.The Rapid Update Cycle in the U.S. is the only 1-h assimilation and mesoscale forecast cycle in the world running operationally as part of an operational numerical prediction center (U.S. National Centers for Environmental Prediction - NCEP). Predictions from the Rapid Update Cycle (RUC) are used heavily as mesoscale guidance for short-range forecasts. Many phenomena important for this application are better predicted with higher spatial resolution, including convection, icing and clouds, turbulence, and surface events influenced by topography, coastlines, and other land-surface variations.. Rapid updating of NWP guidance on a 1-3h frequency has been provided operationally in the U.S. by the RUC since its initial implementation in 1994. Since then, a number of upgrades occurred to improve forecasts through higher resolution, improved data assimilation, model components, and higher update frequency, all targeted to improve needs from the user community, especially for aviation and severe-storm forecasting. In June 2005, another upgrade to the RUC, probably the most significant in the last 7 years, was implemented at NCEP, including a change of the horizontal resolution from 20 km to 13 km (Benjamin et al. 2004a). In this paper, we summarize two milestone changes to the RUC covering the present and the future: • 2005 – The 13-km RUC – higher resolution, assimilation of new observations, improved physics. At least 9h forecasts now produced hourly. • 2008 – Planned implementation of the successor to the RUC, the Rapid Refresh, a new rapid updating system using a version of the WRF model covering all of North American on a 1-h update cycle.
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