Examination of daily rainfall from 222 stations in Mexico and Central America for Jul--Sep, 1979--1997, shows that local Madden Julian Oscillation (MJO) activity plays a large role in modulating the rainfall. During boreal summer, the MJO has a secondary maximum of activity in the eastern Pacific Ocean that extends over southern Mexico and Central America. At 116 stations, precipitation during the positive phase of the local MJO is at least 25% greater than during the negative phase and at 45 stations is more than 100% greater (twice as large). Field significance exceeds 99.9%. Analysis of individual stations in the most-affected region shows that extreme rainfall events are strongly biased toward the positive phase and that the MJO influence is consistent from year-to-year. As the MJO is predictable out to at least two weeks and potentially longer, these results suggest strong potential for subseasonal prediction in the region.