Jul 11, 2023 - Energy & Environment

Nearly 90 million people under heat alerts as "dangerous" event worsens

Computer model image of a heat dome sprawled out from Florida to California on July 18.

Computer model projection showing an expansive and intense heat dome stretching from Florida to the Southwest on July 18. Image: Weatherbell.com

An "extremely dangerous" heat wave is ramping up from Florida to California, and is poised to topple records and threaten public health.

The big picture: The extreme heat has prompted the National Weather Service to issue heat alerts for more than 86 million people, according to heat.gov, with the hottest temperatures destined for the Southwest.

  • An elongated, persistent area of high pressure aloft, known as a heat dome, is causing records to fall, exacerbated by human-caused climate change.

Threat level: Conditions will worsen before they improve.

  • In the Southwest, the extreme heat is cranking up, with Phoenix making a run at their longest streak of days with 110°F high temperatures or greater. Elsewhere, Las Vegas may tie or break its hottest temperature on record.
  • California's Death Valley, one of the hottest places on Earth, may reach between 125°F to 130°F this week or next. This is rare, even for that location.
  • Based on the movement of the heat dome, the heat wave is forecast to extend to much of California by the weekend.
  • The NWS is warning that this heat event will be in the "extreme" category for Phoenix, Las Vegas and other major cities. This is the highest category on the agency's heat wave severity scale.

What they're saying: "The heat will be extremely dangerous and potentially deadly," NOAA's Weather Prediction Center warned Monday, "due to the intensity, longevity and a relatively cool start to summer which may have limited the ability for people to acclimate to more typical hot summer weather in this region."

  • The NWS is also warning of widespread record warm overnight low temperatures in the Southeast and South Central states, due in part to the extraordinarily warm ocean temperatures in the Gulf of Mexico.

Of note: Extreme heat is the number one weather-related killer during an average year in the U.S., according to the NWS.

  • Heat waves can be extremely deadly, particularly when elevated temperatures are persistent and feature a lack of significant cooling at night.
  • A study published Monday, for example, found that heat waves in Europe last year killed 61,000 people, in a demonstration of how inadequately prepared some societies are for climate change-driven temperature spikes.

Context: A new analysis from Climate Central shows that climate change is likely boosting the odds for the upcoming extreme heat in Arizona by at least a factor of four.

  • Meanwhile, the odds have been boosted even more for the coming extreme heat in other states and in Mexico, the research and communications nonprofit found.

Between the lines: In addition to heat emanating from sinking, drying air within the heat dome itself, a feedback loop is yielding extraordinarily and unusually high sea surface temperatures in the Gulf of Mexico, with water temperatures in the low 90s around South Florida.

  • Water temperatures in the Florida Keys are astonishingly high, between 95°F and 97°F at multiple observing sites.
  • Miami has seen 30 straight days with a heat index at or above 100°F, and five straight days at or above 105°F, atmospheric scientist Brian McNoldy tweeted Monday.
  • Marine heat waves tend to translate to land-based heat. They can also pose a mortal danger to coral reefs, which have a narrow temperature range and can be subject to bleaching if exposed to warm waters for long enough.

Zoom out: Meanwhile, the globe had the hottest June on record, is on track for the hottest July, and the past week (from July 3-9) was the Earth's warmest on record, according to the Copernicus Climate Change Service.

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