The heat index, or how it feels with humidity factored in, will rise into the triple digits for millions as the hot dome of high pressure engulfs the central states and eventually spreads eastward.

You’ll hear the term “heat dome” a lot this week as a pattern of very hot temperatures and dangerous heat indices set up across a good portion of the US. A heat dome is strong area of high pressure and warm air that essentially sits over an area for a prolonged period

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