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About the Precipitation Analysis Pages

These pages graphically show the short-term observed and climatic trends of precipitation across the lower 48 United States (CONUS) and Puerto Rico.

Observed Precipitation

"Observed" data is derived from output from 12 National Weather Service River Forecast Centers (RFCs), and is displayed as a gridded field with a spatial resolution of 4x4 km. "Observed" data is expressed as a 24-hour total ending at 1200 Z (same as Greenwich Mean Time, or GMT), with longer periods simply being a summation of multiple 24-hour periods. 1200 GMT is used as the ending time for a 24-hour total, because it is the end of the "hydrologic day", a standard used in river modeling. Additionally, 1200 GMT closely coincides with the reporting time for most of the National Weather Service’s cooperative observers, whose data are used as a quality control on the dataset. 1200 GMT coincides with 8 AM EDT, 7 AM EST, 7 AM CDT, 6 AM CST, etc.

When viewing "ALL RFC DATA" you may notice that "Observed" data extends well beyond the U.S. border, most notably north of Washington and Idaho and west of Texas. Several RFCs located in the CONUS have service areas that extend beyond the U.S. border, in order to model rivers that flow into the United States. Examples include the Columbia River in the Pacific Northwest and tributaries of the Rio Grande along the Texas-Mexico border. Although no forecasts are provided outside of U.S. boundaries, precipitation estimates are created over these areas in order to simulate streamflow along these rivers as they cross into the United States. When viewing "ALL RFC DATA" it may be helpful to turn on the "RFC Boundary" overlay as a geographic reference.

Normal Precipitation

"Normal" precipitation is derived from PRISM climate data, created at Oregon State University. The PRISM gridded climate maps are considered the most detailed, highest-quality spatial climate datasets currently available. The 30 year PRISM normal from 1971-2000 is used for precipitation analysis since 2004. Prior to 2004 the 30 year PRISM normal from 1961-1990 is used.

Puerto Rico PRISM data comes from a separate project by the International Institute of Tropical Forestry (Ref: Daly, C, E.H. Helmer and M. Quiñones. 2003. Mapping the climate of Puerto Rico, Vieques and Culebra. International Journal of Climatology 23:1353-1381.) PRISM data for Puerto Rico covers the time period 1963-1995.

The PRISM data is expressed as a monthly normal rainfall. For durations less than one month, the value for that month is divided by the total days in that month and multiplied by the number of days in the selected field. For example, a 7-day normal for January 10th (ending at 1200 GMT) would be 7/31 of the total normal rainfall for January, while a 14-day normal would be 9/31 of January’s normal plus 5/31 of December’s normal.

Derived Precipitation Products

"Departure from Normal" and "Percentage of Normal" graphics are generated by simple grid mathematics, where the "Normal" dataset is respectively subtracted from or divided into the "Observed" dataset.

Observation Methods

East of the Continental Divide, RFCs derive the "Observed" precipitation field using a multisensor approach. Hourly precipitation estimates from WSR-88D NEXRAD are compared to ground rainfall gauge reports, and a bias (correction factor) is calculated and applied to the radar field. The radar and gauge fields are combined into a "multisensor field", which is quality controlled on an hourly basis. In areas where there is limited or no radar coverage, satellite precipitation estimates (SPE) can be incorporated into this multisensor field. The SPE can also be biased against rain gauge reports.

In mountainous areas west of the Continental Divide, a different method is used to derive the "Observed" data. Gauge reports are plotted against long term climatologic precipitation (PRISM data), and derived amounts are interpolated between gauge locations. The following link provides more information about the process and program used to derive observed precipitation for the western U.S.

Quality of Data

"Observed" data, and by extension the derived data, is vulnerable to inaccuracies that can be caused by either radar or precipitation gages. For radar, problems would include freezing or frozen precipitation, low topped convection, bright banding, the reflectivity/rainfall relationship in use, calibration of the radar, radar location and elevation, range degradation, and the radar's effective coverage. For precipitation gages, problems come from freezing precipitation, windy conditions, gage siting, undermeasurement by tipping bucket gages in high intensity rainfall, and gage maintenance.

Production / Update Times

The precipitation analysis pages are routinely update three times per day, at approximately 9:30am, 12:30pm and 4:30pm Eastern Standard Time. Data for the western U.S. are usually available by the second update. The data for the first two updates are preliminary and subject to change. While the data in the final update are much less likely to change, they are neither official nor certified. Please contact the National Climatic Data Center for certified past weather information.

Data Formats

The precipitation fields are provided in GIF format for viewing, and shapefile and netCDF formats for download and use in other projects and research. More information about using netCDF files is available from the University Corporation for Atmospheric Research (UCAR).