A computer-implemented system and method for generating a probabilistic forecast of photovoltaic fleet power generation is provided. A temporal distance between two locations is determined in proportion to cloud speed within a geographic region. Input clearness indexes are generated as a ratio of irradiance observations for one location, and clear sky irradiance. The clearness indexes are ordered into a time series. A clearness index correlation coefficient is determined as a function of temporal distance. The input clearness indexes are weighted by the clearness index correlation coefficient to form a time series of output clearness indexes. Means and standard deviations of both time series are respectively determined and combined into fleet irradiance statistics. Deterministic fleet power statistics are forecast as a function of the fleet irradiance statistics and photovoltaic fleet power rating. A time series of the forecast power statistics is generated by applying a time lag correlation coefficient.