Extreme heat warnings are in place for large swaths of the United States this week, as an unusual weather pattern eclipses record temperature highs.

In Billings, Montana, residents experienced a sweltering 111F (43.9C) on Sunday, the hottest day in its history. Salt Lake City in Utah also hit an all-time record high on Sunday at 109F (42.8C). Boston has seen more 90F (32.3C) days than average, according to NPR station WBUR-FM.

The high temperatures have been attributed to a so-called “heat dome,” a phenomenon where pressure in the atmosphere traps heat over a region for a prolonged period of time, suppressing clouds and storms and causing hot and dry conditions, according to Cara Schulte, a researcher at advocacy group Climate Rights International.

Officials have warned the public to be on alert for symptoms of heat-related illnesses, which arise when sweltering heat interferes with the body’s ability to cool itself.

Heat exhaustion, for example, can present in symptoms such as profuse sweating, clammy skin and fatigue, according to the National Weather Service. Meanwhile, heatstroke, a medical emergency, can cause slurred speech, a rapid pulse and body temperature over 103F (39.4C), per the agency.

The heat has not subsided at night in certain parts of the midwest and north-east. That lack of nighttime cooling compounded with daytime heat can increase the risk of heat illnesses or cardiovascular strain for vulnerable groups, including older people, Schulte said.

The heatwave comes amid a deadly wildfire season. In late June, three firefighters were killed as they fought to suppress a blaze along Colorado and Utah’s border. This week, a helicopter pilot responding to a fire in Colorado died after his aircraft crashed under circumstances that remain unknown.

Fires this year have scorched about 3.6m acres of land to date, according to the National Interagency Fire Center.

“Heat creates the perfect conditions for fire. Hot and dry weather dries out vegetation and makes it more flammable. Once a fire starts, these types of conditions can cause the fire to grow more quickly and make containment much more challenging,” Schulte said.

“Extreme heat also compounds occupational health risks for firefighters, who are already doing physically intense work in heavy protective gear.”

As of Tuesday, per the National Interagency Fire Center, 46 uncontained fires continue to burn in states including California, Oregon, Colorado, Utah, Minnesota and Idaho.

“Heat waves like this one dry vegetation across the West, turning them into kindling, until a single spark can become a climate disaster,” said Trevor Stankiewicz, another researcher at Climate Rights International.

The National Oceanic and Atmospheric Administration has forecast heat hazards to persist through next Monday. The agency predicts California and several upper midwest states will be at risk of dangerous heat levels for the first half of this week.

High temperatures are expected to blanket areas in the south later in the week.