A method for intra-hour, sub-kilometer cloud forecasting and irradiance nowcasting using a ground-based sky imager at the University of California, San Diego is presented. Sky images taken every 30 s were processed to determine sky cover using a clear sky library and sunshine parameter. From a two-dimensional cloud map generated from coordinate-transformed sky cover, cloud shadows at the surface were estimated. Limited validation on four partly cloudy days showed that (binary) cloud conditions were correctly nowcast 70% of the time for a network of six pyranometer ground stations spread out over an area of 2 km(2). Cloud motion vectors were generated by cross-correlating two consecutive sky images. Cloud locations up to 5 min ahead were forecasted by advection of the two-dimensional cloud map. Cloud forecast error increased with increasing forecast horizon due to high cloud cover variability over the coastal site. (C) 2011 Elsevier Ltd. All rights reserved.