While crews have been able to keep at bay the Parleys Canyon Fire that ignited over the weekend and put at least 6,000 residents out of their homes, they fear an incoming storm could cause the blaze to spread.

Crews achieved 21% containment as of Tuesday morning, and the fire, east of Salt Lake City, has covered 541 acres.

No homes or other structures have been affected by the blaze, other than still being under occupant evacuation order.

But the fire is burning in an area with both a flash flood watch and red flag warnings in effect for Tuesday, according to the National Weather Service.

"We're concerned about the wind," said Jesse Bender, spokeswoman for Great Basin Team 4, which is managing the fire.

If winds reach 40 to 50 mph, as forecasted, and hit the open flames, there's a chance they could push the fire to spread "very rapidly," Bender said.

Potential thunderstorms could exacerbate the winds, she added.

"So that could make the winds erratic as well and very unpredictable, which is difficult for us."

Rain, however, could help in fighting the blaze. But if a lot of rain falls suddenly and quickly, it could trigger flash floods, debris slides and mud flows, Bender said, as the soil of the burned area is more fragile.

The storm could also ground aircraft and create difficulty for firefighters in accessing the area if roads become slick.

Crews want drivers in the area — and throughout the state — to be aware of potentially hazardous conditions on the roads today, Bender said.

Evacuations were extended Monday due to the red flag warning. Many of the estimated 6,000 to 8,000 residents affected by the fire will need to remain out of their homes until Wednesday or Thursday night.

All residents in the Pinebrook area would be able to return to their homes at 8 p.m. Tuesday, according to Summit County Sheriff Justin Martinez. Timberline and Summit Park will remain under evacuation orders, he added. Residents of those areas are expected to need to stay out of their homes through Thursday at 8 p.m.

Photo of the day: Fighting the Parleys Canyon Fire

Lower Pinebrook residents were allowed to return to their homes Monday afternoon.

Fire crews and local officials say they are continuously evaluating the need for evacuations based on fire ever-changing behavior.

The fire along I-80 was ignited Saturday afternoon by hot particles thrown from a vehicle's poorly working catalytic converter. Four smaller fires were ignited, which then quickly grew. Crews initially said Saturday the fire had grown to 2,500-3,000 acres, then said on Sunday that it was closer to 1,500 acres. Crews were able to map the fire area more accurately on Sunday morning and found that the blaze had covered about 600 acres. They have now narrowed it down to 541 acres.

Jordanelle Reservoir will remain closed while water scoopers use it. The Uinta-Wasatch-Cache National Forest is also closing surrounding areas, including Mill Creek Canyon and Winter Canyon.