Avalanche conditions remain dangerous at the mid and high elevations. The snowpack is underlain by persistent layers of weak faceted snow. The chance continues for a single person to trigger slab avalanches involving the snow that has fallen since late November. This could occur even after a slope has been tracked several times. Observations of significant collapse continue to be reported. Evaluating the snowpack to identify the presence and magnitude of the weak layer is essential and takes little time in the shallow, early season snowpack. Keeping to low angle terrain is also recommended to avoid this avalanche problem. Avalanches at low elevations are not expected today.
At the mid and upper elevations, snow that has fallen since November 25th lies over older, weak, faceted snow. In steep avalanche terrain humans could trigger these slab avalanches that could range in depth from one to three feet. At an elevation of 8000 feet, the weak layer is primarily limited to slopes with a northerly component in aspect, encompassing more and more aspects as elevations rise.
An unsettled progressive zonal flow will be the defining feature of the weather though Saturday. A series of shortwaves or ripples in the westerly flow will precede the passage of the main trough on Friday. A winter storm warning is in effect for the mountains of Western Wyoming and Eastern Idaho. A brief break in the snowfall will occur this evening until after midnight when a stronger source of moisture begins to plow into the region. Heavy snowfall is expected to develop in the early morning hours and continue through the day and into the evening. Snowfall totals 1 to 2 feet are likely in the mountains by Friday morning. This snow event will be accompanied by strong winds and mild temperatures. Winds in the mountains will be from the west to southwest at 20 to 30 with gust from 40 to 50 miles per hour. Temperatures will be in the 20's in the mountains and rise into the mid-30’s in the valleys.The new snow and wind will increase the avalanche hazard at all elevations. At the mid and upper elevations an extended loading period will likely cause very large avalanches to fail on the November drought layers. These dangerous slab avalanches are likely to be easily triggered by humans, fail naturally and may run long distances. Up there the general avalanche hazard rating is expected to increase from considerable to high. Travel in avalanche terrain is not recommended during high avalanche hazard conditions. At the lower elevations the general avalanche hazard is expected to increase from low to moderate. Down there, be aware of increasing hazard on steep wind loaded slopes and the potential for very large avalanches at the higher elevations to run into avalanche runout zones in the lower elevations.
