"It is muggy and stormy."
Translation:Il fait lourd et orageux.
Both are the plural form of this adjective, "orageux" for the masculine and "orageuses" for the feminine. "Orageux" also fit for the singular masculine form, and that's what it is here.
If, like here, you have no info about the gender (it is = unformal), it will go for masculine.
For example, "ces tempêtes orageuses" (fem plural) vs "ce temps orageux" (masc sing) vs "cette tempête orageuse" (fem sing).
Hmmm, feels like this is not correct.
Muggy seems to mean "moite", "humide". https://www.linguee.com/english-french/translation/muggy.html
If you use "lourd", it's rather to say the weather is really hot and heavy on you.