1. A computer-implemented system for generating a probabilistic forecast of photovoltaic fleet power generation, comprising: a computer comprising a processor and memory within which code for execution by the processor is stored, comprising: a temporal distance module configured to determine a temporal distance comprising a physical distance between two locations, which are each within a geographic region suitable for operation of a photovoltaic fleet, in proportion to cloud speed within the geographic region; an input sky clearness module configured to generate a set of input clearness indexes as a ratio of each irradiance observation in a set of irradiance observations that has been regularly measured for one of the locations within the geographic region, and clear sky irradiance, and to order the set of clearness indexes into a time series, each member of the times series being successively spaced at an input time interval; a clearness index correlation coefficient module configured to determine a clearness index correlation coefficient between the two locations as an empirically-derived function of the temporal distance; an output sky clearness module configured to weigh the set of input clearness indexes by the clearness index correlation coefficient to form a time series of output clearness indexes, which indicates the clearness for the other of the locations; a fleet irradiance statistics module configured to respectively determine means and standard deviations of the time series of the input clearness indexes and of the time series of the output clearness indexes and combining the means and standard deviations of the clearness indexes into fleet irradiance statistics; a fleet power statistics module configured to forecast deterministic power statistics for the photovoltaic fleet comprising a mean and standard deviation of fleet power as a function of the fleet irradiance statistics and an overall photovoltaic fleet power rating, and to generate a time series of the forecast power statistics for the photovoltaic fleet by applying a time lag correlation coefficient for an output time interval to the power statistics over each of the input time intervals, wherein the steps are performed on a suitably-programmed computer.