Current climate change is characterized by an increase in weather variability, which includes altered means, variance and predictability of weather parameters, and which may affect an organism’s ecology and evolution. Few studies experimentally manipulated the variability of weather parameters, and very little is known about effects of changes in the intrinsic predictability of weather parameters on living organisms. Here we experimentally tested effects of differences in intrinsic precipitation-predictability on two herbaceous plants (Onobrychis viciifolia and Papaver rhoeas). Lower precipitation predictability led to phenological advance and to an increase in reproductive success, and population growth. Both species exhibited rapid transgenerational responses in phenology and fitness-related traits across four generations that mitigated most effects of precipitation-predictability on fitness proxies of ancestors. Transgenerational responses appeared to be the result of changes in phenotypic plasticity rather than local adaptation. They mainly existed with respect to conditions prevailing during early, but not during late growth, suggesting that responses to differences in predictability during late growth might be more difficult. The results show that lower short-term predictability of precipitation positively affected fitness, that rapid transgenerational responses existed, and that different time-scales of predictability (short-term, seasonal, and transgenerational predictability) may affect organisms differently. This shows that the time-scale of predictability should be considered in evolutionary and ecological theories, and in assessments of the consequences of climate change.