More accurate mapping puts the size of the three-day-old Observation Fire at 150 acres.

The forest fire, 10 miles southwest of Hamilton, started Friday and now has at least 70 firefighters on the lines, according to Joni Lubke of the Bitterroot National Forest.

Winds have caused growth of the fire, which started on the ridge above Lost Horse Observation Point. It is burning in steep, rocky areas. The fire has caused the closure of Lost Horse Observation Point road and the Camas Lake Trail.

Most other roads in the area are open, but visitors are advised to expect fire traffic.

A type 1 large helicopter is expected on the scene Sunday, added to three helicopters already assigned to the fire. The firefighting force includes engine crews from the Darby and Stevensville Ranger Districts and Trapper Creek Job Corps Center.