Who Received the Most Rain Today? An Analysis of Daily Precipitation Extremes in the Contiguous US using CoCoRaHS and COOP Reports

Every day, thousands of volunteers across the United States (US) report the amount of precipitation they have received in the past 24 hours. This study focuses on the largest of these volunteer-submitted reports for each day, using the precipitation measurements from the Community Collaborative Rain, Hail, and Snow Network (CoCoRaHS) from January 2010 to December 2017 as well as observations from the United States Cooperative Observer (COOP) Network from January 1981 through December 2017. Results provide clarity on the spatial and temporal variability, and the seasonal cycle of daily Contiguous US (CONUS) precipitation extremes. Sometimes, the daily CONUS precipitation extreme is modest. At other times, it is record-breaking, and even life-threatening. The highest daily rainfall extreme in this study was 58 times as large as the lowest. Coastal states are most prone to high daily precipitation totals, especially those bordering the Gulf of Mexico or Atlantic Gulf Stream. The average intensity of daily CONUS precipitation extreme events varies with the seasons; it is greater than seven inches in late August, and less than four inches through meteorologic winter. The observations also suggest that the location of daily extremes is seasonally variable. For example, 28.5% of February extremes, and 0% of August extremes, fall in Pacific states. These findings demonstrate that there is strength in numbers. The CoCoRaHS and COOP networks together provide a powerful tool for capturing extremes, and for taking a robust spatial sample of storms that produce extremes, such as an intense thunderstorm complex or major hurricane.