Translation:The weather is bad today.
Nouns generally need a following particle, and in this case (since it is the subject, but not the topic) the one used for "tenki" is "ga". Or in other words: because "the thing that is bad" is the weather.
The thing I don't understand is, why is tenki not the topic, this sentence is saying something about the weather right?
Japanese is a very flexible language. In this case, there is no exact rule, it depends what you want to emphasize. You could say "tenki wa warui desu" --> about the weather, it is bad; or "tenki ga warui desu" --> the thing that is bad is the weather! In most scenarios, you can use either particle, but pay attention to the emphasis you're giving to the words, so that your listener/ reader can correctly understand the point you're trying to get across!