The River Forecast Center (RFC) of the National Weather Service, along with cooperating agencies, forecasts natural or unimpaired runoff volumes; that is, observed flows adjusted for upstream water management. This publication, the Guide to Water Supply Forecasting, is meant to assist users in interpreting that forecast effort by unambiguously detailing exactly what adjustments to flow are used and how large those values typically are.
With regard to the magnitude of values, monthly averages are shown that reflect the latest 30 year (1981-2010) period. In the Lower Colorado Basin below Lake Powell, with the exception of the Virgin River, medians are thought to better represent central tendency and are used in lieu of arithmetic averages.