The Rapid Update Cycle (RUC) is an hourly intermittent data assimilation cycle producing high-resolution numerical weather forecasts updated on an hourly basis using latest observations from commercial aircraft, surface stations, satellites, wind profilers, and radars. These forecasts are produced by the NWS's National Centers for Environmental Prediction (NCEP), where the RUC is run operationally. The RUC is developed and tested at the NOAA Forecast Systems Laboratory. The RUC is the only numerical weather forecast system that produces updated predictions on an hourly basis. The RUC currently runs at 20-km horizontal resolution and 50 vertical levels on a domain covering the lower 48 United States and adjacent areas of Canada and Mexico. The RUC has been developed to provide accurate forecastsof wind, temperature, water vapor, cloud and precipitation hydrometeors, and other variables, from which aviation-related variables are derived, including turbulence, in-flight icing, ceiling, and visibility.