Investigating diverse extreme daily precipitation trends over the eastern and western U.S.

Two people with umbrellas walking in the rain.

A previously unexplained feature of extreme daily precipitation trends over the contiguous United States consists of large increases over the East but decreases over the West. New research led by the Physical Sciences Laboratory, published this month in the Journal of Climate, explores the spatial diversity of trends in extreme daily precipitation observed over the last century. Using new suites of climate model experiments, the authors discovered a spatially diverse pattern of climate change impacts on U.S. extreme daily precipitation trends that closely matches observations. The researchers also found that random weather variability – unrelated to climate change – can itself generate substantial century-long trends in extreme daily precipitation, a factor likely important for explaining unusually large upward trends in the East.

The study found that extreme daily precipitation responds to global warming (over the last century), but differently over the eastern compared to the western United States. A 5–10 percent increase in extreme daily precipitation since 1900 occurs over the East in experiments accounting for historical climate change, but little change occurs over the West. One explanation for this finding is that in addition to water vapor increases that occur in a warmer world and cause widespread precipitation increases, atmospheric circulation change can also enhance or mitigate thermodynamic influences. These findings demonstrate a robust, dynamically-driven reduction in extreme precipitation over the West that largely compensates for water vapor increases that favor increased heavy precipitation. Even on one hundred-year time scales since 1900, these findings contain a cautionary tale for attribution due to potentially large confounding effects from random weather variability. This variability can generate appreciable long-term trends in extreme daily precipitation, acting to either greatly inflate or obscure changes that arise from climate change alone.

This study examines whether the historical trends in U.S extreme precipitation trends are indicative of risks for coming decades. The findings are important for planners and decision-makers, so that adaptation actions are informed by science-based attribution of causes for observed changes.

Hoerling, Martin, Lesley Smith, Xiao-Wei Quan, Jon Eischeid, Joseph Barsugli, and Henry F. Diaz (2021): Explaining the Spatial Pattern of U.S. Extreme Daily Precipitation Change. J. Climate, 34(7), 2759–2775,