Look up the symptoms (and causes) of blossom end rot. Uneven watering and a calcium shortage are the most common culprits. Not much can be done about unscheduled thunderstorms, but calcium shortages can be addressed.

From my experience, BER is usually caused by uneven watering as Ray said. When I lived in an apartment and had Tomatoes growing in pots, it was almost impossible to avoid very wet to very dry soil. Keeping soil moist all of the time seems to fix this.

Mike