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The High-Resolution Rapid Refresh is a NOAA/NCEP operational weather prediction system is comprised of a numerical forecast model and an analysis/assimilation system to initialize the model. It is run with a horizontal resolution of 3 km and has 50 vertical levels.
The HRRR was developed to serve users needing frequently-updated high-resolution short- range weather forecasts, especially those in the US severe weather forecasting community. The model is run over the CONUS every hour of the day; it is integrated to 48 hours for the 00/06/12/18 UTC cycles and to 18 hours for every other cycle. The HRRR Alaska sector is run every third hour; it is integrated to 48 hours for the 00/06/12/18 UTC cycles and to 18 hours for the 03/09/15/21 UTC cycles.
The HRRR performs a 1-hour spinup using a first guess from the mean of the HRRR Data Assimilation System (HRRR-CONUS) or the RAP (HRRR-Alaska) and then assimilates radar reflectivity data every 15 minutes during the spinup forecast. This is followed by a Gridpoint Statistical Interpolation (GSI) analysis. The assimilation of radar data combined with the GSI assimilation of cloud and hydrometeor data provides significant skill in short-range cloud and precipitation forecasts. The forecast model is the ARW core of the WRF model, and output is generated every 15 minutes during the forecast for the first 18 hours. Hourly output is generated between forecast hours 18 and 48 for the extended cycles.