The NWS has issued a bulletin warning that 100-degree temperatures will likely hit residents in the middle of the US today. The cause for that thermometer spike is a growing heat dome in the Mississippi Valley that should expand to the northeast by the end of the week, potentially affecting more than 100 million people, per Accuweather.

High humidity and low overnight temperatures will put tens of millions of people under heat alerts over the course of the coming week

The National Weather Service already has issued extreme heat warnings, watches and advisories across a swath of the Midwest and South.