Slab avalanches that were triggered by skiers and snowmoblers on Friday and Saturday were some of the larger events of the season. One of these avalanches was remotely triggered by a skier in the Ullrs slide path on Peak 9,975 in Grand Teton National Park. Another avalanche in the Snake River Range caught four snowmobilers. One did not survive. Northwest winds were strong overnight and continued to load leeward aspects. Persistent weak layers continue to be potential problem layers in some areas at all elevations and aspects. Large slab avalanches could be triggered by humans who venture onto steep avalanche prone slopes. Snowpack collapses are a sign of instability. In these conditions, snowpack tests may be unreliable and the consequences of a poor terrain choice could be severe. Expert route finding skills and conservative terrain choices are essential for safe travel in avalanche terrain.
Strong northwesterly winds have loaded lee slopes and cross loaded gullies and other terrain features. Slab avalanches formed by those winds could be triggered by humans in steep wind loaded avalanche starting zones. These slabs could be one to three feet deep at the mid and upper elevations and up to 18 inches deep at the lower elevations.
On steep avalanche prone slopes, large persistent slab avalanches could fail on buried weak layers of faceted snow. These dangerous slabs could be two to four feet deep at the mid and upper elevations and 1 to 2 feet deep at the lower elevations. These slabs could be triggered by the weight of a single person or by the release of a wind slab.
Dry conditions and sunny skies are expected on Tuesday. Mild inversions are likely in the morning with near zero temperatures predicted for the higher elevations and sub-zero temperatures predicted in the valleys. Daytime highs should reach into the twenties at all elevations. Winds are to be northwesterly with five to fifteen mile per hour averages. The general avalanche danger rating will decrease to Moderate tomorrow above 7500 feet and to Low below that elevation. This is simply due to the decreased sensitivity of dangerous slabs that have become unlikely to release naturally, but could still be triggered by the presence of a single person. As is consistent with persistent slab problems, above 7500 feet, a chance continues for avalanches to be triggered remotely upon approach from a ridge line or from below a slope. Long propagation distances also provides for the possibility of large avalanches. Crown depths could reach as much as four feet at higher elevations and two feet at lower elevations.
