There has been presented the method of yield forecasting using precipitation amounts and mean air temperature based upon plant phenology and probability distribution of both the random variables. The method enables making forecasts for each recorded phenological stage; the time horizon of the longest of them exceeds 100-110 days. The verification test carried out on the grounds of data coming from different regions of Poland proved a high effectiveness of the method. The use of the model has been illustrated on an example of extreme course of weather according to data obtained from the Experimental Station of the University of Agriculture, Wrocław.