81-2010 Normals Implementation
Implementation of NRCS 1981–2010 Climate Normals
Implementation of the 30-Year Period 1981–2010 Climate Normals is Delayed until October 1, 2012
The NRCS Snow Survey and Water Supply Forecasting Program follows the World Meteorological Organization (WMO) conventional standard for climate normals when comparing current data to historic trends. The WMO has established a 30-year reference period "as it is long enough to filter out any interannual variation or anomalies, but also short enough to be able to show longer climatic trends". Climate normals are updated every ten years, and NRCS as well as other agencies like the National Weather Service are currently in the process of changing from the 1971 – 2000 period to the new 1981 – 2010 period. The most recent 30 year period is said to
better represent the current climatological conditions; that is why a 30 period is used instead of a longer period.
The full implementation of the "new" normals for all NRCS applications is planned for the beginning of the 2013 water year on October 1, 2012. In the meantime, all reports and products including streamflow forecasts for the current water year will continue using the 1971 – 2000 normals. Normal values of both medians and averages will be produced for all parameters and available to the public. Please contact the Idaho Snow Survey Office or watch our home Internet page for more information.
1981-2010 Climate Normals will use a Combination of Means and Median
The values presented as the "NRCS official" 1981-2010 30-year normals, whether daily, monthly, annual or seasonal (April – July period for example), will be the
arithmetic mean (i.e., average)
of the 30 individual yearly data
for the following parameters: streamflow volume acre-feet, reservoir storage acre-feet, SNOTEL and NWS precipitation, and snow depth
(manual snow courses only). SNOTEL sites do not have enough years of automated snow depth to produce reliable normals.
However, the normal values presented for the snow water equivalent (SWE) parameter will be the
statistical median of the 30 individual yearly data for both manual snow courses and daily SNOTEL data. This is a departure from previous years when all parameters used the average (mean) values.
Climate data specialists are nearly unanimous in the opinion that the SWE median better represents a normal condition for a parameter that accumulates and dissipates in a seasonal pattern with great variation. As an example, think of a site that hardly ever has snow water on the ground on June 1. The perception is that the "normal" condition is more or less zero snow for June 1. Now, if you add an extreme year, like June 2011, the arithmetic mean value might end up being a couple inches. In this case the mean does not represent the normal or most expected condition; however, the median which would probably be zero, makes a better normal. In 2001 when the averages were updated, the average Julian start and melt out dates were calculated and used to define the start and end points for the 1971-2000 SWE averages in Idaho. This new method of using medians now provides consistency West-wide.