The first major U.S. heat wave of the year starts on Friday. It brings dangerously high temperatures and is expected to last for days.

Maps show where extreme humidity and 90- or 100-degree temperatures will affect around 255 million people across the United States.

Millions of Americans are facing oppressive heat, with forecasters expecting a "heat dome" to develop over parts of the county over the weekend and through next week. The National Weather Service (NWS) warns that heat related illnesses increase significantly during extreme heat events.

A dangerous heatwave is sweeping across the central United States this week, putting millions of residents at risk as temperatures are expected to surge well above normal. Forecasters are warning that temperatures could rise by as much as 20°F above average,