The Coal Hollow Fire, already covering about 19,400 acres and blowing smoke into Utah’s second-most populated county, is again threatening one of the state’s most important east-west corridors.

The blaze moved overnight and on Saturday approached U.S. Highway 6. Leann Fox, a spokeswoman for the firefighting team, said the flames are nearing an area called Knoll Hollow — about 20 miles east of Spanish Fork in Utah County. Firefighters are working to contain or repel the fire there.

“They are concerned today it may jump [Highway ] 6,” she said. “If it does, they will close the highway.”

Location of Knoll Hollow

The latest map shows Highway 6 on the fire’s north flank. Television news crews on Friday night showed firefighters setting controlled burns along the highway to remove fuels before the larger fire reached them.

As of Saturday night, the fire hadn’t crossed the highway, but officials said it was “actively burning” along the corridor.

Fox said the blaze also advanced on its southeast flank Friday night and Saturday morning. The latest measurement places the Coal Hollow Fire at 19,360 acres and 4 percent contained. Lightning ignited the fire Saturday.

The blaze has already closed Sheep Creek Road — a winding mountain lane that runs north from Highway 6 — as well as over a dozen smaller roads throughout the fire area. The fire has been sending heavy smoke into neighboring communities, and it caused Spanish Fork events to be canceled Friday evening, including a movie screening in a city park and a Spanish Fork High School football scrimmage.

The air in Salt Lake, Davis, Weber and Utah counties remained “unhealthy for sensitive groups” Saturday, according to the Utah Division of Air Quality. The division’s forecasts called for the air to remain unhealthy through at least Tuesday.

(Trent Nelson | The Salt Lake Tribune) The Coal Hollow Fire burns along Highway 6 in Utah County, Friday Aug. 10, 2018.

Officials said isolated thunderstorms that developed over the fire Saturday didn’t produce any precipitation, just gusty wind. Storms could move through the area again Monday, meaning a possible and much-needed “wetting rain” over the fire.

A few miles to the south in Sanpete County, firefighters made more progress on the Hilltop Fire. It was listed as 1,861 acres and completely contained Saturday.

Residents in the vicinity of the Hilltop Fire had been warned to be prepared to evacuate. Those warnings are being lifted at 10 p.m. Saturday.

While the Hilltop Fire appears to be tapering out, a new fire started in Utah County on Saturday: the Iron Hill Fire.

Sgt. Spencer Cannon, with the Utah County Sheriff’s Office, said shooters inadvertently sparked the fire when a bullet ricocheted off a target. The men attempted to extinguish the blaze, but soon called 911 when they realized it was getting out of control.

As of Saturday night, the fire had burned 1,400 acres and was considered 25 percent contained, the city of Saratoga Springs tweeted.

Cannon said the two men likely won’t face charges since they were shooting in a legal area and attempted to put the fire out.

Paighten Harkins contributed to this report.