Why is there a heatwave in the United States?
As of June 29, 2026, a powerful heat dome is driving a dangerous and potentially record-setting heatwave across the central and eastern United States. A heat dome occurs when a very hot air mass parks itself over a region and becomes trapped under a high-pressure "lid" above the Earth's surface, preventing the heat from dissipating (1).
The sprawling upper-level ridge of high pressure is responsible for temperatures reaching well into the upper 90s and low 100s degrees Fahrenheit across a vast stretch of the country. High humidity is compounding the danger, pushing heat index values, or "feels like" temperatures, between 100 and 115 degrees Fahrenheit in some locations (2).
Much of the West remains unusually cool, with high temperatures running 20 to 35 degrees Fahrenheit below average across portions of the Northwest and northern Rockies. Meanwhile, the Four Corners region faces dangerous fire weather conditions, and severe thunderstorms continue across the northern Plains and Upper Midwest, with flooding concerns stretching from the Rockies into the Mid-Atlantic.
Which cities or areas are affected by the heatwave?
The National Weather Service says dangerous to record-setting heat will expand across the eastern two-thirds of the United States this week, making it the first widespread heatwave of the summer.
More than 100 million people across the central and eastern United States are expected to experience Major (Level 3 of 4) or Extreme (Level 4 of 4) heat risk (3).
The heatwave is affecting a wide area across the central and eastern United States, including the following regions and cities:
- Michigan: Detroit, Ann Arbor, Flint, Saginaw, Midland, Bay City, and surrounding communities.
- Illinois: Chicago, Peoria, Springfield, Champaign, Decatur, Effingham, Carbondale, and surrounding areas.
- Indiana: Indianapolis, Evansville, Lafayette, Bloomington, and northern Indiana communities.
- Kentucky: Louisville, Lexington, Paducah, and western Kentucky.
- Missouri: St. Louis, Columbia, Jefferson City, Cape Girardeau, and surrounding areas.
- Iowa: Iowa City, Davenport, Cedar Rapids, and eastern Iowa communities.
- Ohio: Northwestern Ohio communities under Extreme Heat Warnings.
- Major East Coast cities: Washington D.C., Baltimore, Philadelphia, New York City, and Boston.
- Southern cities: Dallas, Little Rock, and Memphis.
Richmond, Virginia, is forecast to top 100 degrees Fahrenheit for three consecutive days, potentially breaking daily records each day.
In Washington, D.C., highs of 102 degrees Thursday and 103 degrees Friday would break daily records if the forecast holds, while overnight temperatures could remain above 80 degrees.
New York City could tie both its daily high temperature record and its record warm overnight low on Thursday (4).
Areas including Ohio, parts of North Carolina, and Washington, D.C., face a particular threat to the elderly and those with respiratory issues.
In the meantime, several areas, including Phoenix, Las Vegas, central Texas, and much of the Southwest, were already experiencing temperatures around 100 degrees Fahrenheit heading into this week.
How long will the heatwave last?
As of June 29, 2026, the heatwave is already underway across many parts of the central and eastern United States, with the National Weather Service issuing its warning on June 28. The most dangerous and widespread conditions are expected to build through the week and peak around Thursday and Friday, July 3 and 4, timed to coincide with the July 4 Independence Day holiday weekend.
The heat dome is expected to linger over the East, particularly along the I-95 corridor, through the long Fourth of July weekend. High heat is forecast to continue into next weekend across the Great Plains, the Southeast, and the Mid-Atlantic. Parts of the Midwest and Northeast may see some relief later in the holiday weekend as the upper-level ridge of high pressure weakens and shifts westward, allowing thunderstorms to develop (5).
Above-average temperatures are expected to persist across much of the South into the following week. Forecasters also warn that risks for extreme heat are expected to continue after July 4 and into the West, with daytime temperatures potentially feeling like 100 to 105 degrees and the heat limiting overnight relief.
A critical concern throughout this event is the lack of overnight relief. Overnight low temperatures are forecast to remain in the 70s Fahrenheit, and in urban areas, where concrete and asphalt absorb and slowly release heat, temperatures may struggle to fall much below 80 degrees. This combination of successive hot days and warm nights significantly increases the cumulative health risk for all residents (6).
Are there any alerts in place?
The National Weather Service has issued a range of alerts across a broad swath of the central and eastern United States, reflecting the severity and widespread nature of this heat event. Three tiers of alerts are currently active (7):
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Extreme Heat Watch: In effect from Tuesday afternoon through Thursday evening for much of Michigan, including southeast Michigan (Detroit, Ann Arbor, Flint, Midland, Bay City, Saginaw, and surrounding communities) and northern Michigan (Alpena, Cheboygan, and nearby areas). Heat index values of 100°F to 108°F are possible.
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Heat Advisory: In effect through Thursday or Friday across central and southeast Illinois, south central Indiana, central Kentucky, northwest Indiana, and parts of Missouri. Heat index values of 105°F to 110°F are expected.
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Extreme Heat Warning: In effect through Wednesday, Thursday, or Friday for parts of southern Illinois, southwest Indiana, western Kentucky, southeast Missouri, eastern Iowa, northwest Illinois, northern Indiana, southwest Michigan, and northwest Ohio. Cities under warnings include St. Louis, Paducah, Evansville, and Cape Girardeau. Heat index values may reach 112°F.
Authorities and health officials across the affected states are urging residents to take immediate precautions. The Missouri State Emergency Management Agency has advised everyone to plan accordingly and limit prolonged outdoor exposure where possible, and to take immediate action if anyone shows signs of a heat-related illness.
New York City Health Commissioner Alister Martin warned that heat strokes are "fast, deadly, and almost always preventable," urging residents to use air conditioning, stay hydrated, and check on neighbors.
The National Weather Service and the Centers for Disease Control and Prevention advise all residents and visitors in affected areas to:
- Drink plenty of fluids and stay hydrated
- Stay in an air-conditioned room as much as possible
- Limit outdoor activity, especially during afternoon and early evening hours
- Check on relatives, neighbors, the elderly, and those with chronic illnesses
- Never leave young children or pets in unattended vehicles
- Contact local emergency management for information on cooling center availability
- Know the signs of heat exhaustion and heat stroke, including confusion, dizziness, nausea, heavy sweating, and weakness
- Call 911 immediately if someone is suspected of having heat stroke
Occupational Safety and Health Administration has also recommended that outdoor workers schedule frequent rest breaks in shaded or air-conditioned environments to reduce the risk of heat-related illness on the job.
