The only plural form people will probably know is from the song Mistral Gagnant where the lyrics include les mistrals gagnants but that refers to sweets!
Not sure why anyone would think "les mistraux"... ;)
https://fr.m.wikipedia.org/wiki/Mistral_gagnant_(confiserie)
https://en.m.wikipedia.org/wiki/Mistral-class_landing_helico...