By Andrew Freedman, CNN

(CNN) — An unusually intense heat dome is bringing dangerous temperatures to parts of the West on Saturday, where about 55 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.

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Here’s the latest:

• 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 this weekend could reach 118 degrees in Phoenix, 115 in Las Vegas, and in the mid-90s in downtown Los Angeles.
• 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. 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.
• Extreme fire danger in Northwest: A rare, level 3 of 3 “extremely critical” fire danger is in place for portions of Washington and Oregon on Saturday.

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.

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.

While the heat will be centered on the Southwest for Saturday and Sunday, some of the most unusual high temperatures compared to average will be found in the Plains and northern Rockies, as triple digit heat affects Montana, Nebraska, Colorado and Wyoming.

This summer has seen multiple heat domes affect the US and Europe. Human-caused climate change is making heat waves like this one more likely, as well as more intense and longer-lasting, studies show. Particularly strong and slow-moving heat domes are also possibly becoming more common as the world warms, though this is an area of active research.

In addition to the extreme heat, there is extreme fire danger for the Pacific Northwest, where the wildfire season in Oregon and Washington is already off to a record pace.

A rare, level 3 of 3 “extremely critical” fire danger is in place for portions of Washington and Oregon on Saturday. Multiple large wildfires are already burning in this region, and high winds and drought conditions could cause those blazes, and any new ones, to spread rapidly. Smoke pollution from these wildfires is affecting multiple states as well as Canada.

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CNN’s Emma Balleste contributed to this report.