Backed by cooler temperatures and higher humidity, firefighters worked to get ahead of the state's largest wildfire Thursday before thunderstorms possibly arrive in central Utah.

The Milford Flat fire grew at both ends overnight, jumping a fire break near Clear Lake, fire-information officer Mike Martin said.

"Right now, it's looking pretty quiet. We're working on improving lines," Martin said. "So far, it's a good day."

The federal government, meanwhile, said higher levels of radioactive radon were detected in the area of the fire, although the odorless, colorless gas was not considered a human health threat.

The Milford Flat and Mathis fires were no longer ranked as the two top national firefighting priorities, replaced by fires in Idaho and Wyoming.

Milford Flat has burned 680 square miles since July 6, scorching cattle range and wildlife habitat in Beaver and Millard counties. It grew by nearly eight square miles to 435,759 acres.

The National Weather Service predicted isolated showers and thunderstorms across the fire area, about 120 miles south of Salt Lake City, with a 20 percent chance of rain. Peak temperatures were expected to be far lower than this week's 100-degree days.

"The fire outlook looks pretty good because humidity is high," meteorologist Alex Tardy said. "That same high-pressure system is going to come right back over the heart of Utah Friday and humidity is going to go down and temperatures are going to go up."

The National Nuclear Security Administration recorded radon levels as high as 140 microrems per hour, up from an average of 20 per hour, spokesman Darwin Morgan said.

Radon is a naturally occurring element found in geological formations. Experts believe radon is being released as the fire burns over rocks. One millirem is equivalent to 1,000 microrems.

"A chest X-ray is 10 millirem. This is at 0.14 millirem," Morgan said from Las Vegas. "We're not trying to cause alarm. But we know that from the experience with downwinders, we're very sensitive to concerns that Utahns have about radiation."

Fallout from nuclear-weapons tests in Nevada during the 1950s and '60s has been linked to cancer in Utah.

Once ranked No. 1 by the National Interagency Fire Center, the 3-square-mile Mathis fire was 50 percent contained after firefighters used a bulldozer and intentional burn to slow its growth near a coal mine and methane vents, said Steve Politsch, a fire-information officer.

"If we get adverse conditions, that could affect this fire," he said. "We think we've got some containment lines that will hold, though, no matter what Mother Nature throws at us."

Elsewhere, managers on the Black Rock Gulch fire, about 20 miles south of St. George, said they expect to fully contain the 35-square-mile fire by Friday.