Sweltering heat to expand, affect 170M in central and eastern U.S.

The highest temperatures so far this year will combine with increasing humidity and intense summer sunshine to create dangerous, life-threatening conditions for roughly 170 million people, AccuWeather meteorologists warn.

The heat wave will be the result of a heat dome shifting out of the West and parking over the central and eastern United States.

Summer officially arrives in the Northern Hemisphere Friday night. The heat wave will come into full bloom just after the summer solstice, during some of the longest days of the year and when the sun is most intense.

Energy demands will surge as air conditioners and fans hum and temperatures climb well into the 90s to near 100 degrees Fahrenheit during the day. In some cases, temperatures will challenge daily records that have been in the books for decades.

"It will be exceptionally warm at night with little relief once the sun goes down," AccuWeather Chief On-Air Meteorologist Bernie Rayno said. "Many urban areas may struggle to drop below 80 at night, and that can take a toll on the body without air conditioning."

Heat kills

"Extreme heat is tragically the leading cause of weather-related fatalities in America," AccuWeather Chief Meteorologist Jonathan Porter said.

The conditions, which will feature AccuWeather RealFeel® Temperatures surging past 100 and possibly peaking between 110 and 120 degrees, can be especially dangerous for young children, the elderly and pets. Cooling centers will need to be activated.

"There is an amplified risk of heat-related illnesses because this is the first heat wave of the year for millions of people, and their bodies are not yet acclimated to this type of heat and humidity," Porter said.

Even healthy people should use caution when working or exercising outdoors.

Experts urge people to remain hydrated and take breaks in cool environments when and where possible to reduce the risk of heat exhaustion and heatstroke.

Over the last 30 years, extreme heat has been the leading cause of all weather-related fatalities in the United States annually, according to the National Weather Service. In 2024 alone, nearly 200 people died of heat-related illness in the United States.

The 238 heat-related deaths in 2024 were more than tornadoes, hurricanes and flooding combined (236).

Some of the hottest conditions during the heat wave will occur during the middle to latter portion, as it will take a few days for the ground to dry out following the recent wet conditions. Once topsoil dries out and paved and concrete surfaces become thoroughly warmed, most of the sun's energy will be directed to heating the air, rather than evaporating moisture from the ground.

"The last day of a heat wave, and the day after a heat wave breaks, are often when most heat-related illnesses and emergencies are reported," Porter said. "This is because health impacts from heat accumulate over time."

Rain goes away for many

While most areas within the heat dome shield will be free of rain for three to five days or more, areas along the edges will be vulnerable to thunderstorms, severe weather and flooding downpours. This is known among the weather communities as the "ring of fire."

Swim, have fun, but be careful

Despite the risks and great discomfort for millions of people, many others will rejoice with limited rainfall (finally) and opportunities for summertime activities such as swimming, days at the beach or camping in forested areas.

"This will be the first nice weekend at beaches across the mid-Atlantic and Northeast in weeks," Rayno said.

Breezes, when active, will offer some relief from the heat along the shores of the ocean and large lakes.

AccuWeather meteorologists caution those taking a dip in unheated pools, lakes and the ocean that water temperatures may be lagging due to weeks of cloudy, wet weather to this point. Cold water shock can be dangerous as it can lead to severe muscle cramps, cause breathing difficulties and result in drowning.

Water temperatures typically do not peak until mid- to late August. As of the latter third of June, Atlantic water temperatures range from the icy 40s and 50s along the coast of Maine to the 60s along the southern coast of New England, New York and New Jersey. Water temperatures trend upward through the 70s from Delmarva to Virginia and are mainly in the 80s from the Carolinas south.

Heat wave highlights

"This will be the most intense heat wave Chicago has experienced in the last few years," Porter said. Chicagoans can expect three to five days with high temperatures in the 90s.

Meanwhile, Philadelphia and Washington, D.C., are forecast to experience at least seven days with highs at or above 90 degrees, including at least two days with highs of 100 or higher next week.

Even New Yorkers will experience the swelter, with multiple days with highs in the 90s. Temperatures can touch 100 F on a couple of occasions in Central Park (which tends to run cool) and are likely to exceed 100 across the rivers in New Jersey and Long Island.

Folks in Boston may escape the full duration of the heat wave. However, temperatures on a couple of days can surge well into the 90s to near 100.

Heat will be no stranger in areas from the central Plains to the Southeast states. Kansas City, Missouri, and Oklahoma City will have multiple days with highs in the 90s within the heat dome. St. Louis and Nashville will likely amass at least eight days in a row with highs of 90 to near 100.

Even though Atlanta's and Charlotte's typical high for the latter part of June is only 88, highs in the mid-90s to low 100s will be routine during the heat wave, or 5-15 degrees above the historical average.