An unusually intense heat dome is bringing dangerous temperatures to parts of the West on Sunday, where about 45 million people are under heat alerts. Most of those people are in portions of California, Nevada and Arizona, although heat alerts stretch north to the Canadian border.

• Records at risk: Dozens of record highs are likely to be set this weekend from California to Montana, and it is possible that a few monthly records could be tied or broken as well.
• Searing highs: Triple-digit heat stretches from Las Vegas to Montana, with cooler pockets in between. Forecast high temperatures Sunday could reach 114 degrees in Phoenix and Las Vegas, and the low-to-mid-90s in downtown Los Angeles. These numbers are down only slightly from Saturday.
• Public health threat: The NWS warns of “major” or “extreme” heat risks across much of the region, indicating a widespread threat to public health from heat exposure. “Major” heat risk is not only centered on the Southwest, but will also stretch from Denver to North Dakota. In Los Angeles, which is under an extreme heat warning, cooling centers are open this weekend through Sunday, though above-average temperatures are forecast to extend into next week. Other major cities like Las Vegas and Phoenix have also opened various cooling stations.
• “Critical” fire danger in Northwest: Elevated fire risk is predicted from eastern Washington southward into Oregon and Nevada, on eastward all the way to the Dakotas. “Critical” fire weather risk — a level 2 out of 3 — is projected for much of Montana and parts of Idaho and Wyoming. In Washington, thousands of residents in Spokane County are under an evacuation order due to the Old Trails Fire.

Extreme heat is the number one weather-related killer in the US each year, according to the National Weather Service. One factor that makes heat so dangerous is when temperatures remain elevated at night, and in Phoenix and Las Vegas in particular, low temperatures will only reach the upper 80s and low 90s. Temperatures this high will rob peoples’ ability to cool down and rest if they don’t have access to cooling, increasing the likelihood of heat-related illness.

Note: The map shows four risk categories: Minor affects very heat-sensitive people, especially if they’re outdoors without cooling or enough water; Moderate affects sensitive people, could impact some health systems and industries; Major affects anyone without cooling or enough water, likely to impact health systems, industries and infrastructure; and Extreme affects those without cooling or water during prolonged heat with no cool nights, likely to impact most health systems, industries and infrastructure. Areas not shaded represent very low to no risk from heat.

Source: National Weather Service and the US Centers for Disease Control and Prevention

Graphic: Byron Manley, Matt Stiles and Renée Rigdon, CNN

When extreme heat risk is present, it indicates the heat could be deadly, particularly to those without cooling access. Extreme heat risk also means that health systems are likely to see increased demand, including with visits to the emergency room for heat-related illness.