Severe weather alerts were in place for roughly 40 million people across the western United States on Sunday, as a heat dome scorching the region finally reached its peak.

Forecasters warned of the dangers associated with the unusually prolonged bout of warmth, which they’ve called “exceptionally rare” for some locations, even in mid-July. The harshest effects of the heat dome — an atmospheric phenomenon that essentially traps hot air over a particular location — were being felt in the northern Plains, Rockies and sections of the desert Southwest. But the National Weather Service said hazardous heat would spread eastward at times, while lingering in that central region through next weekend.

The heat dome built in the West after a strong heat wave, the first of the summer to be so large and intense, began to bake the area last week. Now, forecasters said people should brace for widespread temperature highs between 105 and 115 degrees, breaking numerous local temperature records and increasing wildfire risks. They also warned about the heightened chance of heat-related illnesses, including heat stroke.

“Extremely hot daytime highs combined with potentially record-warn lows will result in increasing heat stress over the next several days due to limited relief,” the weather service said in an advisory.

The weather service office in Billings, Montana, said temperatures in the region could reach all-time highs on Sunday, climbing well into the triple digits and, potentially, up to 112 degrees. Extreme heat warnings were in place across much of the state, which, the weather service said, could strain “health systems, industries, and infrastructure” and harm anyone without access to adequate cooling and hydration. 

Salt Lake City could also challenge its all-time high temperature on Sunday, as forecasters predicted it could reach 107 degrees. Most of Utah was under an extreme heat warning, which was scheduled to remain in place through Tuesday morning.

Heat has also spread across the Dakotas, with forecasters also issuing extreme heat warnings for vast sections of both states. In Bismarck, North Dakota, they said conditions would be “dangerously hot,” with heat indices ticking upward to 100 or 110 degrees through Monday, and low temperatures in the 70s offering little reprieve. 

Similar weather was forecast for much of South Dakota, where the weather service advised people to “stay out of the sun” and “check up on relatives and neighbors,” among other recommendations to reduce the likelihood of developing heat-related illnesses.