Knowing more precisely the onset of the rainy season in a region would reduce risk at crop establishment. To evaluate this issue, the study proposes a simple procedure based only on daily rainfall data to estimate the onset of the wet season, by analyzing records of seven meteorological stations in the Province of Córdoba between the years 1961 and 2006. The onset of the rainy season was defined according to the simultaneous occurrence of two conditions: (i) the amount of rainfall in 3 consecutive days equal to or greater than 20 or 30 mm, (ii) a period without rain of 7 to 16 d during the subsequent 30-d period. Onset of the rainy season shows strong interannual variability throughout the region. An increase from 20 to 30 mm of rain in 3 successive days delays the onset date, which on the average reaches a maximum of 8 to 13 d, according to location. By allowing a longer period without rain after the initial event, the onset date is systematically earlier by16 to 35 d, depending on initial conditions. Furthermore, the relationship between the onset date and cumulative rainfall in the subsequent 30 d was evaluated with the bivariate normal distribution to estimate the risk level that influences the early sowing decision.