The emergence of heat and humidity too severe for human tolerance

Again, I don't know the exact formula but that sounds pretty damn close to the conditions I saw referenced in the article and there's about 2000 people hanging out outside for a swim meet all day today.

Granted, we do have ample shade that most adults are staying under but many of the kids are actively running around playing.

And the current conditions are still a good bit lower than what we will typically see at the peak of summer towards the end of July/early August (hitting 100f-105f is not uncommon and likely going to happen this early week).