LOLO, Mont. -

An inversion that stuck around late Wednesday helped firefighters battle the Lolo Creek Complex fire overnight.

The fire is now 9,500 acres. That's about 1,000 more acres more than Wednesday.

Fire information officer Dixie Dies reports it was "a good night."

Massive numbers of people on what has been called the nation's top priority fire, are busy preparing for a Red Flag day Thursday afternoon, which could bring 50 mile per hour winds.

The fire is burning about 4.5 miles west of Lolo.

Five houses were lost to fire Monday. but fire officials report no structure losses since.

Two-hundred-fifty homes are evacuated.

But Dies said as many as 1,200 houses could be threatened.

The Missoula County Sheriff's Department said there is a mandatory evacuation for children who live in the active fire zone.

So far, it has not been an issue, said Sheriff's department information officer Paige Pavalone. "But we have an obligation to protect kids."

On the Montana side, the Lolo Creek corridor on Highway 12, becomes more densely populated the further east you go. According to Lolo Complex Fire command center, the road is closed from Graves Creek at milepost 16 to the junction with US 93 at Lolo due to wildfires. The duration of the closure is unknown at this time.  Eastbound traffic from Idaho is being detoured at milepost 16 north to I-90 near Alberton on Forest Service Road - Graves Creek/Petty Creek. All semi-trucks and large motor coaches must use an alternate route due to road condition and gravel surface. All other travelers use detour at your own discretion. US-12 westbound traffic must use alternate route.

Five-hundred personnel are on the fire.

There are 9 helicopters, dumping both water and retardant , 31 engines, 14 dozers and 9 water tenders.

The Lolo fire's status as top priority may be a mixed blessing.

It means we are "getting the resources we need," said Dies.

"The toughest part of fighting this fire," said Dies, " is the steep terrain. There's not a lot of good access," said the information officer.

Dies explains the perimeter map shown in this story.

The fire is designated in red. The black dots are houses.