Heat index is more commonly used and more frequently now in media (hmmm, wonder why?) but I think this is just some intern or AI that ran across a local midwest slang and ran with it to embellish their news story about the current heat wave to fill it out to a larger word count. I'm not from the midwest, so I've never heard that before.
Even though heat index is often used, temperature is still the main one talked about, which is fine under normal conditions. But it doesn't take a very high temperature with high humidity to become very dangerous. Any time I think about this my mind goes back to the first chapter of "Ministry of the Future" and how horrible now already can be for some places, and the future will just expand where that happens.
The cheerful colors make it even more surreal. Just have to imagine that the next floor looks the same. And the next. And the next...