Mapped: The deadly U.S. heat wave marches on

• Extreme heat in the West this week has already resulted in more than two dozen fatalities, according to the Washington Post, though this is likely an undercount. Fatalities have also been reported in Washington, D.C., where heat indices, or feels-like temperatures, have reached or exceeded 110°F at times.

Zoom in: The unusually strong heat dome that has dominated the weather pattern across the West is unlikely to fully loosen its grip, despite the expansion of the unusually hot weather to other parts of the country.

• One of the main metrics used to determine heat-related health risk is the wet bulb globe temperature, which includes air temperature, solar radiation, humidity and wind. NOAA data shows how people in parts of the Southwest, Midwest, South, and East Coast will feel the heat through July 16.
• This metric doesn't reflect all hot areas in the country, since some spots may have high air temperatures, but low humidity values, for example.

Context: Human-caused climate change has greatly increased the odds and severity of heat waves and led them to be longer lasting as well.

• According to the Climate Shift Index from the research group Climate Central, human-caused climate change has made daily high and low temperatures in much of the West at least five times more likely to occur, when compared to a pre-industrial atmosphere.

By the numbers: The heat has toppled hundreds of daily temperature records, including both daytime highs and overnight minimum temperatures. Monthly milestones have also been broken.

• All-time records have fallen in Palm Springs (124°F), Las Vegas (120°F), and Redding, Calif. (119°F), among other locations.
• As expected prior to the event, what stands out is the duration of the scorching temperatures. Las Vegas broke its record for the longest stretch of days with temperatures above 115°F, with five such days.
• Locations in the Sacramento and San Joaquin Valleys have seen temperatures climb into the 100s°F to 110s°F for two consecutive weeks, and heat warnings have been extended across the West through Saturday.
• "This level of heat for many people will create an extreme risk of heat-related illnesses when access to adequate cooling or hydration is not available," the Weather Service stated in a forecast discussion Thursday morning.

• "Be sure to follow proper heat safety, which includes staying hydrated, wear light clothing, avoid outdoor activity, and using air conditioning."

Zoom out: Nationally, June was the second-warmest such month on record in the Lower 48 states, according to data NOAA released this week.

• The oceans and atmosphere agency found that 24 million people in the West, South and Northeast had their warmest June when it comes to overnight minimum temperatures.
• This is a key variable in determining the health risks from a heat wave, since a lack of cooler temperatures at night can deprive people of the ability to cool down and recharge, boosting their odds of experiencing heat-related illness.
• In general, overnight temperatures are increasing faster than daytime highs, in keeping with expectations tied to human-caused climate change.

Between the lines: Many cities are currently on track to have their hottest summers on record, including Raleigh, Atlanta, Baton Rouge, La., Corpus Cristi, Texas, Phoenix, Flagstaff, Ariz., Las Vegas, Fresno and Sacramento, among others.

• Given weekly to monthly temperature outlooks, many of these cities are likely to solidify their rankings as time goes on.

Go deeper: "Heat islands" worsening extreme temperatures across the U.S.