(CNN) Coastal areas in the Pacific Northwest have seen the worst of recent record-breaking heat, but millions in eastern Washington and Oregon are still under heat advisories through the upcoming weekend.
Hundreds of deaths have been reported across the Pacific Northwest and Canada amid a heat wave that has also swept through parts of America's Northeast.
At least 486 sudden deaths have been reported in Canada, a 195% increase in deaths that would typically happen in a five-day period.
"While it is too early to say with certainty how many of these deaths are heat related, it is believed likely that the significant increase in deaths reported is attributable to extreme weather (British Columbia) has experienced and continues to impact many parts of our province," British Columbia Chief Coroner Lisa Lapointe said in a statement.
And while the heat is expected to let up on the US East Coast, more than 4.5 million people in eastern Washington and Oregon are under heat alerts that are likely to last through at least Friday and in some areas until Sunday, according to CNN meteorologist Michael Guy.
Six western states remain under heat alerts Thursday, with record-breaking heat likely to continue from eastern Washington and Oregon to Montana and south into Nevada and northern California.
The most extreme heat will be centered over eastern sections of Washington and Idaho where temperatures of 110 degrees and higher are possible again.
Record heat will reach the Northern Plains and the Upper Midwest this weekend with the Dakotas and Minnesota reaching well into the 90s.
The Canada heat, which reached the country's highest recorded temperature of 121 on Tuesday in British Columbia, is expected to subside by early next week.
"A colder air mass will be moving into eastern Canada by Tuesday and the Canadian portion of the heat wave will be over by then," Guy said.
While Boston and New York have experienced record-breaking heat, temperatures on the East Coast dropped Wednesday night, and heavy rain is expected for that region on Thursday.
Both the US and Canada are experiencing wildfire threats.
There are 78 active fires burning in British Columbia, Canada, and 55 of those have were ignited in the last two days, according to the B.C. Wildfire Dashboard. The province saw a total of 464 fires so far this year, the B.C. Wildfire Service said.
The West in the US remains hot and dry, creating perfect conditions for wildfires to flourish. According to the National Interagency Fire Center, 44 wildfires are currently burning more than 667,000 acres across a dozen states, including Arizona, California and Utah. Across the entire country, wildfires have burned 1.4 million acres so far in 2021.
Scientists have told CNN the heat wave is a clear sign of the climate crisis, and say that similar extreme heat events will happen more frequently in the future.
The heat wave has expanded and worsened drought conditions in the Western US, especially in the Pacific Northwest, according to the US Drought Monitor.
"Unprecedented heat in the Northwest, combined with another week of dry weather, led to worsening drought conditions across the region," according to the summary of this week's drought numbers released Thursday morning.
Temperatures ranging from 20-30 degrees Fahrenheit above average have increased evaporation, "further drying out soils and vegetation" and worsening the drought in the region.
Many areas in the Northwest saw drought classifications worsen by a category, with expansions of the areas of severe and extreme drought. The Southwest remained unchanged, except for some expansion of extreme drought in central Arizona.
Currently, drought coverage in the West has climbed to an all-time high of 93%, with nearly 60% in extreme or exceptional drought, the two highest categories. There are five states completely in drought conditions; California, Nevada, Oregon, North Dakota and Utah.
Temperatures are expected to remain above normal over much of the Northwest heading into the weekend, but the record-setting heat will slowly subside.
Welcome relief has pushed into Seattle and Portland where highs were in the 70s and 80s Thursday. Warmer temperatures are expected for the weekend.
Portland had set an all-time, record-high temperature three days in a row, topping out at 116 degrees on Monday.
The Medical Examiner's Office in Multnomah County, which includes Portland, has identified 51 deaths related to excessive heat since Friday, June 25, according to a county news release Thursday.
The people who died ranged in age from 44 to 97, with an average age of 67. Many had underlying health conditions and were found alone, without air conditioning or a fan, the release said.
Hyperthermia, which the release called "an abnormally high body temperature caused by a failure of the body to deal with heat coming from the environment," was cited as the preliminary cause of death, according to the county.
The county noted that there were only 12 deaths from hyperthermia for the entire state between 2017 and 2019.
"I know many county residents were looking out for each other and am deeply saddened by this initial death toll," said Multnomah County Health Officer Dr. Jennifer Vines. "As our summers continue to get warmer, I suspect we will face this kind of event again.''
The Washington Department of Health reported 1,648 emergency department visits statewide for suspected heat related illness since June 25, according to public Information Officer Corey Portner.
Seattle hit 108 degrees, also breaking the all-time record it set just a day earlier. At least two locations in Washington reached 118 degrees, which, if confirmed, would tie the state temperature record that dates back to 1928.
The King County, Washington, medical examiner's office on Wednesday reported 11 heat-related deaths and one drowning. There were also 105 emergency department visits for heat-related illness on Tuesday, with 32 admitted to hospitals.
Two people died in Spokane after experiencing symptoms consistent with heat-related stress, city spokesman Brian Coddington said. A man in his 70s, was found by firefighters in an apartment building downtown, Coddington said.
Spokane hit 109 degrees, breaking the previous record of 108 degrees set on August 4, 1961, according to National Weather Service Spokane.
In Oregon, a farm worker in St. Paul, about 30 miles south of Portland, died Saturday, during the heat wave, a spokesperson for the state Occupational Safety and Health told CNN. The man was working on a crew moving irrigation lines when he was found unresponsive in the field, the spokesperson said.