Millions at risk amid widespread heat wave

• Much of the eastern U.S. is at "major" or "extreme" risk of heat-related impacts this week, per the National Weather Service's experimental HeatRisk index.

By the numbers: Nearly 370 daily high temperature records were tied or broken between last Monday and Saturday, according to NOAA data.

• Boulder, Colorado and nearby Fort Collins broke 131-year and 130-year daily records, respectively, with highs of 98°F in both cities.
• A 119-year daily record fell in Paradise, Arizona with a high of 100°F, while a 108-year daily record in Provo, Utah was toppled at 102°F.

Zoom in: The extreme conditions are being driven by a "heat dome," with high pressure trapping warm, humid air over vast areas.

Between the lines: Human-caused climate change is making heat waves both more likely and more intense, researchers say.

• While daytime highs often make the headlines, average nighttime temperatures are also increasing nationwide — meaning evenings can offer less respite.

Threat level: People with underlying health conditions, outdoor workers and those without air conditioning are especially vulnerable to heat waves.

• Heavy A/C use, meanwhile, may strain electric grids and cause blackouts.
• Power outages were affecting thousands of people in Queens, New York as of Monday afternoon, per local station PIX11.

What they're saying: "Heat tends to have cumulative effects on the body, and we are currently forecasting three consecutive days of 95°F+ and 5-6 days of 90°F+ taking into account temperatures this past weekend," reads a Monday afternoon forecast discussion from the NWS' Mt. Holly, New Jersey office.

• "Take frequent breaks if outside, and drink plenty of fluids."

What's next: Relief isn't expected until the end of the week in some places.