Chance of a Thunderstorm?

I’m working on a weather app for the iPad. We are using the phrase “Chance of a Thunderstorm” to state that there is a chance of a thunderstorm. However, that phrase sounds strange to me. Is it more common/proper to say “Chance of Thunderstorms”? I googled both phrases and the singular has far more hits. Which would you say is more widely used/correct? We can only use one or the other as we don’t have much space and cannot write out the entire sentence.