Washington Governor Bob Ferguson said it was a “nightmare situation”, adding: “This is likely the worst natural disaster in Spokane history.”
No deaths or injuries have been confirmed.
But the city’s mayor Lisa Brown said there was still “the potential for lives to be lost”.
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 Tuesday saw lighter winds and cooler weather, temperatures could rise again later this week.
Farinacci, who was arrested over the Old Trails fire, was being held on a charge of arson in the first degree. He was held on a $1m (£744,000) bond.
He appeared in court on Tuesday via video call, but did not enter a plea to the charge.
Residents reported seeing Farinacci near some train tracks before they saw flames in the area, according to charging documents.
Authorities stopped Farinacci near the scene. He was found with a pack of cigarettes, a butane lighter and a box of what authorities called “heavy duty waterproof matches”, say the court papers.
He spoke with authorities and denied starting the fire or being in the area where it started, saying witnesses who saw him didn’t know “the whole thing”.
He was initially let go by authorities as they investigated the cause of the blaze.
Fire officials eliminated other possible causes, like train activity or electrical infrastructure, and found it likely to be caused by vegetation with ignition by matches or a lighter.
Court records reveal Farinacci came under scrutiny from authorities in two other arson investigations for fires last summer, though he was never charged.
Sheriff Nowels said the Spokane resident also has a manslaughter conviction.
Farinacci, then aged 21, shot and killed his father in 2010, according to court records cited by US media outlets.
He pleaded guilty to manslaughter and was released from prison in 2020.