A heat dome moving across much of the United States will lead to scorching temperatures in several cities this Fourth of July. Kalshi weather markets are forecasting four cities most likely to reach 100 degrees.
Today's maximum temperature market currently shows better-than-even odds of 100-degree temps for Phoenix (99%), Las Vegas (99%), Washington, D.C. (80%) and Philadelphia (80%).
Dallas and Oklahoma City remain far less certain. Both have swung repeatedly between the 30s and 60s throughout the session, making them too volatile to call as favorites in either direction.
What’s causing the extreme heat?
In the eastern U.S., more than typical summer warmth is driving these temperatures.
The Washington Post reported that a heat dome started to gain strength above the Ohio Valley on Wednesday, July 1, 2026, that caused the National Weather Service to issue heat alerts for more than 150 million Americans in the Midwest and surrounding regions.
That mix of desert cities and East Coast metros shows how far the heat and humidity are expected to extend beyond the Southwest.
That same Washington Post report found that on Tuesday, June 30, 2026, stretches of Illinois, Indiana, Iowa, Minnesota and Wisconsin “experienced humidity that rivaled or even exceeded that of Dubai,” a notoriously humid city on the Persian Gulf, based on that day's highest dew points.
What created the heat dome?
That same Washington Post analysis traced the unusually high dew points mainly to tropical moisture moving up from the southwest Atlantic and eastern Pacific, with corn sweat contributing a smaller, secondary effect.
Scientific American describes corn sweat as a form of evapotranspiration, explaining that “corn, soybeans and other crops release moisture into the air as temperatures soar,” a phenomenon The Cincinnati Enquirer also highlighted in its coverage of Ohio's heat wave.
A separate Scientific American report notes that the sheer volume of water vapor released by crops across the region pushes dew points higher, adding to the mugginess.
As The Cincinnati Enquirer explained, sweating cools the body, but when the air is already saturated with moisture, sweat can't evaporate as easily, making it harder to cool down. The moisture only condenses once temperatures fall.
How to prepare for extreme heat
The National Weather Service advises homes with vulnerable populations to take extra precautions during a heat advisory. These include young children and infants, older adults, people with chronic medical conditions, and pregnant women.
Ready.gov also recommends several precautions you can take before the heat wave hits your home:
Identify places in your community where you can access cool temperatures such as public libraries or shopping malls
Cover windows with drapes or shades
Weather-strip doors and windows
Install window air conditioners with insulation and don't rely on fans to stay cool
Contact your local health department or the Low Income Home Energy Assistance Program (LIHEAP) for additional help.
Whether these forecasts hold will become clear by the end of the day, but Kalshi markets currently favor Phoenix and Las Vegas as the surest picks for triple-digit heat, while unusually intense conditions push into the East Coast.
The takeaway:
Kalshi markets now predict
Follow Danny Gallagher on Instagram: @writerdannygallagher
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