Historical Perspective on Hurricane Harvey Rainfall

Hurricane Harvey produced catastrophic rainfall, exceeding 1000 mm (40 inches) in some locations, along the southeast Texas coast during a 5-day period in late August 2017. While point values of rainfall were astounding and approached Probable Maximum Precipitation design values, the real story of Harvey is the areal coverage of extreme rainfall. Over the core area, rainfall averaged 400 mm (16 inches) over an area of about 100,000 km². Our research compared Harvey with other large area, long duration events in the southeast United States. This comparison reveals the extremity of Harvey. For an area size of 100,000 km², Harvey rainfall was greater than any other event since 1949 and 40% greater than the 2^nd ranked event. An analysis of the top 100 events indicates that about a quarter were caused by tropical cyclones. Most of the rest were caused by quasi-stationary fronts associated with extra-tropical cyclones. The estimated return period of Harvey is >1000 years. We evaluated the timing of these large events as related to both Gulf of Mexico sea surface temperatures and greenhouse gas concentrations. When taking these factors into account, the estimated return period of an event like Harvey in 2017 is reduced by a factor greater than 2.