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Climatic and Hydrologic Normals

This page reviews the NRCS normals, describes how normals are calculated, and provides access to current and historic normals.

The Snow Survey and Water Supply Forecasting (SSWSF) normals are site-specific measures of central tendency (either the median or average) for a data type, such as snow water equivalent (SWE). The statistics are calculated over a 30-year period and updated each decade, in agreement with World Meteorological Organization (WMO) standards. This 30-year reference period was chosen to characterize the current hydroclimatology at each station. The most recent medians and averages have been updated to include data for the water years 1991-2020. The National Water and Climate Center (NWCC) also provides medians and averages for the 1981-2010 and 1971-2000 reference periods for stations with sufficient data. The normals available from the NWCC include the median and average for SWE, snow depth (snow courses only), precipitation, volumetric streamflow, and reservoir storage. Values are calculated from data collected by NRCS-managed stations and external agencies such as the U.S. Geological Survey (USGS), National Weather Service (NWS), state agencies, and private organizations. Normals are calculated for various durations including daily, month-to-date, semi-monthly, monthly, seasonal, and annual based on the data type.

1991-2020 Climatic and Hydrologic Normals

1991-2020 Normals Overview
Calculation Methods
Differences Between Current and Previous Normals
Median vs. Average

Retrieving 1991-2020 and Historic Normals

NRCS normals are available from several NWCC applications. Following are some of the primary sources for retrieving these data. The Report Generator can be used to view 1991-2020, 1981-2010, and 1971-200 normals (where available), for all data types at single or multiple sites. The Interactive Map can be used to view normals for multiple sites on a date of interest.


Seasonal Streamflow Normals Comparisons for NRCS Forecasted Stations

Caution is recommended when making inferences from comparisons between the old and new reference periods. A shift in normals may occur for several reasons including: 1) change in underlying data due to different 30-year reference periods; 2) change in calculation methods. View comparisons on the page SRVO Normals Comparison.