National Water, Climate Center releases new snowpack and precipitation normals
Charts provided by the United States Department of Agriculture Natural Resources Conservation Service and National Water and Climate Center
Charts provided by the United States Department of Agriculture Natural Resources Conservation Service and National Water and Climate Center
Charts provided by the United States Department of Agriculture Natural Resources Conservation Service and National Water and Climate Center
Charts provided by the United States Department of Agriculture Natural Resources Conservation Service and National Water and Climate Center
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WASHINGTON, D.C. — The Snow Survey and Water Supply Forecasting (SSWSF) Program of the USDA has published new 30-year hydroclimatic normal for snowpack and precipitation from western U.S. monitoring stations, including Flathead Lake.
These “normals” are specific to each site, and measure either the median or average for each data type. Calculated over a 30-year period, these statistics are updated each decade. Most statistics include data from 1991-2020. Also included are medians and averages for external agency reservoir storage and streamflow volumes used for water supply forecasting.
Gathered by automated SNOTEL (snow telemetry) stations at hundreds of sites, the data helps inform decisions for water managers, reservoir operators, and other groups.
The Washington Department of Ecology explains “many communities rely on a snow-fed water supply to provide safe and clean drinking water. The irrigated agriculture industry, which helps drive the local and state economy, relies on water to irrigate crops. That same water also feeds rivers and streams that support salmon.”
However, updated calculation methods for 1991-2020 may contribute to differences between current and previous normals. Additionally, the underlying data used to compute statistics may not be the same between two 30-year periods.
Calculations for the normals of 1991-2020 do not use any estimation techniques. Normals for the previous decade, however, used estimation methods for sites and snow courses with less than 20 years of data. A seven-day smoothing algorithm was also applied to the previous decade’s daily average and median, while normals for this decade are not smoothed for any data type or duration. 1981-2010 also required serially completed data for daily normals, meaning omission of an entire year of normal calculations of a site could occur if any data from that year was missing. 1991-2020 normals do not require serial completeness. Finally, the method for calculating peak Snow Water Equivalent (SWE) normals differs in 1991-2020 by calculating the average and medians for a subset of seasonal peaks for SWE and the corresponding date determined each year during the reference period, as opposed to being derived as the maximum value from the set of daily SWE medians as was done for 1981-2010 normals.
Caution is recommended when making inferences when comparing reported normals from 1991-2020, 1981-2010, and 1971-2000.
