Expanding heat wave prompts alerts for 115 million people

The big picture: This heat wave is already notable for its scope, intensity and endurance.

• An area of high pressure aloft, also known as a heat dome, is moving into place over the Southwest, but will influence a much broader area.
• The Southwest will see the longest-lasting impacts, and parts of Texas are coming off their hottest June on record.
• "Unfortunately, the long term outlook for the region shows a continued heatwave through this weekend and into next week," the NWS said of the Southwest in an online forecast discussion.
• Alerts are up from the Atlantic to Pacific coasts, and include nearly all of Texas, Oklahoma, Louisiana, Missouri, Nevada, California and Arizona.

Zoom in: Cities in the Southwest that are synonymous with heat are poised to break daily, monthly and potentially all-time high temperature records during this event, as well as set milestones for the longest streak of exceptionally hot days.

• For example, Phoenix is on track to either tie or break its record of 18-straight days with highs of 110°F or above, having had their 13th such day in a row on Wednesday. The city may also break its record string of days with 90°F or above overnight low temperatures.

Threat level: Phoenix has a chance of seeing temperatures climb to 120°F this weekend. If this occurs, it would only be the fourth time since reliable records began there in the 1920s — and close to the city's all-time record of 122°F.

• "Widespread areas of Major Heat Risk will increase to Extreme levels this weekend," the NWS' Phoenix office said.
• Another record is in jeapoardy in Phoenix: the title of the city's hottest week on record, for the July 13-19 period.
• Las Vegas may see its hottest temperature on record of 117°F tied or exceeded this weekend as well.
• Cooling stations were being activated across the Las Vegas Friday through Monday in response to the NWS warning the city on Thursday that the city was entering a period of potentially deadly "extreme" heat risk — the highest on the agency's scale.

Of note: In Death Valley, Calif., one of the hottest places on Earth, the temperature is predicted to climb into the rare territory of 129-132°F this weekend.

• If it exceeds 130°F, this would set a record for the highest reliably recorded temperature on record for anywhere on Earth.

• The heat dome itself may also peak at record intensity for the Southwest at this time of year, indicating a highly unusual weather pattern.
• Florida is seeing extremely high heat indices, in part because Gulf of Mexico water temperatures are hot tub-like, in the mid-to-upper 90s, which is unheard of for any time of year.

Between the lines: Extreme heat is the top weather-related killer during an average year in the U.S., per the NWS.

• Heat waves can be extremely deadly, particularly when high temperatures are persistent and there is a lack of significant cooling at night.
• Heat waves are quiet killers, with deaths often going uncounted until well after an extreme weather event has subsided.
• A study published Monday found that European heat waves killed over 61,000 people last year — underscoring how inadequately prepared many places are for climate change-driven temperature spikes.

Context: New analysis, using peer-reviewed methods, from Climate Central shows climate change is likely boosting the odds for the upcoming extreme heat in Arizona by at least a factor of four.

• Many studies show heat waves are more likely, longer lasting and more intense as global average surface temperatures increase.
• The main cause of long-term climate change is human-caused emissions of greenhouse gases, such as carbon dioxide, due to burning fossil fuels such as coal and natural gas.
• Some research has shown that without climate change, certain heat waves would have been virtually impossible.

Editor's note: This article has been updated with new details on weather alerts, temperatures and additional context.