Two Heat Domes Set to Merge Into a Major US Heatwave Next Week

If you live anywhere between the Gulf Coast and the Great Lakes, you're going to want to read this.

Two separate heat domes are currently sitting over different parts of the country — one parked over the Desert Southwest, the other hovering over the subtropical Atlantic. A heat dome, in plain terms, is a bubble of high-pressure air that acts like a lid on a pot, trapping heat close to the ground and keeping it from escaping. They're a normal summer phenomenon, but they're usually not this well-coordinated.

Here's the part that matters: these two separate heat domes — one over the Desert Southwest and another over the subtropical Atlantic — will merge over the eastern half of the country by the middle of next week. When they combine, they don't just add up — they amplify each other, creating one giant heat event with no easy exit for the trapped air.

The result will be several days with heat index values between 100 and 110 from the Gulf Coast to the Southern Great Lakes. Places like Chicago and Detroit may see heat index numbers over 100, and some parts of the Mississippi Valley could peak at 115. The heat index — what it actually feels like when you factor in humidity — is the number you want to pay attention to here, not just the air temperature.

The heatwave looks to last all week long, and even into the weekend in the Southeast. Dozens of record hot days and record-warm mornings will likely be challenged. That second part — the warm mornings — is actually a big deal. When overnight temperatures don't drop, your body never gets a chance to recover from the daytime heat, which is when heat-related illness becomes genuinely dangerous.

This isn't just a Sunbelt problem. AccuWeather senior meteorologist Chad Merrill said it will turn very hot and humid in the East by early next week (June 29–30), with multiple days of highs well into the 90s likely along the I-95 corridor in the mid-Atlantic. So if you're in D.C., Baltimore, Philadelphia, or New York, don't think you're off the hook.

Heat domes are large bulges of sinking warm air that can stretch up to 1,000 miles in summer, driving temperatures 30 degrees above normal and creating hazardous, drying conditions that often lead to deadly, multi-day heat waves.

One more wrinkle: within the core of the heat dome, little to no thunderstorm activity is likely — but rounds of thunderstorms will be possible on the rim of the heat, from portions of the Plains to the Upper Midwest and Northeast. Translation: if you're right under the dome, you won't even get the relief of a storm to cool things down.

While the heat dome in the central states is forecast to ease up during the second week in July, temperatures will still remain a few degrees above the historical average in much of the Plains and the Midwest. So even after the worst passes, don't expect a full reset.

Bottom line: this is the kind of week to check on elderly neighbors, keep pets inside during peak hours, and seriously reconsider that outdoor July 4th run you had planned. The timing — right before the holiday weekend — means a lot of people will be outdoors. Plan accordingly.