Time series datasets since 1981 are modeled using climatologically-aided interpolation (CAI), which uses the long-term average pattern (i.e., the 30-year normals) as first-guess of the spatial pattern of climatic conditions for a given month or day. CAI is robust to wide variations in station data density, which is necessary when modeling long time series.
The datasets are available based on either monthly or daily modeling. As with any averaging technique, the yearly average obtained from monthly values may not exactly match that based on daily values. Moreover, since 1 January 2002, the precipitation observations used for daily modeling includes radar measurements, which monthly modeling does not take into account. This may cause dramatic local differences between the two datasets in monthly/annual totals for areas east of the Rocky Mountains.
These datasets use all of the station networks and data sources ingested by the PRISM Climate Group, not just those that have existed for long periods of time. See PRISM datasets for more information; the data available from this page is "AN81m" (monthly) or "AN81d" (daily). Note that individual grids contain non-climatic variations due to station equipment and location changes, stations openings and closings, and varying observation times. These datasets should be considered "stable" (unlikely to change until a major version change). Datasets from 1895-1980 will be found in the Historical Data section.