MARFC is in the process of updating the monthly county precipitation "normals" used in our precipitation departure maps and other products and displays.
Background: The National Climatic Data Center (NCDC) computes new normal precipitation values for official observing stations every ten years, based on the previous thirty years of data. NCDC's most recently released station normals are for the period 1981 through 2010. However, development of analogous values for geographic or political areas (counties, hydrologic basins, etc.) has been at the discretion of each River Forecast Center, depending on the requirements of users within their respective service areas. In past years, a variety of methodologies, datasets, and update schedules have been used in the development of MARFC's areal normals. Our previous set of normals, developed approximately ten years ago, included data from both official and unofficial precipitation gauges with periods of record ranging from only a few to as many as 50 years of observations. The areal averaging method involved a coarse grid sampling of plotted maps, hand-contoured to account for reasonableclimatic and terrain variability between the observational sites.
New updated normals: MARFC's new set of areal normals is based on a more objective approach developed by the PRISM (Parameter-elevation Regressions on Independent Slopes Model) Climate Group at Oregon State University, in part funded by the National Oceanic and Atmospheric Administration and the U. S. Department of Agriculture. PRISM data uses 1981-2010 point estimates of precipitation originating from multiple sources which are quality controlled by NCDC and the PRISM Climate Group. A high resolution (800-meter) gridded dataset is generated using regression analysis based on the point estimates, topography, and other localized factors. PRISM data are well suited to regions with mountainous terrain, because the model incorporates a conceptual framework that addresses the spatial scale and pattern of orographic process. PRISM data are increasingly being used as the National Weather Service's standard for geo-spatial analyses of precipitation, temperature, and other climatic data. You can visit http://www.prism.oregonstate.edu/ for more information on PRISM.
In general, the new set of MARFC county normals are not drastically different from the previous set (average difference = 4%), but the values do vary geographically and seasonally. Any differences between the old and new set should be interpreted with caution, as the effects attributable purely to the different methodologies are difficult to quantify and could be significant. Drawing conclusions regarding climatic trends is not recommended.
As in previous updates to MARFC's precipitation normals, the new values are being introduced into our departure computations one month at a time in order to minimize any potential discrepancies with maps and products containing longer duration accumulations. The monthly implementation began in January, 2013, and will be completed in December.