Seasonal Trends and Changes of Extreme Precipitation Events over the Continental United States

Understanding current and projected trends in extreme precipitation, and climate model capability to simulate such events, is essential for evaluation of present and future climate change impacts on agriculture, life, and property throughout the United States. While several studies have examined annual trends in extreme precipitation, little is known about seasonal trends and projections. Annually, the Extreme Precipitation Index (EPI) shows that over the contiguous U.S. (CONUS), as a whole, extreme precipitation events have been increasing in frequency over time but with large disparities among various regions in the U.S. We expand on our previous study (Janssen et al., Earth's Future, 2014) by evaluating trends in seasonal extreme precipitation events, for the CONUS, from observations and available historical and projected simulations from CMIP5 climate models. Based around variations of the EPI, seasonal changes and trends are examined using two methods. The first method looks at each year as a whole and tracks where within each year the changes in frequency of extreme precipitation events are happening. This determines within which season(s) the observed and simulated annual changes are occurring. Using this method we discovered that increases in events during the summer give the greatest contribution to the annual increase in the Midwest region, with fall showing the greatest increase for both the Northeast region and for the CONUS overall. The second method, rather than considering all the seasons collectively for each year, tracks changes individually by analyzing each season directly relative to itself throughout the time period. Thus, summer is compared to previous summers, winters compared to previous winters, and so on. This shows that each season, when taken in relation to itself, has individually experienced an overall increasing trend in extreme precipitation events over the last century. Model performance was also analyzed by season showing similar results to those seen annually. This presentation will examine the findings of historical and projected seasonal trends for extreme precipitation over the CONUS, with the objectives of identifying trends and evaluating climate model performance on a seasonal timeframe.