30-Year Normals Resources
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.
| Data Type |
Default Normal |
Average Available? |
Median Available? |
| Precipitation |
Average |
Yes |
Yes |
| Snow Water Equivalent |
Median |
Yes |
Yes |
| Snow Depth |
Median |
Yes |
Yes |
| Streamflow |
Location Dependent* |
Yes |
Yes |
| Reservoir Storage |
Average |
Yes |
No |
*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 Snow Telemetry (SNOTEL) and snow courses. SNOTEL are automated data collection sites, and snow courses are sites where manual measurements are taken. In addition, precipitation data are measured at SNOTEL sites, and snow depth data are measured at snow courses.
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 courses 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.
- 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 1981-2020 Normals
Caution is recommended in using the 1971-2000 and 1981-2010 normal sets for climatic comparison, for two important reasons:
- 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 were estimated in the 1971-2000 normals.
- 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
A double mass analysis was used to determine if the data collection at a site had been altered due to a significant disturbance, such as fire. If so, the normal may be created from the shortened period of record using the ratio adjustment method described previously.
