It was already answered, but here's a hint about remember it. 불씨 = 불(fire) + 씨(seed) => embers. The word is sometimes used for "something that starts or triggers a big thing", since 불씨 can start a fire.

a meaning of a word '불씨' in the weather forecast Korean is embers which can cause in a miserable blaze. that's why they referred you to be careful with it.