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Climatic and Hydrologic Normals

Current and historic 30-year normals calculations are updated every ten years. This page reviews the most current normals, how normals are calculated, and more information.

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

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

Comparing 1981-2010 and 1991-2020 Normals

The Interactive Map provides access to charts comparing the official normals used for 1981-2010 with the official 1991-2020 normals. At each of the links below, click on a station or basin of interest, and in the resulting popup follow the link labeled ‘1991-2020 vs 1981-2010 Normals’. Comparisons for SWE and some streamflow locations will show 1981-2010 median and 1991-2020 median. All other comparison charts show the 1981-2010 average and 1991-2020 median. As noted above, caution is recommended when making inferences from comparisons between the two reference periods.

SNOTEL SWE and Accumulated Precipitation

Snow Course SWE and Snow Depth

Streamflow Volume

Reservoir Storage

Basin-wide SNOTEL SWE

Basin-wide SNOTEL Precipitation