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What’s Behind the Heatwaves Impacting the United States?

Virtually all of the United States has experienced above-normal temperatures in the past week. More dangerously hot weather is in the forecast this week.

More than 85 million Americans are under excessive heat warnings or heat advisories issued by the National Weather Service. Excessive heat, severe storms and flooding are all expected this week.

Forecasters warn this week’s extreme heat will linger until the weekend on the West Coast. Temperatures could break daily records in Seattle, Portland and northern California by Tuesday and climb to the highest level since a heatwave last year that killed hundreds of people across the Pacific northwest.

Many homes in the often-rainy region lack air conditioning and authorities cautioned that indoor heat is likely to build through the week, increasing the risk of heat-related illnesses, something emergency medical officials in Boston also warned of.

The extreme heat is fueling a fast-moving wildfire in California, near Yosemite National Park, forcing thousands of people to evacuate.

The heatwave in the US follows record heat in Europe that killed hundreds of people and sparked wildfires.

Here is an explanation of what is causing the heatwaves in the US:

What is a heatwave?

A heatwave has no single scientific definition. Depending on the climate of a region, it can be determined by a certain number of days above a specific temperature or percentile of the norm.

Arctic warming and jet stream migration

The Arctic is warming three to four times faster than the globe as a whole, meaning there is ever less difference between northern temperatures and those closer to the equator.

That is resulting in swings in the North Atlantic jet stream, which in turn lead to extreme weather events like heatwaves and floods, according to Jennifer Francis, senior scientist at the Woodwell Climate Research Center.

Heat domes

Warmer oceans contribute to heat domes that trap heat over large areas. At the end of this week, the heat dome will stretch from the southern plains of the Oklahoma/Arkansas area all the way to the eastern seaboard, according to the US Weather Prediction Center.

Scientists have found the main cause of heat domes is a strong change in ocean temperatures from west to east in the tropical Pacific Ocean during the preceding winter.

“As prevailing winds move the hot air east, the northern shifts of the jet stream trap the air and move it toward land, where it sinks, resulting in heatwaves,” the US National Oceanic and Atmospheric Administration says on its website.

Source : Aljazeera