The heat and humidity that gripped Minnesota Tuesday and Wednesday were expected to give way to more comfortable air Thursday, but forecasters warned that northerly winds will bring smoke from Canadian wildfires back into the state.

The Minnesota Pollution Control Agency issued an air quality alert for nearly all of Minnesota that will run till 3 p.m. Friday. Fine-particle levels are expected to be unhealthy for sensitive groups in an area that includes Detroit Lakes, International Falls, Duluth, St. Cloud, Marshall, Rochester and the Twin Cities, as well as the tribal areas of Fond du Lac, Upper Sioux, Leech Lake, Red Lake, Mille Lacs and Prairie Island.

The alert was issued on a day where temperatures soared near 100 degrees. The National Weather Service in Chanhassen issued an excessive heat warning Wednesday, said Melissa Dye, an NWS meteorologist.

Clouds and lingering showers kept temperatures down in the morning. But the sun broke out in early afternoon, sending the mercury into the mid to upper 90s in the Twin Cities and across much of central and southern Minnesota. Dew points reached levels more common in the tropics.

Late Wednesday, thunderstorms broke out in the east metro and parts of western Wisconsin. Some downed trees and minor damage were reported.

A dangerous heat

From May 1 to July 27 of this year, the heat has led to 40 hospitalizations at Hennepin Healthcare facilities, according to data shared Wednesday by the hospital.

That's far more than years past. In 2020, there were 28 heat-related hospitalizations in the same period, in 2019, 30; in 2018, 36, and in 2017 there were 25.

With the dangerous heat comes the potential for heat-related illnesses, particularly for those working outside or participating in outdoor activities, she said. Anybody outside for long periods of time should take breaks in the shade or in air-conditioned buildings and drink lots of water, Dye said.

The Salvation Army has opened all seven of its metro locations for people who need a place to get in from the heat and to get a bottle of water, spokesman Dan Furry said. The centers are open during regular business hours.

"It's something we normally do, even if somebody just needs a place to rest," he said. "It's critical during a heat wave if they need some place to go during the heat of the afternoon."

Behind the front, temperatures and humidity will drop on Thursday to create more comfortable conditions, Dye said. But smoke from wildfires and the resulting poor air quality will be the trade-off, Dye said.

The rest of the week will bring high temperatures in the 80s and lows in the 60s, with little chance of rain.

Staff writer Tim Harlow contributed to this report.

Alex Chhith • 612-673-4759