This tabular data set represents the 30 year average (1961-1990) number of days of measurable precipitation per month, compiled for two spatial components of the NHDPlus version 2 data suite (NHDPlusv2) for the conterminous United States; 1) individual reach catchments and 2) reach catchments accumulated upstream through the river network. This dataset can be linked to the NHDPlus version 2 data suite by the unique identifier COMID. The source data for the 30 year average (1961-1990) number of days of measurable precipitation per month was provided by the Environmental Protection Agency and is included with this data set (prism-wetdays-monthly.zip provided by Ryan Hill, EPA, written commun., 2012). Units are days per month. Reach catchment information characterizes data at the local scale. Reach catchments accumulated upstream through the river network characterizes cumulative upstream conditions. Network-accumulated values are computed using two methods, 1) divergence-routed and 2) total cumulative drainage area. Both approaches use a modified routing database to navigate the NHDPlus reach network to aggregate (accumulate) the metrics derived from the reach catchment scale. (Schwarz and Wieczorek, 2016).