The jet stream is bringing fire weather to the West and a chill to the East

A supremely wavy, loopy jet stream pattern is responsible for the hot, dry weather that’s returned to the western half of North America, and for the shock of cold headed for the middle and eastern parts of the U.S.

“The weather pattern across the United States is about to get wild,” wrote meteorologist Guy Walton last week. In his more than 30 years of weather forecasting, he’s never seen such an extreme pattern set up at this time of year. His predictions have started to become reality.

“We’re looking at a whole week of record heat in the West, from Sunday all the way through the following Saturday,” he says, along with a “great blue trough” of cold air coming down from the Great Lakes to the Southeast.

The heat spells another round of dangerous fire weather for California and much of the West, which has already smashed past previous records for burning; more than 5.8 million acres have gone up in flames already this year. But in the East, millions will feel weather to match the calendar.

(How do fires create their own weather, like dangerous fire tornados?)

Autumn is settling in across the Northern Hemisphere, so it’s no surprise that weather patterns are shaking up. But this round of extreme heat, influenced by a jet stream pattern that is pulling hot air from the tropical Pacific region northward, is yet another wave the fire-prone West can’t afford.

Scientists are busy trying to understand whether these kinds of ultra-wavy jet stream events, which often end up with systems “stuck” in place for days or weeks, are changing, intensifying, or becoming more common as human-caused climate change continues to reshape the planet.

A heat dome grows

A loopy jet stream is not a particularly unusual weather phenomenon, but its effects can be dramatic. A particular flavor of waviness has been implicated in many serious extreme heat events, from the deadly 2003 European heat wave to this spring and summer’s record-breaking Siberian heat wave, or the one in early September that hiked Death Valley temperatures to a jaw-dropping 130 degrees Fahrenheit, says Kai Kornhuber, a climate scientist at the Lamont Doherty Earth Observatory.

“From those weather events that made headlines this year, almost all of them were linked to an elongated ridge-trough pattern that built up over the U.S.,” he says.

The jet stream is essentially a river of air flowing fast high above Earth’s surface, pushed from west to east by the spin of the planet. Sometimes it flows in a relatively neat doughnut around Earth, wavering only slightly north to south on its path. Other times, the wobbles grow large, with troughs and ridges that stretch down near the equator and far up toward the pole, like a snake bending its sinuous body.

The jet stream picks up hot southern air and carries it north, and cold northern air and pulls it south. When the wobbles are big and reach farther south, the air that jet stream waves pick up and carry north can be quite warm; the same, in reverse, happens on the northern end as the stretched-out wave grabs very cold northern air and brings it south. That’s what’s happening now. Very warm air is getting pulled northward along California and the western U.S. and cold northern air is being carried down on the southward limb of the wobbly path, descending through the middle and eastern part of the country.

If it seems like the West has been sweltering for a while, that’s because it has been. For several stretches during the past few months, a stable area of high pressure, called an atmospheric “ridge,” appeared, steering hot air over the region.

“The West has been under one of these ridges, and it allows all the warm air to come up from the south…Way back in June, there was the first major heat wave and it really kind of set the stage for more, and for the very dry conditions that led to the forest fires that we’ve been having,” says Jennifer Francis, a climate scientist at the Woodwell Climate Research Center.

“I keep waiting for this pattern to break, and it just hasn’t,” says Zachary Labe, a climate scientist at Colorado State University.

The effects of the siege of heat waves over the summer have been dire. The heat has dried out soils and vegetation, priming living and dead plants to burn and kicking off a dangerous feedback loop for warming.

(Learn how climate change has amped up this year's fires).

“The moisture acts like an air conditioner, because to evaporate water takes a lot of energy, so extra heat goes into evaporation instead of warming,” explains Francis. “When you lose the moisture, the heat builds up, setting up a vicious cycle in the atmosphere.” More heat creates a stable “heat dome” over the region, which in turn makes the pattern harder to break.

Climate change may be involved in more ways than one

Over the past few decades, as the human fingerprint on global temperature rise has become ever clearer, climate change has raised the fire risks and stakes for the West.

The first effect is relatively straightforward: California has warmed about 3 degrees Fahrenheit since the 1900s. But the risks of both heat extremes and fires have grown more than that number might suggest because temperatures continue to reach new highs, creating new fire dangers and potentially toying with the jet stream. Globally, during the summer, the hot spells are now so much more frequent that some scientists wonder whether changes in atmospheric dynamics, such as the shape and speed of the jet stream, might also be at play.

A second potential effect of climate change is still being hotly debated in the climate science community. But some evidence suggests that the speed of the jet stream winds might be slowing during the summer, and that its shape may get wavier and “stuck in place” more often. So weather systems—heat waves, storms, or cold spells—would move less quickly past a location than they used to, like cars stuck in a traffic jam.

The basic idea why that might occur is intricately linked to a reality of climate change called “Arctic amplification,” and it relies on some simple logic.

The speed of the west-east flow of the jet stream winds depends largely on the temperature difference between the warm tropics and the cold poles. Warm flows toward cold; if the temperature difference is big, the air flows faster. This differential is like a power source, in a way, generating the winds of the Northern Hemisphere.

The shape of its waves and bends is also partly controlled by that temperature gradient. When the difference is small, the waves may be able to meander more widely from north to south.

The Arctic has warmed somewhere between two and four times faster than the rest of the planet, and the warming begets more warming. That’s partly because higher air temperatures lead to melted snow and sea ice earlier and earlier in the season. Dark, exposed land and sea absorb more solar heat over the summer, and cold winters can’t fully compensate for the extra heat pumped into the system. The warming compounds, leading to Arctic amplification.

That decreases the difference between temperatures at higher and lower latitudes, particularly during the summer. In theory, the shrinking gap leads to changes in the behavior of global winds like the jet stream, slowing them down and making weather patterns persist longer, at least in the Northern Hemisphere’s warm season.

“The ‘motor’ of the circulation is losing its main driving force” as Arctic amplification intensifies, says Kornhuber. “That essentially is weakening it.”

During the summer, this dynamic can also nudge the jet stream into a super wavy shape, with more lobes than usual. The waves can interact with others in the atmosphere, creating something resembling a standing wave, like a pinged guitar string that seems like it’s vibrating in place. Once these waves are in resonance, they’re relatively stable—meaning that any weather patterns that flow through them are likely to be stuck until the wave systems gets knocked out of place.

It’s crucial to remember that many other complicated weather and climate phenomena interweave with any changes, making them difficult to discern or perhaps overwhelming them altogether, says Ruth Petrie, a climate scientist at the Rutherford Appleton Laboratory, in the U.K. Scientists are still debating the existence of these phenomena, let alone how much of an effect from them we can see today.

An ongoing debate is underway about how the character of the jet stream might be shifting in winter because of Arctic climate change. Some research suggests that—somewhat counterintuitively—a warming Arctic can lead to more cold-weather outbreaks in the mid-latitudes, as the jet wanders south, carrying wintery Arctic air.

The debate, and the research, is marching forward. Climate scientists, says Labe, are very certain of the air temperature increases they've seen and of their causes, and that the rise will continue into the future. They’re “not yet certain about changes in the dynamics—in the jet stream, the movement of storm systems, the amount of blocking that will happen,” he says. In other words, all the smaller-scale weather events that make up many people’s everyday experiences.

But they’re getting closer every year, both because the science leaps forward and because climate change forces signals to appear from the noise.