Marc Alessi coauthored this post.
Friday marks the official first day of summer, but Danger Season is already in full swing. Our team refers to the warm months, when climate-driven weather extremes tend to concentrate and do greatest harm, as “Danger Season.” Here in mid-June, these extremes have already hit much of the country—just since May 1, 78% of population has faced at least one of the Danger Season alerts we track—and the eastern half of the US is headed into a prolonged and potentially unprecedented stretch of extreme heat.
This post is the first installment in our Danger Season Weekly Outlook. We aim to post these each week with a short summary of (1) the extreme events we’re tracking, (2) the implications for people and communities, including how current cuts to federal resources make it harder for people to cope and calls to action, and (3) what we see shaping up next week.
What we’re tracking weather-wise: buckle up for dangerous, historic heat
There’s a lot going on out there—wildfires burning for weeks already in Canada, poor air quality from smoke in the Midwest and Northeast, pockets of catastrophic flooding—but extreme heat is about to dominate them all.
- A heat dome is building in the central and eastern parts of the United States starting this weekend and lasting, according to our team’s analysis of the models, at least through the end of next week. A heat dome is an exceptionally warm air mass that forms when a strong high pressure system in the upper atmosphere “sits” in one spot for days or even weeks. High pressure systems usually limit mixing of hot air at the surface with cooler air aloft, allowing surface heat to build and a heatwave to unfold.
- The heat, yes, but the humidity… During this heat wave, temperatures are forecast to surpass 90°F across most of the central and eastern US. Many areas, including Boston, NYC, and Washington, DC, may surpass 100°F. But the game changer in this event is the humidity. While a heat dome prevents mixing between the surface and upper parts of the atmosphere, winds closer to the surface will continue to bring significant amounts of moisture from the Gulf of Mexico and Atlantic Ocean into the central and eastern parts of the country, leading to high humidity. High humidity and temperature combine to create an experience of heat—the “feels like” conditions captured by the heat index (below)—that can make a heat wave truly oppressive, dangerous, and even deadly.
- The forecast includes locations in the Mid-Atlantic reaching a heat index of 115°F+ by Tuesday. According to the National Weather Service’s Heat Risk map, parts of the Midwest and Northeast will experience “extreme” heat risk next week, the highest risk according to the product.
- The climate connection is clear. It is well documented that heatwaves are becoming more frequent and lasting longer in the US, and this potentially historic event is no exception to that trend. Our friends at Climate Central have developed a tool that quantifies the role of climate change in the near-term heat forecast—and generally finds it to be ubiquitous and overwhelming. But what about the ocean-fed humidity? Currently, the Gulf of Mexico and Atlantic coast are warmer than usual for this time of year—the temperature anomalies across large areas of the Gulf this week are made 30x more likely by climate change—allowing for more evaporation of moisture into the atmosphere, which then leads to higher dew points and humidities over the eastern US. The fingerprints of climate change are all over this event.
Some of the impacts we’re concerned about
Heat like this is dangerous for anyone, even deadly. But there’s a spectrum of risk. Some groups of people—young children, elderly people, and people with pre-existing conditions—have trouble tolerating heat at relatively low thresholds. On the other end of the spectrum, most healthy adults can tolerate higher temperatures, provided they have access to rest, water, shade, and cool spaces. But any healthy person, if exposed to temperature and humidity combinations that prevent their body from being able to cool itself, will succumb to heat illness.
And then there are the situations we live in that make coping with heat harder, including working outdoors; being at school or at work without cooling; being detained or incarcerated in un-airconditioned spaces; living in an urban heat island; being unhoused; being pregnant; being on medication that reduces heat tolerance; the list is long. Read more about heat risk here and its inequities here.
In a major heat wave, serious risks are everywhere, but some of the things we’re watching this week are:
- Acclimatization. Each year, people tend to struggle with heat at the start of summer, then acclimate as the season goes on. (For this reason, the NWS tends to issue fewer heat alerts in August than in June for similar heat conditions.) With the widespread, sudden onset of extreme temperatures, millions of people may not yet be acclimated and at greater risk of succumbing to heat illness.
- Outdoor workers. Amidst this coming heat, outdoor workers will be unprotected by heat safety standards in all but seven states (and each state varies on the industries covered and the requirements for employers). After years in the making, the first-ever proposal for an OSHA federal standard for worker heat protections is going through a public hearing process (which is being held virtually from June 16 through July 2, 2025, you may wish to tune in, here’s the full schedule) — but whether the Trump administration will finalize a strong rule is a big question especially as it has fired the entire team of heat experts at National Institute for Occupational Safety and Health (NIOSH), and gutted the heat centers at the multi-agency National Integrated Heat Health Information System (NIHHIS). Much of what our taxpayer dollars had built to keep people safe from heat is being dismantled as the summer heats up. We need to increase the pressure on and advocate for OSHA to finalize a strong heat protection rule.
- Here are some calls to action: Join UCS and sign on to this coalition letter asking members of Congress to reintroduce the House and Senate versions of the Asunción Valdivia Heat Illness, Injury, and Fatality Prevention Act: the deadline to sign on is 6/20. For local actions and contacts see heatjusticenow.org and to advocate for state level heat actions, join UCS in signing on to the Alliance for Heat Resilience and Health (AHRH) letter to the National Governors Association.
- Grid reliability. Keeping the AC on depends on the ability of our power supply to keep up with demand and there’s reason to think that, in this heat, it might be unable to in some places. For example, in their summer outlooks, the Independent System Operators (ISOs) that manage the electricity transmission grids for New England and New York identified extreme heat conditions that would require emergency measures to maintain reliability and avoid blackouts, . With this heat wave, substantially hotter and longer heat conditions are in the forecast.
- To go deeper, read our new report, Keeping Everyone’s Lights On.
- Lack of access to cooling—Households with fewer resources, or those who live in substandard housing, may not have air conditioning or may not be able to pay for the costs of running air conditioning. This can be deadly, especially for those who are elderly or mobility impaired and may not be able to go to neighborhood cooling centers.
- New escalation of ICE activities in major cities. Given that ~170 million people will be living in extremely dangerous heat in the coming two weeks, this is a time for prioritizing public health, safety, and general calm. Studies have shown that exposure to extreme heat increases people’s stress levels and even instances of violence. The Trump administration’s 6/16 announcement that ICE will escalate activities in major blue-state cities is mobilizing advocates. Given the determination people have shown to defend their neighbors from unlawful ICE actions, the risk of new tension, conflict and unrest during this deadly heat could be high.
- Being good neighbors. Related to the point above, during this heat, we can each make a point to check in on neighbors and people we know to be at higher risk.
What are you watching?
Let us know in the comments. It is forecast to be a challenging season, both weather-wise and socio-politically. Let’s stay alert, keep each other informed, and weather it as best we can.