HOW A HEAT DOME IS PUSHING EXTREME TEMPERATURES TO NEW HEIGHTS IN THE WEST ~ The Washington Post

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The extreme and unforgiving heat wave in the West, which has set hundreds of records since Sunday, made history again on Thursday as temperatures surged to their highest levels yet. It is poised to maintain its grip on the sizzling Southwest through Saturday before slowly subsiding.

The heat wave, like most, is the result of a sprawling zone of high pressure popularly known as a heat dome. Common in summer, heat domes are often found over the Four Corners region of the Southwest United States, where intense heating occurs over deserts.

However, the heat dome wreaking havoc across the western United States this week is striking for its incredible strength, geographic scope and persistence. Evidence suggestshuman-caused climate change is making these heat domes more intense over time.

This lengthy list of temperature records on Thursday, from California to Nebraska and far from comprehensive, is a testament to this heat dome’s might:

  • Palm Springs, Calif., matched its highest temperature ever recorded, soaring to 123 degrees.
  • Death Valley, Calif., hit 128 degrees, the highest temperature measured anywhere on the planet so far this year and just one degree off its June record of 129 set on the 30th in 2013.
  • Phoenix hit 118 degrees, a record for the date, and the earliest the city has observed a temperature this high. “Only 18 other days in Phoenix’s period of record have reached 118° or greater,” tweeted the National Weather Service office in Phoenix.
  • Denver reached at least 100 degrees for the third straight day, the earliest occurrence of such a streak on record. “[A]ll of the 100°F streaks in Denver history lasting three or more days have occurred in the past 32 years,” tweeted Bob Henson, a meteorologist and weather journalist. “Denver’s climate record goes back 150 years.”
  • Tucson reached at least 110 degrees for a sixth straight day, tied for the longest streak on record.
  • Las Vegas’s low temperature of 90 degrees was its warmest on record so early in the season. It also set a daily record high of 114 degrees.
  • Sacramento set a daily record high of 109 degrees, shattering the previous record of 102.
  • Omaha set a daily record high of 105 degrees, its hottest June day since 1953 and its third highest June temperature.


People wait in line for snow cones during a heat wave in Dallas on Thursday. Texas is pushing homes and businesses to conserve electricity for a second day in a row to stave off blackouts. (Kathy Tran/Bloomberg News)

This flurry of heat-dome-driven records follows temperatures that matched all-time highs on Tuesday in parts of Utah, Wyoming and Montana. Salt Lake City, Sheridan and Laramie, Wyo., and Billings, Mont., all made history, soaring to 107, 107, 94 and 108 degrees, respectively.

On Wednesday, Las Vegas soared to 116 degrees, one shy of its highest temperature ever recorded, while Grand Junction, Colo., hit 105, matching its highest temperature ever observed in June.

How a heat dome works

Hot air masses expand vertically into the atmosphere, creating a dome of high pressure that diverts weather systems around them. One way to gauge the magnitude of a heat wave is to measure the height of the typical halfway point of the atmosphere — at the 500 millibar pressure level. For this pressure level to stretch to heights of 600 dekameters, or 19,685 feet, is quite rare, but that marker was forecast for this week, and it was indeed reached in Flagstaff, Ariz., on Tuesday.

Alex Tardy, a meteorologist with the National Weather Service in San Diego, noted in an email that what is unusual about this particular heat dome is its strength and size, and the fact that it is only mid-June. A weather balloon in San Diego measured a record temperature of 89.2 degrees in the lower atmosphere on Thursday, which Tardy called a “very significant” reading for this location and time of year. That is translating into scorching temperatures for inland areas and deserts.

As the ground warms, it loses moisture, which makes it easier to heat even more.

As a high-pressure system becomes firmly established, subsiding air beneath it heats the atmosphere and dissipates cloud cover. The high summer sun angle combined with those cloudless skies then in turn further heats the surface.

Hot air masses expand vertically into the atmosphere, creating a dome of high pressure that diverts weather systems around them. One way to gauge the magnitude of a heat wave is to measure the height of the typical halfway point of the atmosphere — at the 500 millibar pressure level. For this pressure level to stretch to heights of 600 dekameters, or 19,685 feet, is quite rare, but that marker was forecast for this week, and it was indeed reached in Flagstaff, Ariz., on Tuesday.

Alex Tardy, a meteorologist with the National Weather Service in San Diego, noted in an email that what is unusual about this particular heat dome is its strength and size, and the fact that it is only mid-June. A weather balloon in San Diego measured a record temperature of 89.2 degrees in the lower atmosphere on Thursday, which Tardy called a “very significant” reading for this location and time of year. That is translating into scorching temperatures for inland areas and deserts.

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As a high-pressure system becomes firmly established, subsiding air beneath it heats the atmosphere and dissipates cloud cover. The high summer sun angle combined with those cloudless skies then in turn further heats the surface.

But the vicious feedback loop doesn’t end there: the combination of heat and drought is working to send this heat wave into truly extreme territory. With very little moisture in soils right now, heat energy that would normally be used on evaporation — a cooling process — is instead directly heating the air and ground surface.

Jane Wilson Baldwin, a postdoctoral researcher at the Lamont-Doherty Earth Observatory at Columbia University, said that given the severe drought in the West right now, many feedbacks between the land and the atmosphere are combining to produce an unusually persistent extreme.

“When the land surface is drier, it can’t cool itself through evaporation which makes the surface even hotter, which strengthens the blocking high further,” she said in an interview.

The situation is greatly amplified by increasing background temperatures due to the burning fossil fuels.

“You would be hard-pressed to come up with a metric of heat waves that isn’t getting worse under global warming,” she said, adding that the increasing intensity and duration of heat waves is particularly clear.

Heat waves are often high mortality disasters, but those deaths are preventable, she said, with advance warning, air conditioning, cooling centers and neighbors checking on neighbors.

However, avoiding heat-related disasters also depends on the resilience of the electrical grid, which can fail if electricity demand due to air conditioning use exceeds supply. As a result, there is the double risk of infrastructure failure and health impacts from temperature extremes, as occurred during the Texas freeze of February.

More record heat into Saturday before heat dome weakens

Excessive heat warnings remain in effect for much of California away from the coast and the mountains, western and southern Arizona, southern Nevada and southern Utah.

Temperatures on Friday and Saturday in places like Phoenix, Las Vegas and Sacramento will be about as hot as they were on Wednesday and Thursday.

The hot, dry air underneath this heat dome has created tinderbox conditions conducive to the spread of wildfires. On Friday, the Weather Service warned of a “critical” fire threat from eastern Utah into southern Colorado. The concern is dry thunderstorms that unleash lightning igniting blazes. “Fuels are extremely susceptible to lightning starts given ongoing drought and record heat,” the Weather Service wrote.

By Sunday, computer models indicate the intensity of the heat dome will begin to wane some and drift southeast into northern Mexico and west Texas early next week. At that point, temperatures will still be above normal, but not record challenging.

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