• A storm system crossing California is affecting parts of Montana
  • Dense fog may reduce visibility to a quarter of a mile or less
  • Riming ice can result in slick roads, making travel risky

Heavy snowfall continued across the western United States after another storm lashed California on Monday. Weather forecasters in Montana have warned of riming ice on roads and dense fog that will make travel hazardous.

"Unsettled weather will continue across much of the Intermountain West and including the Four Corners region tonight and Tuesday as a storm system currently crossing California advances farther inland," the National Weather Service's Weather Prediction Center (WPC) said in a report Monday.

The western third of Montana is part of the U.S. Intermountain West.

While snow remains an issue in areas affected by the California storm system, officials are even more concerned about dense fog and icy roads.

"Watch for slick roads especially during the overnight and early morning periods," NWS Missoula warned.

Weather officials said riming ice may accumulate on surfaces, "making them extra slick."

Riming ice is an "opaque coating of tiny, white, granular ice particles caused by the rapid freezing of supercooled water droplets on impact with an object." Fog, on the other hand, is considered dense when visibility "is lowered to a quarter of a mile or less," according to NWS. The agency issues a dense fog advisory when this weather event is observed over a large area.

Fog can lead to accumulations of a light glaze of ice on roadways, causing hazardous travel conditions. Hence, travelers and commuters were asked to exercise extra caution.

The NWS warned of difficult driving conditions due to reduced visibility, combined with slippery sidewalks and roads, "especially bridges and overpasses," in Great Falls.

Dense fog advisories effective through 11 a.m. Tuesday:

  • Helena Valley, Hill County, Fergus County below 4,500 feet, northern Blaine County, western and central Chouteau County
  • Eastern and western Toole, Liberty, central Pondera, eastern Glacier, southern Blaine and Bears Paw Mountains
  • Northeast Montana – central and southeast Phillips, Garfield, Daniels, eastern Roosevelt, Prairie, McCone, northern Valley, Richland, northern and southern Phillips, Wibaux, Sheridan and Dawson
  • Petroleum County
  • Treasure County, southern and northern Big Horn, southwestern and northeastern Yellowstone, Musselshell Village, Powder River, northern and southern Rosebud, northern Carbon, Custer, Bighorn Canyon, Fallon, northern Stillwater, Carter and Golden Valley

Winter weather advisories effective through 11 a.m. Tuesday:

  • Kootenai – Highway 93 Eureka to Whitefish, Highway 37 Eureka to Libby, Highway 56 Bull Lake Road and Highway 2 Kalispell to Libby
  • West Glacier region – Essex, Polebridge, Marias Pass, Highway Bigfork to Swan Lake and Bad Rock Canyon
  • Flathead Lake, Mission Valley, Polson and Flathead Valley
  • Lower Clark Fork region – I-90 Lookout Pass to Haugan, Evaro Hill, Highway 200 Trout Creek to Heron and Highway 200 Thompson Falls to Plains
  • Missoula Valley and Bitterroot Valley
  • Sapphire Mountains
  • Blackfoot region – Butte, Highway 12 Garrison to Elliston, MacDonald Pass, Georgetown Lake and Homestake Pass
  • Seeley Lake region – Potomac, I-90 East Missoula to Bearmouth, Highway 83 Seeley Lake to Condon and Highway 200 Bonner to Greenough
Several cars were involved in a crash on a snowy Michigan Interstate Highway
Representative image. In this photo, Heavy snow makes driving conditions treacherous on the M62 motorway on Dec. 29, 2017, in Huddersfield, England. Getty Images/Christopher Furlong