The Flathead National Forest issued an avalanche warning for Northwest Montana on Jan. 5.

“Substantial continued snowfall, moderate to strong winds, and rising temperatures created very dangerous avalanche conditions,” a press release says. “Travel in avalanche terrain and run-out zones is not recommended. Natural- and human-triggered storm and wind slabs are likely. Avalanches involving a persistent weak layer 2-5 feet deep are possible with this new, rapid load. Avalanches involving this layer could be large and destructive.”

The Flathead Avalanche Center rates the danger at high for elevations above 3,500 feet.

“The well advertised storm slowly nudged its way in yesterday, but overnight it came in with a vengeance,” the center posted on its Web site. “This moist storm system with subtropical origins — read “wet” — pushed out most of the cold air mass from the region leaving substantial snowfall, warming temperatures and increasing winds.”

Some mountain weather stations went from below zero to 28 degrees in the past 24 hours, the center reported, and snow amounts overnight ranged from 6-14 inches with snow-water equivalent amounts from 0.7 to 1.8 inches.

“The Noisy Basin SNOTEL site in the Swan Range reported 1.1 inches of snow-water equivalent in just six hours overnight,” the center said.

Snowfall is expected to continue through today into tonight, ranging from 10 to 20 inches throughout Northwest Montana, with increasing winds from the southwest at 10 to 20 mph and gusts to 40 mph. This situation compounds hazards that already existed on mountain slopes.

“Prior to the onset of this storm, we found a thin layer of weak snow near the surface (near surface facets) that formed last week,” the center posted. “This new snow will now create a slab on top of that potential weak layer.”

The center reported that backcountry skiers in the Middle Fork corridor reported excellent skiing conditions on Jan. 4 with some sloughing of the new snow.

“Expect these avalanche conditions to change today with the snow and wind overnight continuing through today,” the center posted. “With rising temperatures overnight, new snow has fallen on top of lighter snow. This lower density snow is not likely to support the newer more dense slab on top. The persistent weak layer that showed signs of improvement may become reactive again with this new load.”

For more information visit online at http://flatheadavalanche.org/advisories or call the Flathead Avalanche Center at 406-387-3835.