A long-lasting extreme heat wave is reaching its dangerous crescendo Tuesday, bringing the hottest day in a decade to some major East Coast cities and putting millions of already fatigued Americans at risk.

A massive heat dome – a stationary high‑pressure system trapping soaring heat and humidity – has parked over DC. This "lid" on the atmosphere allows relentless sunshine to bake the region day after day.

The National Weather Service has issued an extreme heat warning starting from 11 a.m. to 9 p.m., as temperatures are forecast to reach highs between 97 and 102, with a heat index between 105 and 110. “It’ll be the hottest day in nearly a year for D.C.,” said 7News First Alert Chief Meteorologist Veronica Johnson.