Washington Governor Bob Ferguson said it was a "nightmare situation", adding: "This is likely the worst natural disaster in Spokane history."
The city's mayor Lisa Brown said there was still "the potential for lives to be lost" and that it was too early to say when people might be able to return home.
No deaths or injuries have been confirmed.
The destruction, however, has left local residents in shock and despair.
"I shouldn't have been back here, but I had to see my house," Ken Anderson told NBC News, standing near the skeletal remains of his home.
His wife, Lisa, said: "It seems like it's a nightmare that we are just gonna wake up from and it's all gonna be OK, but I don't think it's going to be OK."
Online fundraisers for those affected by the wildfires tell similar stories.
Jane Biernat, who is six-months pregnant with her second child, safely escaped the wildfire but lost "everything" when her apartment in Spokane burned down, according to a verified GoFundMe page set up by her family.
"In just a few minutes, their home, clothes, furniture, baby's belongings, and all their cherished memories were gone," the account said.
"Without renters insurance, they are now faced with the daunting task of starting over from nothing."
Authorities are trying to make contact with 14 people they have been unable to reach by mobile phone but are not yet considered missing.
Sheriff John Nowels said it was likely many of them did leave but had lost signal or had depleted phone batteries.
The three fires in the city - Old Trails, Fairview and Autumn Lane - are among a total of 16 wildfires in Washington state, according to the state's department of natural resources.
Firefighters have made progress against two of them but while lighter winds and cooler temperatures have been forecast on Tuesday, temperatures could rise again later this week.
Farinacci, who was arrested over the Old Trails fire, is being held on a charge of arson in the first degree and held on a $1m (£744,150) bond.
Nowels said the Spokane resident had a previous manslaughter conviction.
Farinacci, then age 21, shot and killed his father in 2010, according to court records reported on by several US news outlets. He pleaded guilty to manslaughter and was released from prison in 2020.
Buildings, including homes and businesses, have been destroyed, with videos from the city showing flames tearing through houses as helicopters fly above, trying to put out the flames.
Resident Anna Andreachi, who has lost her home, told BBC News partner CBS News: "All of our memories are gone.
"My dad just died, and all of his stuff was still in there too."
One man who saved his home by digging a fire line said he felt "survivor's guilt" after his neighbour lost theirs.
"We're going to open our home to our neighbours," Joe Cvancara said. "I know it's going to take a long time to rebuild."
Governor Ferguson has issued a statewide ban on all outdoor fires.
Commissioner of Public Lands Dave Upthegrove said: "Our firefighters are already stretched thin across the state, and with more heat and wind expected this weekend and for the rest of the summer, any new spark on our tinder dry landscapes can lead to another devastating wildfire.
"I'm urging all Washingtonians to not be that spark."
More than 900 firefighters have been assigned to the Washington fires, with reinforcements expected from other states soon.
Wildfires have also been burning north of Washington state in Canada's British Columbia, where more than 8,000 people have been evacuated and over 230 homes have been destroyed, Canadian officials said on Tuesday.
It comes as the province's interior region deals with extreme draught conditions, and as temperatures in the Okanagan are expected to hit a high mid-30C (86F).
More than 100 fires were started last week alone due to the hot and dry weather, officials in BC said.
Scientists have said that human-induced climate change makes the warm, dry conditions that lead to wildfires more likely - as well as leading to more frequent and intense heatwaves and potential summer droughts.
With additional reporting from Nadine Yousif in Toronto.
