Saturday is delivering a double dose of dangerous weather across the north-eastern US, with smoky skies from Canadian wildfires giving way to severe thunderstorms that have already triggered a flash flood warning as torrential rain pounds parts of the region.

After a brief break on Friday, smoke from the wildfires moved back into New York City and surrounding communities on Saturday, sending air quality back into unhealthy levels.

Conditions are expected to gradually improve as shifting winds push the smoke away, but those same changing conditions are fueling a new threat: strong to severe thunderstorms that are expected to sweep across much of the region, clearing the haze while bringing the potential for dangerous weather.

The first wave of powerful storms moved through northern New Jersey and New York City on Saturday, prompting a flash flood warning through 2pm. Videos shared on social media showed water pouring into parts of the city’s vulnerable infrastructure, including Penn Station.

By the afternoon, more than 2.5in (6cm) of rain had fallen in some locations, with flooding either under way or expected in areas covered by the warning. Neighborhoods including Jamaica, Crown Heights, Forest Hills, Ozone Park, Brooklyn Heights, Park Slope, Woodside and Greenpoint were among those affected.

Forecasters warn the storms could produce torrential rainfall capable of causing flash flooding, damaging wind gusts and even isolated tornadoes before the day is over. Hail is also possible. The National Weather Service has placed much of the tri-state area under a level 3 (out of 5) risk. Unlike the more typical scattered severe storms, this level indicates numerous severe storms are likely.

Meanwhile, Texas continues to battle the aftermath of days of destructive storms and historic rainfall. Although the prolonged rain has ended across Texas Hill Country, forecasters are now closely monitoring major rivers expected to surge south through the weekend.

The Devils River in south-west Texas was forecast to reach major flood stage on Saturday morning and could climb as high as 22ft (6.7 meters), according to the National Weather Service; the flood stage for the Devils River is 5ft (1.5 meters) in that area, the NWS said.

The Rio Grande is expected to peak at 21ft (6.4 meters) by Sunday morning; minor flooding in the Laredo area is at 8ft (2.4 meters).

Emergency crews carried out more rescues on Friday as additional heavy rain expanded the flood threat. The flooding has killed at least two people and left hundreds more needing rescue.

More than 2ft (60cm) of rain fell in some areas during a week of relentless downpours. While rainfall was expected to ease, another round of showers pushed already swollen rivers even higher and flooded rural communities near the Mexico border that had largely escaped the worst of the earlier damage.

Near Ozona, roughly 200 miles (320km) west of San Antonio, flood waters overtopped Interstate 10. More than 50 people were rescued by boat from flooded apartment complexes, and an RV park was overwhelmed by rising water. In Uvalde county, part of a bridge collapsed over the Nueces River after the region received months’ worth of rainfall in just a matter of days.

In Cincinnati, Ohio, residents of the Hyde Park neighborhood said it took just 20 to 30 minutes for heavy rain to escalate into a dangerous flooding situation, reported WCPO.

Across several states, communities are continuing cleanup efforts and assessing the damage left behind by powerful flooding. Long sections of highway were submerged, forcing numerous drivers to pull over as flood waters covered the roadways.