A heat dome happens when hot air is trapped over an area due to a persistent high pressure system, William Gallus, a professor of atmospheric science at Iowa State University, said. Heat domes can last for days to weeks and can stretch over multiple states.

Millions of Americans are under weather advisories this week. See which major cities have heat, flood or storm advisories.

Central Pennsylvania is bracing for a significant heat wave that will peak Friday, with temperatures and heat index values expected to reach dangerous levels, potentially triggering a heat advisory and heat risk across the region.

Heading into Wednesday night, the comfortable nights are over, the humidity is still rising, and with lows in the low 70s, overnights will start feeling similar to how last week was. Skies will remain mostly clear throughout the night.

Hotter temperatures and feels like temperatures are increasing in the next few days. This heat is dangerous and it will be important to reduce time outdoors in the hottest parts of the day.