A powerful Heat dome is expanding across the United States, bringing dangerous temperatures to millions of Americans and raising concerns about heat-related illnesses.

The weather pattern, characterized by persistent high pressure that traps hot air near the surface, is expected to push temperatures well above normal across parts of the West before spreading into the northern Plains and Midwest.

The developing heat wave comes as communities across the country continue to deal with increasingly intense summer weather. Forecasters are warning that extreme temperatures can become especially dangerous when they persist for several days, limiting the body's ability to cool itself and increasing the risk of heat exhaustion and heatstroke.

What Is a Heat Dome?

A heat dome occurs when a strong area of high atmospheric pressure settles over a region and remains in place. The high-pressure system acts like a lid, suppressing clouds and precipitation while allowing heat to build near the ground.

The stagnant pattern can also produce weak winds and, in some areas, higher humidity. When humidity rises, sweat evaporates less efficiently from the skin, making it harder for the human body to release heat.

Heat domes can last for days or even weeks. During prolonged events, temperatures can climb substantially above historical averages, creating dangerous conditions for people who work or exercise outdoors as well as those without reliable access to air conditioning.

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Extreme Heat Spreads Across the West

The interior West is expected to experience some of the most intense temperatures as the heat dome strengthens. Temperatures in parts of the region could reach 5 to 10 degrees Fahrenheit above historical averages.

Phoenix and Palm Springs are forecast to reach the mid-110s, while Las Vegas could see temperatures in the low 110s. Salt Lake City is expected to approach or exceed 100 degrees, while temperatures in parts of Idaho, Montana, and Wyoming could also reach triple digits.

Coastal areas are expected to experience more moderate conditions. Los Angeles could see temperatures in the 80s, while San Francisco and Seattle are forecast to remain considerably cooler.

The sharp contrast illustrates the importance of geography during major heat events. Dry desert areas can experience extreme afternoon temperatures, while coastal communities may benefit from marine air and cooler conditions.

Heat Creates Serious Health Risks

Extreme heat can affect virtually anyone, but older adults, young children, people with certain medical conditions, and individuals who work outdoors can face heightened risks.

According to the Centers for Disease Control and Prevention, warning signs of heat-related illness can include heavy sweating, headaches, dizziness, weakness, nausea, muscle cramps, and shortness of breath.

People are encouraged to limit strenuous outdoor activity during the hottest parts of the day, drink plenty of fluids, and spend time in air-conditioned buildings when possible.

In dry portions of the Southwest, there is another danger. Low humidity can cause sweat to evaporate quickly, meaning people may not realize how much fluid they are losing. Dehydration can develop rapidly even when a person does not appear to be sweating heavily.

When a Heat Dome and Thunderstorms Collide

The heat dome will not necessarily bring clear skies everywhere. As the high-pressure system shifts, moisture from Mexico is expected to move into the Southwest and interior West, helping fuel the North American monsoon.

Thunderstorms could develop across parts of the Northwest, Great Basin, Southwest and High Plains. Early storms may produce little rainfall but generate strong wind gusts and lightning.

That combination can create additional hazards.

Storm Hazards

Lightning strikes from relatively dry thunderstorms can ignite wildfires, while strong winds can spread flames and carry embers into new areas. Gusty winds can also produce large dust storms, sometimes called haboobs, that reduces visibility and create dangerous driving conditions.

As moisture increases, storms could begin producing heavier rainfall. In mountainous areas and desert terrain, heavy rain can trigger flash flooding and debris flows, particularly along normally dry washes and arroyos.

Heat Dome Moves Toward the Midwest

The heat dome is expected to shift northward toward the Rockies and Plains before potentially expanding farther east.

Parts of the Dakotas and Montana could experience their first triple-digit temperatures of the year. The system could then push into the Midwest, bringing significant heat to cities including Minneapolis, Chicago, and Kansas City.

In some locations, temperatures could reach 10 to 20 degrees above historical averages. Higher humidity across the central and eastern United States could make conditions feel even hotter than the actual thermometer readings indicate.

That combination of heat and humidity is particularly concerning because the body relies on sweat evaporation to regulate its temperature.

Staying Safe During Heat Dome Weather

Weather officials urge residents to take heat warnings seriously, particularly during prolonged heat waves. People should avoid unnecessary strenuous activity during the hottest portions of the afternoon, drink fluids regularly, and take frequent breaks in air conditioning or shade.

It is also important to check on elderly relatives, neighbors, and others who may not have adequate cooling. Never leave children or pets in a parked vehicle, even for a short period.

Extreme heat can become dangerous quickly, and symptoms of heat illness should not be ignored. Moving someone to a cooler location and seeking medical assistance when symptoms become severe can be critical.

Related Resources

• Centers for Disease Control and Prevention: Heat and Health
• National Weather Service: Heat Safety
• AccuWeather: What Is a Heat Dome?
• AccuWeather: Next Heat Dome Forecast
• National Weather Service: HeatRisk