The latest surge of smoke pollution from Canadian wildfires is smothering parts of the Northern Plains and Midwest, resulting in unhealthy air for a second day in Minnesota and parts of neighboring states.

Reminiscent of last week's historic smoke incursion into the Northeast, the air quality in Minneapolis plummeted to its worst level on record Wednesday evening. It reached the Code Purple range, meaning very unhealthy air for the entire population. As smoke lingered Thursday, air-quality alerts covered Wisconsin; Minneapolis and southern Minnesota; eastern Iowa; Green Bay and Madison, Wis.; and Cedar Rapids, Iowa.

Some of the smoke in the Upper Midwest may seep into the Northeast on Friday, and there are no signs that outbursts of smoke from Canada into the Lower 48 states will end soon.

Canada's historically bad fire season, in which 13.1 million acres have burned, is young, and weather projections suggest the hot, dry conditions that have fueled these blazes could intensify next week. Then, steering currents in the atmosphere could ferry yet more smoke into the northeastern United States.

"It's not if but rather when the northern Ontario and Quebec will make headlines again," tweeted Anthony Farnell, the chief meteorologist at Canada's Global News.

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Where smoke is now

On Thursday, wildfire smoke in the Lower 48 states was thickest in eastern North Dakota, southern Minnesota, eastern Iowa and parts of Wisconsin and Illinois and was drifting toward the south-southeast. At midday, the Air Quality Index was in the Code Red range - signifying unhealthy air for most people - in locations including Marshall, Minn.; Waterloo, Cedar Rapids, Iowa City and Dubuque, Iowa; and Sterling, Ill.

The air quality in Chicago could deteriorate for a time Thursday, although a passing area of low pressure should switch wind directions there, shunting smoke away.

Smoke at higher altitudes - which doesn't tend to affect air quality near the ground - was much more expansive Thursday, shrouding sunshine in portions of more than two dozen states across the eastern half of the country. One plume stretched from the Plains to Pennsylvania on Thursday morning. While much of this smoke was suspended high in the sky, some had settled closer to the ground, degrading air quality as far east as Pittsburgh.

As in recent episodes of compromised air quality associated with Canadian wildfire smoke, the main concern for human health is related to PM2.5, or particulate matter that is 2.5 microns and smaller. These fine particles can travel deep into the lungs and enter the bloodstream, leading to a number of detrimental health consequences.

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Next week's fire and smoke forecast is ominous

On Friday, some of the smoke over the Upper Midwest could bleed into the Northeast, but it shouldn't be nearly as thick as it was last week. But forecasters are becoming increasingly concerned about next week.

A zone of high pressure, also called a heat dome or upper ridge, is in its early stages over Canada's central provinces. It is expected to expand northeastward and intensify.

"The upper ridge developing over Quebec is concerning," said Mike Flannigan, a professor and wildfire expert at Thompson Rivers University in Kamloops, British Columbia, in a message to The Washington Post.

Temperatures under this ridge or heat dome are forecast to be unusually high, even for the beginning of summer, across much of Ontario and into Quebec - into the upper 80s and 90s.

Tomer Burg, a Ph.D. student in meteorology at the University of Oklahoma, shared an image on Twitter showing substantial probabilities for record-breaking temperatures in Ontario and Quebec next week. The abnormally high temperatures could begin Tuesday and last for days.

In addition to the heat, little or no rain is predicted underneath the heat dome from the Great Lakes toward the southern Hudson Bay, including much of Ontario and a large swath of Quebec. The predicted lack of rain will dry out the land surface and make it more combustible in this fire-prone zone.

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Pathway may open for smoke to seep into the Northeast U.S.

Forecasting smoke specifics outside a few days is difficult, but steering currents in the atmosphere next week may allow additional incursions of smoke into the Lower 48 states.

Computer models project a zone of low pressure to become stuck over the Mid-Atlantic states. The counterclockwise winds around it, combined with clockwise flow beneath the high-pressure zone over southeast Canada, would direct smoke along two potential pathways, according to Burg: 1) Eastward into northeast Canada and the northern Atlantic Ocean, and 2) into the northeast United States.

It is too soon for computer models to simulate how much smoke may develop and its timing.

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Fire situation may improve in western Canada

While eastern Canada may roast under a high-pressure ridge next week, fueling its fires, a parade of low-pressure systems arriving in portions of western Canada may offer much-needed rain.

Much of Canada's west is in moderate to extreme drought, which has substantially contributed to the severity of its fire season. Alberta is already experiencing its worst fire season since at least 1950. Meanwhile, British Columbia's Donnie Creek Fire is close to its largest on record, having burned 1.2 million acres.

Computer models project the potential for up to a few inches of rain in the next two weeks, which would be welcome but may not significantly reduce the overall summer fire risk.

Flannigan cautioned that the peak season for fire in British Columbia isn't typically until mid-July to early September.

"These larger fires in Alberta/British Columbia/Saskatchewan/Northwest Territories will burn through the summer and some will burn through the winter," he said.

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The Washington Post's Jason Samenow contributed to this report.