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.

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

Temperatures are expected to rise to their hottest point so far this summer and it could be spawned by a heat dome. Here's why it matters.