Wildfire smoke blanketed the eastern US from the Great Lakes to Washington, DC on Friday, floodwaters tore through Texas's Hill Country for a third day, and new fires erupted in the Pacific Northwest overnight, with 68 large blazes now burning in 15 states.

Millions of Americans faced hazardous conditions and orders to stay indoors, as the country's summer weather extremes converged on three fronts at once: a smoke-choked East, rising water in the South, and fast-spreading flames in the West.

Firefighters are now battling 68 large fires nationwide, up by nearly two dozen from a day earlier. Some 17 new blazes broke out in the Pacific Northwest after a run of lightning strikes made it the most active fire region in the country, according to a summary from the National Interagency Fire Center.

More than 17,400 personnel, 140 helicopters and four military C-130 air tanker crews are now deployed across the US to battle wildfires, with record-low snowpack in the Mountain West and drought pushing fuel conditions to the type of dry and fire-prone levels normally not seen until mid-August, according to the NIFC.

Nearly 3.72 million acres (1.51 million hectares) have burned nationwide so far this year, outpacing last year's mid-July tally by over 1 million acres.

Jesse Berman, a professor at the University of Minnesota School of Public Health whose research focuses on how extreme weather affects health, said simultaneous disasters like those playing out now can make them more dangerous.

"These are compound events, and that can sometimes make the impacts of them far worse than what we would experience with any one of these events individually," Berman said.

University of Pennsylvania climatologist Michael Mann said the extreme weather events were linked by a wave pattern in the jet stream that may be a phenomenon known as "resonance." This occurs when large waves in the jet stream become amplified and trapped, causing extreme weather to persist over a region for longer periods, creating more chaos.

Mann said his research shows that human-driven climate change has led to a tripling of these stalled jet stream events since the 1950s.

Climate scientist Jonathan Overpeck, dean of the University of Michigan's School for Environment and Sustainability, said rising temperatures are driving disasters through the same basic mechanism, even though the effects can look opposite.

A warmer atmosphere pulls moisture out of soil and vegetation more aggressively, which has left Canada's landscape drier and more fire-prone. At the same time, a warmer atmosphere can hold more water vapor before releasing it, which is producing the kind of intense, heavy rainfall now battering Texas.

“What climate change is doing is it's rolling the dice so that you’re going to get drier conditions and more wildfires at the same time as you’re going to get more rain,” said Overpeck.

Hazy skies

The smoke from wildfires in Canada has turned skylines from Minneapolis to Washington orange-brown and pushed dangerous air quality into the Midwest, Northeast and Mid-Atlantic. Over 100 million Americans -- nearly a third of the population -- were under some level of National Weather Service air quality alert, ranging from warnings to refrain from exercising outdoors to instructions to stay inside out of the smoke.

Chicago's air quality was second-worst in the world on Friday, according to Swiss air quality technology company IQAir. Local officials closed parks and beaches along Lake Michigan for the immediate future, cancelling or moving parks department activities indoors.

The closures limited options for residents without air conditioning, which according to the Civic Data Atlas comes to about 4% of citywide households. Temperatures in the city were expected to reach above 90 degrees Fahrenheit (32 degrees Celsius) with heat index values up to 97 F, the National Weather Service said, activating the city’s community service cooling centers.

Earlier in the week, Detroit, Minneapolis and Toronto ranked among the most polluted cities on Earth.

A heat dome parked over the Carolinas has set up northwesterly winds funneling smoke from Minnesota and Canada into the country's most populous corridor. Rain forecast for the weekend could finally begin clearing the air.

Texas gets soaked

The NWS forecasts rains to begin easing in Texas on Friday, with hot and dry weather expected in the coming week.

But the state's Hill Country endured a third consecutive day of catastrophic flash flooding, with more than 27 inches of rain falling in some areas since Tuesday, according to the National Weather Service.

Texas Governor Greg Abbott urged residents to remain vigilant about the dangers of floodwaters. He said at a Friday afternoon press conference that even if the worst of the rain is behind the state, "the rivers are going to continue to rise." Forecasters expect some major rivers to crest over the weekend.

Abbott said the state confirmed that two people died there in this week's floods: a 65-year-old man swept away in his RV near the town of Comfort, and a 74-year-old man who drove into floodwaters in Uvalde County. Rescuers have pulled hundreds of people from rising water this week.

The Texas flooding comes two weeks after the anniversary of last July's flood on the Guadalupe River, which killed at least 135 people in the towns that are once again flooding.