shift is attributed to anthropogenic climate change, which has fundamentally altered the temperature dynamics of the continent. The warming trend is not uniform across North America but exhibits pronounced polar amplification, particularly in high latitudes and during winter months.  One of the outcomes of this temperature shift is the reduction in the severity of extreme cold events. The IPCC also highlights that this change is not limited to temperature records alone. It has a broader impact on the region's climate dynamics, including alterations in precipitation patterns, snowpack, and other related climatic variables. These shifts in climate variables have cascading effects on various aspects of North America's environment, such as changes in snowpack and glacier extent, reductions in sea and lake ice. This shift carries profound implications for various sectors, from agriculture to infrastructure planning, as North America adapts to this changing climate landscape.

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 January 2024  USA winter storm, we have low confidence in the robustness of our approach given the available climate data, as the atmospheric pattern that led to the cold spell is largely unique in the data record.

ClimaMeter Analysis

We analyse here (see Methodology for more details) how events similar to the USA January 2024 Snowstorm have changed in the present (2001–2023) compared to what they would have looked like if they had occurred in the past (1979–2001)  in the region [-130°E -60°E 20°N 65°N].  Surface Pressure Changes show limited areas of negative changes over the Gulf coast and positive changes on the northeastern US, both around 5 hPa. Temperature Changes show that similar events have not changed overall, besides temperatures up to 5 °C warmer over Florida and parts of the west Coast in the present than in the past. Precipitation Changes show that present events are 6 to 12 mm/day wetter than in the past in the most affected areas, while Windspeed changes show 6 to 12 km/h less windy conditions in the present than in the past over the Midwest. All combined, these changes suggest that these events have slightly changed in a way consistent with expectation in a warming climate, with higher precipitations and warmer temperatures, although only in limited or very limited areas. We also note that Similar Past Events have become more common in February than in January, as opposed to what happened in the past. The analysis for Changes in Urban Areas shows that Denver has been less windy, Chicago drier, and Miami wetter, warmer and less windy than what they would have been in the past in similar events.

Finally, we find that sources of natural climate variability, notably the Pacific Decadal Oscillation, may have heavily influenced the event. This means that the changes we see in the event compared to the past may be influenced by the natural climate variability.

Conclusion

Based on the above, we conclude that Winter storms resembling the January 2024 Snowstorm in the Western USA are experiencing 6 to 12 mm/day higher precipitation levels in the current timeframe compared to historical records, spanning much of the Eastern USA and portions of the West Coast. Additionally, these storms exhibit comparable temperatures now as they did in the past. We interpret this winter storm as a largely unique event whose characteristics can mostly be ascribed to human driven climate change.

Contact Authors

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

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