New Delhi, June 28 -- A powerful heat dome is expected to expand across much of the central and eastern United States this week, bringing days of dangerous heat, extreme humidity and unusually warm nights to more than two dozen states through the Independence Day weekend.

Forecasters warn that temperatures in many cities will climb into the 90s Fahrenheit, with some locations topping 100 F (38 C). Combined with high humidity, it will feel even hotter, increasing the risk of heat-related illnesses.

A heat dome is a weather pattern in which a strong area of high pressure traps hot air over a region for several days or even weeks.

The sinking air beneath the high-pressure system suppresses cloud formation and rainfall, allowing the sun to...