Translation:The lightning flashes.
Because lightning isn't plural, it's non-countable so you use the 3rd person conjugation.
That said, "lightning strikes" is a better term for this, I've already flagged it.
Cause there is grammar involved. Like this is legit sentence right here. You have the subject with 가 and you have 네요 as 'is....-ing' You can't just turn that into a timeless and rather contextless word like lighting flash.