An intense heat wave will bring extreme temperatures and humidity beginning Friday to more than 250 million Americans in cities across the Northeast and along the Atlantic coast of the country, including New York and Washington, conditions that could last at least a week.

Forecasters estimate that in the next few hours a so-called “heat dome” – a phenomenon of high atmospheric pressure that traps high temperatures – will move from the center to the east, where it will expand and cause record temperatures, abundant sunshine and tropical levels of humidity.

This combination could be potentially dangerous, because it promotes the risk of heat stress and medical emergencies, which is why the U.S. National Weather Service has already issued extreme heat warnings and advisories for portions of the Southwest, Midwest and Northeast of the United States.

Meteorologists predict that temperatures will exceed 89 degrees Fahrenheit in areas where some 220 million people live, indices that could reach or exceed 100 degrees in localities in the Northeast and the Atlantic coast, affecting another 30 million Americans.

According to the National Oceanic and Atmospheric Administration (NOAA), starting Friday the Extreme heat and humidity will increase the risk of heat stress in Colorado, Nebraska, Iowa, Minnesota, and South Dakota.

These conditions will spread south and east through the weekend. In the Midwest, the heat risk is forecast to reach the highest level on the NOAA heat index Saturday and Sunday.

Temperatures will exceed 95 degrees Fahrenheit—and in some areas will reach 100 degrees—in several East Coast locations, including the densely populated cities of Baltimore, Washington, Charlotte, and Norfolk.

Portions of Ontario, Canada, are also forecast to reach 89 degrees Fahrenheit.