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).

Infinite, the major press outfits will be reporting everything like business as usual until they die from heat stroke

The weather models are built around long-term average. Short term observations even if they are consistent and low variance can be unexpected outliers for the models. This is a common concern in academia where people have been raising concerns about models having a bias and consistently underpredicting extreme events.