TUCSON, Ariz. (AP) — Hundreds of foothills homes on the outskirts of Tucson remained under an evacuation notice Friday as firefighters supported by helicopters worked to keep a wildfire from moving downhill from canyons and ridges in mountains in a national forest.

Most of the western United States is experiencing extreme dryness or drought, creating challenging conditions for wildfire season, Bryan Henry, meteorologist with the National Interagency Fire Center, said in a recent fire season outlook.

Authorities have said southern Arizona's dry, hot weather and the steep, rocky topography have been the main challenges in fighting the fire. Friday was the hottest day yet for crews fighting the blaze, with temperatures forecast to reach 107 (41.6 Celsius) in the afternoon.

Brian Francis, senior forecaster for the National Weather Service in Tucson, said temperatures were projected to remain in the triple digits through the weekend and into early next week with a slight chance of dry thunderstorms that bring only a trace of moisture but can kick up gusts of wind.

Fire officials said the fire’s growth slowed overnight Thursday but that it remained a threat. Flames have been visible from Tucson at night, and smoke from the fire plumed over a suburb Friday morning.

The firefighting effort will get help from Federal Emergency Management Agency, which officials say has granted Arizona’s request for federal funds.

Pima County officials on Thursday issued an evacuation notice for approximately 300 homes in a area south of the Coronado National Forest, and numerous other residents were told to prepare to evacuate.

Over 400 firefighters battled the week-old lightning-sparked fire that as of Friday had burned 11 square miles (29 square kilometers) of mostly brush and tall grass.

Fire officials said specialized crews were assessing potential threats to homes and other structures to identify access routes and develop contingency plans in case the fire spreads into residential areas.

In the neighborhoods closest to the fire, sheriff's deputies checked IDs to confirm that people trying to enter were residents, as helicopters dropped water on hot spots.

Sheriff's officials went door to door in neighborhoods on Thursday to notify residences of the evacuation notice, which a sheriff's spokesman said Friday wasn't mandatory.

“People are not required to leave, however we are telling people we may not be able to come back and assist if things start to burn," said Deputy James Allerton.

No injuries or structural damage was reported, Allerton said.

Travis Mayberry, an operations section chief for the fire management team, said Friday much of the fire was burning in terrain too rugged for grounds crews to work safely, particularly if they needed to escape quickly.

”Ït's not a great place for us to put people in to safely work," he said. “Ït's just too steep.""

Mayberry said crews continued to clear lines to keep the fire away from homes south of the forest and might set additional fires to burn vegetation to slow the wildfire by depriving it of potential fuel.

In Oro Valley, on another side of the Santa Catalina mountains, dozens of people parked their vehicles Friday afternoon alongside State Route 77 to watch three helicopters battling flames descending the slopes as smoke poured into the sky.

The fire, sparked June 5 by lightning, was only 10% contained as of Friday morning, and Mayberry said reaching full containment would be a slow process.

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Associated Press writers Anita Snow and Paul Davenport in Phoenix contributed to this report.