The Contreras Fire, burning southwest of Tucson, Ariz., grew to about 20,000 acres as of Monday as crews worked to keep the northern edge of the fire south of Arizona Highway 86.

The fire started June 11 on a remote ridge of the Baboquivari Mountain range, north of the Baboquivari Peak, 20 miles east of Sells, Ariz., on the Tohono O’odham Indian Reservation. It destroyed four structures on site of the Kitt Peak National Observatory last Friday, but did not damage the telescopes or other scientific facilities, officials have said.

Fire officials were worried Monday about the weather, especially lightning that could spark other fires. Wetter weather was expected to move into the area later this week, according to an update from the Eastern Area Incident Management Team.

In the north portion of the fire, ground crews were being aided by aircraft dropping retardant and water to keep the fire south of Arizona 86 and west of the community of Pan Tak, which was previously evacuated. Crews were working with the communities of Pan Tak and Hayhook Estates to develop a structure protection plan.

West of Pan Tak and Pan Tak Pass, bulldozers and hand crews will remove brush and grass on existing road systems to reduce the threat of fire spread. The fire is expected to flank toward Kitt Peak Road and Coyote Mountain Wilderness due to a shift in winds. Crews will focus on defensive firing operations below Kitt Peak Observatory.

Very little smoke can be seen in the area. Monitoring of the fire in the south and the west will remain a priority, but very little fire activity is anticipated.

On Kitt Peak, crews completed defensive firing operations on the observatory property Sunday and began mop-up operations.

The fire has grown to 20,360 acres as of Monday, an increase of 3,079 acres from Sunday. It is about 40 percent contained.

There are more than 350 personnel assigned to the fire as well as numerous aircraft.