The demand for agricultural products is expected to increase as a result of population growth, changed diets and demands for bioenergy. At the same time, agricultural yields in many regions are expected to be negatively impacted by climate change. Here we show, how altered winter conditions in combination with an extreme weather event impact reproductive development and yield of oilseed rape (Brassica napus L.). Winter oilseed rape plants were grown for twelve weeks in climate chambers simulating three winter treatments, control (2–4 °C), shorter winter (6 weeks at 2–4 °C and 6 weeks at 10–12 °C) and warm winter (10–12 °C). During seed maturation, a thunderstorm with heavy winds and rain had a drastic negative effect on yield. We capitalized on this, presenting yields after a combination of altered winter conditions and an extreme weather event. Both warmer and shorter winters reduced the production of flowers and axillary shoots and thereby decreased yield compared with control winter conditions. Both treatments significantly differed from controls, but the effect was particularly strong for the warm winter treatment. The extreme weather event had strong negative effects on yield but effects from the winter treatments remained qualitatively similar. Our findings reveal detrimental effects of altered winter conditions in combination with extreme weather events on the development and production of oilseed rape, which suggests that climate change may have negative consequences for the production of winter crops in many regions of the world.