More than 100 million people across the United States face dangerous heat this weekend as a heat dome shifts eastward, bringing extreme temperatures and humidity. Heat advisories are in effect along the north-eastern coast from Portland, Maine, to Wilmington, North Carolina, with daytime heat index values 10 to 15°F above average in some areas.
The National Weather Service warns that overnight temperatures will remain very warm and oppressively muggy, offering little relief. While thunderstorms may break the heat in the north-east by Friday night, the south-east and Tennessee Valley will experience the most persistent and dangerous conditions through the weekend, with heat indices potentially exceeding 115°F.
New York City is forecast to swelter under a heat index of 106°F on Friday afternoon, slightly higher than Phoenix, Arizona. The combination of high temperatures and humidity increases the risk of heat exhaustion, heat illness, and death, particularly for children, older people, those with pre-existing health conditions, outdoor workers, the unhoused, and individuals with substance use issues.
A heat dome occurs when hot air is trapped by a stalled high-pressure system, allowing the sun's rays to directly heat the ground with little cloud cover. Climate scientists note that heat domes are becoming hotter due to global heating from fossil fuel combustion. In the corn belt, a phenomenon called 'corn sweat'—water vapor released from maize leaves—can boost the heat index by up to 10°F.
Meteorologists also forecast flash flooding from north-east Kansas to much of Indiana, and scattered thunderstorms across parts of New England, the northern mid-Atlantic, and North Dakota. Meanwhile, dry, windy conditions have triggered red flag wildfire warnings for parts of Utah and Oregon.
