"One predicts a thunderstorm."
Translation:Men voorspelt onweer.
As Maquis shows in the first comment, "onweer" becomes countable when you refer to "een onweersbui". It's similar to the English "We are expecting rain" and "We are expecting a rain-storm".
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Yes, or "Men voorspelt een onweersbui", the word thunderstorm suggests that there is a storm, "een bui" so to speak.