The first of two rounds of extreme heat is underway for parts of the eastern half of the United States, the opening act for a potent and widespread heat dome that will send temperatures to dangerous, record-breaking levels over the weekend and into next week.

More than 15 million people from the Washington, DC, area to parts of Florida are under a level 3-of-4 major heat risk through Thursday, according to the National Weather Service. Such heat “affects anyone without effective cooling and/or adequate hydration,” the agency warns.

Many places will see highs in the low to mid-90s but it will feel hotter, with the heat index — how it actually feels outside, including humidity — in the triple digits. Humid air this week will make it feel especially sticky, hotter and limit overnight cooling.

This week’s heat risk is being driven by an area of high pressure over the western Atlantic Ocean that is funneling hot, moist air up from the Caribbean into the United States. That tropical origin, bringing particularly extreme humidity, is a key reason why the heat will feel so intense — and linger.

While the most intense heat will center on Virginia and the Carolinas Thursday, temperatures will also climb across parts of the Northeast and central US in a prelude to next week’s more expansive and intense heat dome.

The heat dome is forecast to expand across the central and eastern US by the weekend, covering parts of the southern Plains to the Midwest, Mid-Atlantic and Northeast. High temperatures across these regions are expected to soar at least 15 degrees above normal and will mark the hottest temperatures of the year to date, rising well into the 90s. With humidity, it could feel as hot as 110 degrees — especially in the Mid-Atlantic.

A level 4-of-4 extreme heat risk — signaling rare, long-lasting heat with little overnight relief — is forecast early next week for parts of those regions, including St. Louis, Chicago, New York City and Washington, DC.

The heat will first surge into the central US on Friday, when Denver is forecast to reach a record for the date of 101 degrees. Chicago follows this weekend, with highs of 95 expected on Saturday and 96 on Sunday.

The heat will peak in the Northeast and Mid-Atlantic early next week. Forecast highs of 95 and 96 degrees in New York City on Monday and Tuesday come within a degree of daily records, and in Washington, DC, the temperature could reach 100 — matching or breaking records on both days. Boston is expected to hit 92 degrees on Monday and 94 on Tuesday, just shy of daily records set in the mid-90s.

Over 150 daily temperature records could be broken on Monday and Tuesday, including both record highs and record warm lows — many at sites with data going back to the early 1900s.

Heat remains the deadliest form of extreme weather in the US, contributing to more than 800 deaths annually on average since 1999, a 2023 study found.

Days of intense heat disproportionately drive more severe public health impacts, even in places accustomed to summer heat. Emergency room visits for heat-related illness surge on “major” and “extreme” risk days, particularly when high humidity and warm nights prevent the body from cooling, according to the National Weather Service.

Nighttime temperatures are also warming faster than daytime highs due to climate change. This makes it harder for the body to cool and recover and increases the risk of heat-related illness and death.

This stretch of heat is building just as the seasons officially change. The summer solstice occurs on Friday at 10:42 p.m. ET, the moment the Northern Hemisphere is tilted closest to the sun. The solstice brings the highest sun angles and longest days of the year, meaning more solar energy is available to fuel extreme heat.

The solstice is the start of astronomical summer, but meteorologists mark the beginning of summer on June 1, using calendar months to better match seasonal weather patterns.

The hottest weeks of the year still lie ahead. For nearly all of the country — outside of parts of the Desert Southwest in Arizona, New Mexico and West Texas — the warmest days typically occur in July or August.

This early-season event is likely just a preview of what’s to come, though this stretch of heat is expected to ease by the end of next week.