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In this work, we present the construction of a rainfall index for Zafra (southwestern Spain) during the period 1750–1840 from documentary sources. For this purpose we used the correspondence of the Duke of Feria that contains a quasi-weekly report of the weather in Zafra. In order to check its reliability, the Zafra rainfall index was compared with the short rainfall series available from relatively close stations, including Lisbon, Mafra, Seville and Gibraltar, all of which show a similar pattern.We also studied the atmospheric circulation over North Atlantic and western Europe in the period 1750–1840 for the months characterized by extreme values of the percentage of rainy weeks in the period 1750–1840, specifically those months above the 95th percentile or below the 5th percentile. The average large-scale atmospheric circulation maps corresponding to the months with high percentages of rainy weeks have negative Sea Level Pressure (SLP) anomalies around southwestern Iberian Peninsula (IP) and higher than usual SLP around Iceland. On the other hand, the maps corresponding to the winter months with low percentages of rainy weeks are dominated by an anticyclonic circulation over the south Europe affecting southwestern IP. The monthly rainfall index anomalies for the study period were calculated and results show two wet period (from 1782 to 1789 and from 1799 to 1807) and two dry ones (from 1796 to 1799 and from 1816 to 1819) in accordance with other regional precipitation series available.