The index of runoff potential before a storm data is the antecedent runoff condition (ARC). ARC is an attempt to account for the variation in CN at a site from storm to storm. The CN for the average ARC at a site is the median value as taken from sample rainfall and runoff data. For more information on the CNs for the average ARC, which is used primarily for design applications, see "Runoff Curve Number"-426. See the National Engineering Handbook (U.S. Soil Conservation Service 1969) and TR-55, Urban Hydrology for Small Watersheds (U.S. Soil Conservation Service 1986b) for more detailed discussion of storm-to-storm variation and a demonstration of upper and lower enveloping curves.