30-Year Climatic and Hydrologic Normals (1981-2010)

Definition of a Normal

The Snow Survey and Water Supply Forecasting (SSWSF) normal is a measure of central tendency for a data type (such as snow-water equivalent) at a site location, over a 30-year period.  The 30-year interval was chosen in agreement with World Meteorological Organization (WMO) standards.

Depending on the data type, the central tendency measure available may be the median, the average or both.  The SSWSF Program has chosen a default normal with the best representation of central tendency for a particular data type.  The default normal appears in pre-determined reports.

The table lists the data type, the chosen default normal and the availability of either average or median for that data type.

* In general, Median for Lower Colorado, otherwise Average

Data Sources

Snow-water equivalent is determined from the data record of the Natural Resources Conservation Service SSWSF SNOTEL and Snow Course sites.  SNOTEL, which stands for SNOw TELemetry, are automated data collection sites, and Snow Courses are sites where manual measurements are taken. In addition, precipitation data is measured at SNOTEL sites, and snow depth data is measured at Snow Course sites.

Streamflow and reservoir storage normals are determined from data networks available outside of the SSWSF Program.  The sources include Federal, state and private agencies. The normals presented are generated by the SSWSF Program.

Creation Method

• All streamflow normals were determined from the 1981-2010 period of record, regardless of the length of record within that period. 

• For SNOTEL and Snow Course sites with a minimum of 20 years of record, the period of record was used to create the normal. 

• For SNOTEL sites with less than 20 years of record, a ratio adjustment method was applied to the normal of the station with the short period of record, based on the normal of a neighboring site with a long period of record.  Missing years of the 1981-2010 period were not replaced in the historical record with estimates.

• For Snow Courses with less than 20 years of record, a correlation relationship, based on a neighboring station with a long period of record was used to fill in missing years to create the normal.  The estimated missing years are not saved in the historical record.  Normals were not created for Snow Courses with less than 10 years of record.

• Snow depth normals were determined from the period of record at snow courses.  If a snow depth record was less than 10 years, no normal was created.

• Finally, a seven-day smoothing algorithm was applied to SNOTEL snow-water equivalent average and median values in creating the final reported normal. This was done to create a consistent relationship to normal from day to day, without experiencing daily oscillation.

Differences: 1971-2000 and 1981-2010 Normals

Caution is recommended in using the 1971-2000 and 1981-2010 normal sets for climatic comparison, for two important reasons.    First, SNOTEL sites did not exist in 1971.  The first major installation of SNOTEL sites occurred in the late 1970s to early 1980s.  Therefore, data at SNOTEL sites for the 1970s was estimated in the 1971-2000 normals.  Second, the procedures to determine normals for sites that did not have a complete 30-year record were different in the two datasets.

Sites with Significant Disturbance, such as Fire

A double mass analysis was used to determine if the data collection at a site had been altered due to a significant disturbance.  If so, the normal may be created from the shortened period of record using the ratio adjustment method described previously.

If a Site is not Listed

• Most normals have been completed, although some sites are still being reviewed. 

• Sites that have been recently installed will not have a normal.

Frequently Asked Questions