Alternative Precipitation Normals Based on NEXRAD Quantitative Precipitation Estimates

A climate "normal" of a particular variable (e.g., daily average temperature, monthly average precipitation) is traditionally defined as a 30-year average. Official Climate Normals for the U.S., computed by NOAA/NCEI, are currently based on the period 1981-2010. NOAA/NCEI also produces Alternative Normals, which supplement the official Climate Normals by providing normals that are more representative of the current state of the climate at the time of reporting. These include normals computed over shorter time periods. Planning is underway to make an update of the official Climate Normals for 1991-2020 in 2021. Included in this update will be normals computed from remotely sensed observations over shorter timeframes within this 30-year period.

In this study we use precipitation estimates that are derived from the Weather Surveillance Radar – 1988 Doppler ground-based radar sites through the conterminous U.S. (commonly referred to as the Next Generation Weather Radar – NEXRAD). The NEXRAD precipitation processing subsystem of the National Weather Service consists of several steps, or stages to generate various levels of gridded precipitation estimates. We use one of these stages of data – the stage IV gridded daily CONUS-wide data that is generated and disseminated from the National Centers for Environmental Prediction (referred to as NCEP stage IV)

Data from the NCEP stage IV data have been routinely produced since 2003 and there exists a relatively continuous archive of daily and hourly gridded precipitation over CONUS since then. We provide an overview of methodologies for computing radar-based Climate Normals and include a comparison of our results with traditional in-situ based normals. These include gridded normals for monthly average precipitation, daily average precipitation and standard deviations. Finally, we describe the probability of occurrence of these variables and include a description of their distributions.