Dome casings

Thermal dome dominates US weather as “derecho” threat looms in Midwest

The latest in a series of relentless heatwaves on Wednesday brings dangerously warm temperatures to the central United States and will contribute to a violent outbreak of thunderstorms in the upper Midwest. The heat will spread towards the end of the week.

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

  • This heat wave, which is expected to continue through the end of the week in many areas, is part of a series of extreme heat episodes that have become fatal this summer in the United States and Canada in particular.
  • A total of 81 large forest fires are burning in the west amid extreme heat and drought, regularly forming towering pyrocumulus clouds above the fires, with high altitude winds carrying smoke over 1000 miles east, obstructing air quality as far as Maine.
  • The heat is 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, further drying out the environment and contributing to warming in a feedback loop.

Details: Portland, Ore. – the epicenter of a deadly heat wave in late June – is under excessive heat watch Thursday and Friday, as temperatures could once again reach the century.

  • The Pacific Northwest heat wave in June killed hundreds and bore the fingerprints of man-made global warming, scientists found.
  • An excessive heat warning is in effect Wednesday for the Twin Cities, where high temperatures could reach 100 ° F as well as heat indices, which include humidity levels, as high as 110 ° F, the National Weather Service said. .
  • In Missouri and Mississippi, heat indices of up to 115 ° F are expected Wednesday through Saturday, forcing the weather service to warn of “significant heat stress” conditions.
  • And in California, where inland areas experience warmer than average conditions, the state’s power grid operator has declared a “flexible alert” asking residents to conserve energy due to an anticipated increase. demand and limited excess capacity that can be brought in from out of state.

How it works: This latest heat wave is caused by another “heat dome,” which is an area of ​​high pressure aloft that helps keep the weather hot and dry. The last heat dome sits above the western plains, encouraging the descending air.

  • As the air moves down, it gets warmer and also crushes showers and thunderstorms that could temporarily break up the heat.
  • This is not the case along the periphery of the high pressure zone, however. With elongated thermal domes like this, there is usually a strong jet stream flowing along the border between warm air and colder air to the north.
  • This can create an ideal environment for severe thunderstorms, which meteorologists call a “ring of fire” weather model, since storms erupt on the edges of the high pressure area.
  • The Storm Prediction Center on Wednesday designated the Upper Midwest as a Level 4 of 5 severe weather threat, primarily due to the likelihood of well-organized storm clusters that could cause severe wind damage to multiple states.
  • It is weather conditions 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 cause destructive straight winds over long distances.
  • A derecho that hit Iowa in 2020 was the costliest severe thunderstorm in history, causing $ 7.5 billion in damage. Much of Wisconsin, northern Illinois and Michigan lie on the potential track of these storms on Wednesday.

The bottom line: The heat observed this summer is no accident, say scientists. Instead, heat waves are among the most obvious consequences of global warming, with their probability, severity, and duration increasing as the average temperature of the planet rises due to emissions of gases. greenhouse effect of human origin.

  • A study released Monday found that extreme heat such as the Pacific Northwest event, in which Portland hit an all-time high of 116 ° F, will likely be much more common in the years to come as the global warming rate is accelerating.

Go further: Study: Prepare for many more record breaking heat waves

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