The Observation Fire on the Bitterroot National Forest 10 miles southwest of Hamilton has burned 237 acres, as of Monday evening.

The fire has caused the closure of Lost Horse Road at the junction with Lick Creek Road, which means Twin Lakes, Schumaker Campground and Bear Creek Pass are not accessible. That's added to the current closure of Lost Horse Observation Road and the Camas Lake Trail.

Visitors are cautioned to watch for fire traffic on Lost Horse Road.

The firefighters are constructing fireline along the ridge at Observation Point, according to fire information officer Dixie Dies. The 80 firefighters are working mainly on the east edge of the fire.

Four helicopters are dropping water. Two 20-person hotshot crews and numerous engines are on the firelines.

Hot, dry weather and possible thunderstorms this week are expected to affect the fire behavior.

The fire is the fifth forest fire on the Bitterroot National Forest this season. The other four fires were kept to less than a half acre.