Heat dome dominates U.S. weather as a "derecho" threat looms in Midwest

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The latest in a series of relentless heat waves is bringing dangerously hot temperatures to a the Central U.S. on Wednesday, and will contribute to a severe thunderstorm outbreak across the Upper Midwest. The heat will expand in scope toward the end of the week.

The big picture: Heat watches, warnings and advisories are in effect across 19 states, from Portland, Oregon east to Minneapolis, and running all the way south to New Orleans. Temperatures of between 10°F and 15°F above average in these areas along with high humidity poses a public health threat.

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  • This heat wave, which is forecast to continue through the end of the week in many areas, is part of a series of extreme heat events that have turned deadly this summer across the U.S. and Canada in particular.

  • A total of 81 large wildfires are burning in the West amid extreme heat and drought, routinely forming towering pyrocumulus clouds above the blazes, with upper level winds carrying smoke more than 1,000 miles east, fouling air quality all the way to Maine.

  • The heat is being exacerbated by an ongoing extreme drought — the worst so far this century — in the West. Dry soils allow incoming solar radiation to heat the air more efficiently, thereby drying out the environment even more, and adding to warming in a feedback loop.

Details: Portland, Oregon — the epicenter of a deadly heat wave in late June — is under an excessive heat watch Thursday and Friday, when temperatures could reach the century mark yet again.

  • The Pacific Northwest heat wave in June killed hundreds and bore the fingerprints of human-caused global warming, scientists found.

  • An excessive heat warning is in effect for the Twin Cities on Wednesday, where high temperatures could hit 100°F along with heat indices, which include humidity levels, of up to 110°F, the National Weather Service said.

  • In Missouri and Mississippi, heat indices of up to 115°F are forecast Wednesday through Saturday, causing the Weather Service to warn of "significant heat stress" conditions.

  • And in California, where inland areas are experiencing hotter-than-average conditions, the operator of the state's electric grid has declared a "Flex Alert" to ask residents to conserve energy due to an anticipated surge in demand, and limited excess capacity that can be brought in from out of state.

How it works: This latest heat wave comes courtesy of another "heat dome," which is an area of high pressure aloft that helps lock in place hot, dry weather. The latest heat dome is sitting over the western Plains, encouraging sinking air.

  • As the air descends, it warms up, and also squelches any showers and thunderstorms that might temporarily break the heat.

  • That is not the case along the periphery of the high pressure area, however. With elongated heat domes like this one, there tends to be a strong jet stream flowing along the boundary between hot and cooler air to the north.

  • This can create an ideal environment for severe thunderstorms, which meteorologists refer to as a "ring of fire" weather pattern, since the storms erupt on the edges of the high pressure area.

  • The Storm Prediction Center has designated the Upper Midwest as a Level 4 of 5 severe weather threat on Wednesday, mainly for the likelihood of well-organized clusters of storms that can bring severe wind damage to several states.

  • It's weather patterns like these that often lead to so-called derecho events, which can be extremely destructive. A derecho is characterized by an organized group of severe thunderstorms that brings damaging straight-line winds across long distances.

  • A derecho that struck Iowa in 2020 was the costliest-ever severe thunderstorm, causing $7.5 billion in damage. Much of Wisconsin, northern Illinois and Michigan lie in the potential path of these storms on Wednesday.

The bottom line: The heat seen this summer is no fluke, scientists say. Instead, heat waves are among the clearest consequences of a warming planet, with their likelihood, severity and duration increasing as the planet's average temperature climbs due to human-caused greenhouse gas emissions.

  • A study published Monday found that heat extremes such as the Pacific Northwest event, during which Portland hit an all-time high of 116°F, are likely to be far more common in coming years as the rate of global warming quickens.

Go deeper: Study: Get ready for many more record-shattering heatwaves

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