New snow and strong winds are forming wind slabs on a variety of sliding surfaces including suncrusts, faceted snow and surface hoar. Backcountry riders and skiers could easily trigger these slabs in steep leeward terrain above 8000 feet this morning. As heavy snowfall continues today the size and sensitivity of these slabs will increase and natural avalanches will become possible this afternoon. Weak snow near the base of the snowpack could also fail, resulting in large persistent slab avalanches. Larger triggers such as cornice drops, snowmachines, or a triggered wind slab may be necessary to trigger such an event. Below 8000 feet, isolated pockets of soft slab could be triggered steep wind loaded avalanche starting zones. Dangerous avalanche conditions exist today and cautious route finding and conservative decision making is essential for safe travel.
Strong west to southwest winds continue to transport new and recent snow into avalanche starting zones. These wind slabs will be sensitive to backcountry travelers and will increase in size and sensitivity as the day progresses. Slab depths could reach eighteen inches by the end of the day at the higher elevations.
Above 8000 feet, weak snow lies near the base of the snowpack. This weakness is most likely to be found on northwest through northeast aspects although it does exist on other aspects at the higher elevations. New snow is stressing this layer, making it more susceptible to failure. If triggered, it would be difficult to survive one of these hard slab avalanches that could have crown depths to three feet.
( Discussion is generated from the previous day's afternoon forecast. )The National Weather Service has issued a winter storm warning for the area, effective at 11pm tonight and continuing through 5am Thursday. The storm is to provide light accumulations through this evening before increasing in intensity as the new year arrives. By morning, approximately six more inches of snow is expected in higher terrain. The heaviest snow is expected Wednesday morning, continuing through the afternoon. Total snowfall amounts could reach twenty inches by Wednesday evening when snowfall will start to diminish. Temperatures at 9000 feet on New Year’s Day are to start in the low teens and rise into the low twenties. West winds are to be strong, particularly during the heavy snowfall and particularly in the southern forecast area. Expect averages on the summits to be between twenty and thirty-five miles per hour with higher gusts. The general avalanche hazard above 7500 feet is expected to be Considerable tomorrow morning and could be High by the afternoon. There will be two avalanche problem types. The first will involve the new snow. Given the rapid snowfall rates and strong winds expected, wind slabs will quickly form on leeward slopes. These slabs are forming upon faceted snow and crusts. They will be sensitive in the morning and will increase in both size and sensitivity as the day progresses. The other problem type involves the weak basal snow. Failure of this layer can result in large avalanches. Avalanches involving this layer will become more likely as the snowpack is loaded by the new snow and could be triggered by backcountry travelers or more likely, after it is impacted by a larger trigger such as the movement of one of the surface slabs. Below 7500 feet the avalanche danger is expected to be Moderate where initialy small wind slabs grow in size and sensitivity throughout the day in steep avalanche paths prone to wind loading.
