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(Bloomberg) — Smoke and haze from the worst Canadian wildfires on record have returned to New York City and the US Northeast and will likely linger until cleansing rains reach the region early this week. 

After briefly improving early Saturday, the air quality across most of the eastern US was downgraded to moderate. It’s worse in some pockets as the raging fires in Canada belch smoke into the atmosphere. 

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The air was unhealthy for sensitive groups in the Bronx on Sunday morning. The Delaware Valley Regional Planning Commission, which covers nine counties in Pennsylvania and New Jersey, issued an air quality alert for metro Philadelphia.

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“It’s come back in,” Jim Connolly, a meteorologist with the National Weather Service’s office in Upton, New York, said by phone. “You can see the haze out there.”

The worsening conditions come after a respite Saturday, when much of the Northeast awoke to clear skies and clean air. That shifted by the end of the day as winds from the south blew smoke back across the region, though not as intensely — or visibly — as last week when Manhattan skyscrapers disappeared into orange haze. 

Read more: Wildfire Smoke Increases the Risk of Contracting Covid-19

Wildfires have become more common as climate change leads to hotter summers that make forests more vulnerable to blazes. It’s one more indicator that that extreme temperatures are making the planet less hospitable, with drought, stronger hurricanes and heat waves becoming more severe.

The smog in the eastern US will likely remain for a few days until a storm front arrives Monday evening to drive out the smoke and deliver up to two inches of precipitation. 

The front was near St. Louis early Sunday and will be passing through the Ohio Valley before reaching the Northeast and New England. 

“That will wash out some of the smokiness,” said Zack Taylor, a meteorologist with the US Weather Prediction Center in College Park, Maryland. 

—With assistance from Brian K. Sullivan.