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Well, one, but it’s also not literally “unprecedented” in the Japanese they’re translating from. They’re quoting the JMA, which, in the official warning and in related press events, described the event as “like you haven’t experienced before” in order to get the point across that it was a more dangerous event than usual (keep in mind that this part of Japan gets a yearly rainy season as well as typhoons; the baseline of experience is already pretty catastrophic by many other places’ standards).