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A fresh storm system, heavy with snow and propelled by winds gusting near 50 mph, triggered avalanches and caused road closures as it pounded Utah's central mountainous spine from north to south on Monday.

By mid-afternoon, southbound US-89 in South Weber was closed Interstate 84 due to snow and traction issues, the Utah Department of Transportation reported. Motorists were advised use southbound I-15 at Riverdale.

Also, US-89/91 at Sardine Summit was closed at Dry Lake due to semi truck slide-off, UDOT reported at 3:45 p.m. Motorists were advised to use State Road 30 via Beaver Dam.

The icy fist of winter first pummeled northern Utah at dawn before spreading south along the Wasatch Front, central and then southern mountains. Utah State University, as well as Logan and Weber, Box Elder and Cache counties' school districts, delayed the start of Monday's classes by two hours.

A foot or more of new snow was forecast for the mountains by late Monday night, along with 2 to 5 inches in valley locations. A Winter Storm Warning was in effect through 4 a.m. Tuesday from Logan extending south through Brigham City, Ogden, Salt Lake City, Park City, Provo, Nephi, Richfield, Cedar City and Bryce Canyon, as well as Zion National Park.

A Winter Weather Advisory also was in place for the northwestern, northeastern, southeastern and a pocket of southwestern Utah desert through 10 p.m. Monday. The National Weather Service warned of 1-5 inches of valley snowfall, along with stiff winds, in those areas.

Traffic along Interstate 80 slowed through the passes east of Salt Lake City due to accumulating snow and slush Monday morning; the freeway was closed at one point near Evanston due to the weather.

The storm also closed down Logan and Little Cottonwood canyons for avalanche control work, and Utah's State Road 143 between Brian Head and Mammoth Creek.

In Little Cottonwood Canyon, control work produced about 40 avalanches, seven of which crossed the road, UDOT reported. Crews briefly reopened the road to local traffic for two hours Monday afternoon before closing it again; it was not expected to open again until sometime Tuesday morning.

Logan Canyon was closed for an unknown duration due to hazardous weather, UDOT said.

Two reported snowslides shut down the North Ogden Canyon Road on Monday, and the Weber County Sheriff's Office said the stretch between North Ogden and the town of Liberty could remained closed until Wednesday.

And later Monday, nearby State Road 158 to the Powder Mountain ski resort, was closed to all traffic due to severe weather. Skiers at the resort were to be escorted out, UDOT said, but an afternoon avalanche trapped both skiers and employees, possibly overnight

By 9 a.m., the Utah Highway Patrol had responded to some two-dozen accidents, none involving serious injury.

Warmer temperatures in the Salt Lake and Tooele valleys at least meant the frosty welcome to the new work week would see much of the new snowfall melted by late Monday afternoon. But after Monday's highs flirted with 40 degrees, Tuesday's forecast called for daytime temperatures only in the upper-20s — meaning another 2 inches of snow likely will stay on the ground.

Southern Utahns will shake off Monday's icy start, though rain and thunderstorms are expected periodically through Tuesday. Highs in Utah's Dixie will be in the mid- to upper-40s.

More snowfall means increased risks for potentially deadly backcountry mountain snowslides, however. The Utah Avalanche Center on Monday reported the danger as "high" for the Unitas as well as mountains above Logan, Ogden, Salt Lake, Provo and Moab. The state's Skyline and Abajo districts were rated at "considerable" risk for avalanches.

You may be warily watching the snow-laden slopes, or huffing through snow shoveling duties, but at least those deep breaths will be healthy. The Utah Division of Air Quality awarded "green" grades statewide through Tuesday.

For more extensive forecast information visit the Tribune's weather page at http://www.sltrib.com/news/weather/.

Twitter: @remims