Anthropogenic Impacts on the Exceptional Precipitation of 2018 in the Mid-Atlantic United States

Flash flooding surged through the old district of Ellicott City, Maryland, on 27 May 2018, turning Main Street into whitewater rapids, upending cars, destroying businesses, and leading to one death (Campbell and Rentz 2018). As of May 2019, damage from the Ellicott City flood had cost $12 million, and the city was considering flood mitigation plans that ranged from $63 to $175 million (Logan 2019). Ellicott City was just one of 12 significant heavy precipitation and flooding events that occurred between May and September of 2018, shattering precipitation records across the region (National Weather Service 2019). Other notable events included 3 June, when eight West Virginia counties declared states of emergency after intense precipitation flooded bridges and washed out roads (Maher 2018), and 21–24 July, when historic rainfall across the Washington, D.C., Metropolitan Area and northern Baltimore County in Maryland resulted in more than a dozen high-water rescues of motorists stranded by flash flooding (Halverson and Samenow 2018). These exceptional heavy rainfall events are consistent with expectations from global warming (Pendergrass 2018) and observed increases in extreme precipitation across the broader northeastern United States (Huang et al. 2017; Hoerling et al. 2016; Frei et al. 2015).We examine the mid-Atlantic states of Pennsylvania, New Jersey, Maryland, Washington, D.C., Delaware, and West Virginia, which all experienced remarkable total and extreme (99th percentile wet days) precipitation in 2018 that contributed to flooding. For the years 1920–2018, 2018 has the highest or one of the three highest January–September total precipitation amounts at 33% and 62% of stations (Fig. 1a), respectively, and the highest or one of the three highest May–September extreme precipitation amounts at 6% and 13%, respectively (Fig. 1b). Spatial-ly averaged, 2018 has the highest total precipitation on record (1-in-99 year event), while extreme precipitation is the fourth highest (4-in-99 year event), shown in Figs. 1c and 1d. Here, we assess the fraction of 2018 total and extreme precipitation risk attributable to anthropogenic forcing using station observations and a large ensemble of climate simulations.