snowpack remains weak around Yellowstone

The Gallatin National Forest Avalanche Center issued its latest avalanche advisory on Thursday, Feb. 16, with the following information issued by Mark Staples:

Mountain Weather:

Clear skies overnight helped drop temperatures into the single digits to low teens F on Thurdsday. The recent period of abnormally calm winds may be ending. Morning westerly winds were averaging 10-15 mph with gusts in the 20s except in the Bridger Range and Hyalite Canyon where ridgetop winds were blowing 20-35 mph. Thursday’s temperatures were into the high teens and low 20s F. Westerly winds blew 10-15 mph with gusts of 20-30 mph. Overnight, moisture descended from the northwest and produce an inch or two of new snow.

Snowpack and Avalanche Discussion:

The mountains around Cooke City:

During dry, clear weather last week, the snow surface formed a new weak layer which is now buried about one foot deep. Recent natural avalanche activity occurred on this layer. Slopes receiving wind-blown snow will be the best places to trigger avalanches on this layer. Fortunately this instability is easy to assess since it is not buried deeply.

Weak snow near the ground remains a concern. Recent avalanches have not broken on this layer, but it remains a serious concern because avalanches occurring on this layer will be hard to trigger, difficult to predict, and potentially deadly. This situation is a “low probability, high consequence” scenario, and not enough time has passed for me to feel comfortable ignoring this layer. As a snowmobiler, I would be boondocking today not hill climbing. As a skier I wouldn’t be skiing slopes much steeper than 30 degrees and avoiding any wind loaded areas. For today, a considerable avalanche danger exists on wind-loaded slopes. All other slopes have a moderate rated danger.

The southern Madison and southern Gallatin Ranges and the Lionhead area near West Yellowstone:

In the southern Madison and Gallatin ranges and areas near West Yellowstone, there have been many avalanches this season. Unfortunately the snowpack structure responsible for this avalanche activity hasn’t changed much. During the last six days, light snowfall has not stressed the snowpack enough to get avalanches. Winds have been calmer in these areas than in the northern half of the advisory area. Until winds increase or more snow falls, triggering an avalanche is not likely but remains possible. The avalanche danger is rated as moderate.

The Bridger, northern Madison and northern Gallatin Ranges:

Plenty of weak snow exists in the mountains near Big Sky and Bozeman but there has been less snowfall and fewer avalanches in these areas.

Two days ago Eric found stable conditions on Mt. Ellis and Doug found stable conditions in Beehive and Bear Basins. Although the snowpack is weak in many places, it is not unstable because it does not have a load. In Hyalite the snowpack is generally stronger, but has this layer of near surface facets. Westerly winds increased mostly in the Bridger and northern Gallatin ranges. With some fresh wind slabs potentially resting on small facets, heightened avalanche conditions exist on specific terrain features.

This avalanche problem is not widespread yet. For Thursday, the avalanche danger was rated as moderate on all wind-loaded slopes and on all slopes steeper than 35 degrees. Non wind-loaded slopes less than 35 degrees have were rated as having low avalanche danger.