Friday marks the start of summer and the first significant heat wave of the season, according to the National Weather Service, leaving much of the country bracing for temperatures that will feel like they're in the triple digits.
"Summer has taken its time arriving in the Northeast, but it's coming in HOT," the NWS wrote in a post on X. "Triple-digit heat indices will be common through next week, with oppressive humidity."
The city of Philadelphia, for example, has issued a code red warning for its residents in order to transfer homeless people to indoor locations, member station WHYY reports. The notice is put into effect when the heat index is forecast at above 95 degrees for three or more days in a row.
Further west, St. Louis Public Radio reports that that city could go on a days-long streak of temperatures feeling like up to 108 degrees.
In Wisconsin, heat indexes could peak at between 100 to 105. NWS Meteorologist Ben Miller told Wisconsin Public Radio that since "it's been a pretty mild spring," people aren't acclimated the same way they are later in the summer.
"It's certainly going to feel like middle-of-summer humidity," he said. "I mean, we're talking low-to-mid 70-degree dew points, and that's going to feel pretty uncomfortable."
The heat index is a measure of how hot it actually feels — based on the temperature and the humidity — compared to the actual number on the thermometer.
A peak in scorching temperatures is expected to begin in the Great Plains and creep across the Midwest and Great Lakes and into the East Coast.
Summer Solstice – the start of the astronomical summer – is tonight at 10:42pm EDT. Though the hottest parts of summer are weeks away, this year’s solstice will usher in dangerously hot conditions in the Plains and Midwest this weekend, which will work to the East Coast by… pic.twitter.com/KZUopNMDxc
— National Weather Service (@NWS) June 20, 2025
The heat and humidity are not expected to abate until late next week.
"Don't wait until heat arrives to begin protecting yourself," the NWS warned. "If the forecast calls for extreme heat, begin preparing now."
Everyone in high heat environments is at risk of suffering from heat-related complications — including death — but pregnant people, children, those suffering from chronic illness and the elderly are at the highest risk of experiencing dangerous symptoms as a result of high temperatures.
The NWS encourages people to never leave anyone alone in a closed car, use air conditioners and the shade when possible, and drink plenty of water, even if not thirsty, as some measures to avoid falling ill as a result of the heat.
⚠🌡 Everyone is at risk from the dangers of extreme heat, but these groups are more vulnerable than most. Ensure that your loved ones and neighbors are safe from the heat and remain #WeatherReady. pic.twitter.com/Iro2D90Y9p
— NWS Huntsville (@NWSHuntsville) June 19, 2025
Heat waves are lasting longer and peaking at higher temperatures than in the past due to climate change. The average number of heat waves the U.S. experiences today has doubled since the 1980s, and the length of the dangerous heat wave season has increased from about 40 days to roughly 70.
The summer of 2024 was the hottest on record since countries began documenting their temperatures in the 1800s. Last year unseated 2023, the previous record holder for dangerously high temperatures.
Hotter weather can have immediate repercussions on human health, but the long-term effects of climate change also threaten the planet's plant and animal life and natural landscapes.
In order to combat the changing climate, at a 2015 conference of the United Nations in Paris, most member countries — including the United States — agreed to try to reduce their greenhouse gas emissions in order to rein in the potential calamitous effects of climate change.
But under President Trump, the United States has withdrawn from its commitment, with Trump calling the Paris agreement "unfair" and a "rip-off."
Copyright 2025 NPR