A wildfire on the Colville Indian Reservation, west of Spokane, has burned about 15,000 acres and is only 20% contained. Firefighters are concerned the weather could make it harder to predict where and how fast the blaze will spread. Tuesday is expected to be the hottest day of the week, hitting 100 degrees near the Columbia River with humidity in the low teens, and a heat advisory is in effect.

The Williams Flats Fire started Friday when early-morning thunderstorms brought lightning to the eastern portion of the reservation, igniting dry brush, grass and timber in an area with warmer and less humid conditions than usual, according to InciWeb Incident Information System.

The fire is burning south of Silver Creek Road, within the designated game reserve, and roads in the area, including Kuehne and Canteen Creek roads, have been closed, according to InciWeb.

The fire has been active at night due to low humidity and wind patterns, InciWeb reported, and firefighters’ efforts have been challenged by steep slopes and limited ways to access the area. Nearly 550 people were fighting the fire as of Monday night, according to InciWeb.

Smoke from the fire was reported to be affecting air quality in downwind regions.

Colville tribal natural resource law-enforcement officers are asking boaters to stay clear of the firefighting efforts at Lake Roosevelt Recreational Area.

Fire officials predict it could take more than a week to contain the blaze.