Snowpack List: Avalanche assessments from around the U.S.: Week of December 18

The first few weeks in December brought with them a nasty round of avalanches, due to instabilities buried deep in the snowpack across the western U.S. This week brings in more of the same, with the Pacific Northwest’s danger rating spiking to high, and the Rockies still dealing with persistent slab. Here is the regional report for this week.

Crested Butte Avalanche Center (CBAC)/Colorado Avalanche Information Center (CAIC)

Currently, the Rockies are speckled with areas of moderate and considerable avalanche danger. Where there is greater danger, the culprits are weak layers toward the bottom of the snowpack with a reactive slab sitting above. In the Crested Butte region, forecasters warn on the Colorado Avalanche Information Center website that, “The touchiest slopes will be north through east, to south facing, and particularly those in the alpine that continue to be actively wind-loaded.” This is a common avalanche factor throughout Colorado but is more pronounced in the San Juan Mountains, due to a recent storm that’s creating heightened danger.

“Avalanches in the recent storm snow could be large and dangerous, and they could trigger deeper weak layers. These avalanches can be triggered from a distance or from below,” The CAIC explains on their website about the San Juans.

While some regions list low danger below treeline, the CAIC makes it clear that because of the widespread nature of these early season instabilities, caution should be taken on all aspects and at all elevations.

To find out more about Colorado avalanche conditions, visit avalanche.state.co.us, and cbavalanchecenter.org. 

Northwest Avalanche Center (NWAC)

The summary for the regions under the purview of the NWAC as of Tuesday, December 20 is, in a few words, BE CAREFUL. Avalanche danger is rated high for all aspects and elevations. The NWAC has issued an official warning, stating, “An Avalanche warning for HIGH danger is in effect through 6 p.m., Tuesday, for the west slopes of the Cascades, including the Cascade Passes.”

This avalanche danger is a result of a cold spell in the area between the December 13 and 18 that allowed surface hoar and surface faceted snow to form.

“These layers are expected to act as widespread weak layers or sliding surfaces for the upcoming moderate to heavy snowfall,” says the NWAC about the facets.

With new snowfall headed into the region along with westerly winds, multiple aspects and elevations are seeing storm slabs form.

The final warning states, “A natural avalanche cycle should occur Monday night through Tuesday morning during peak warming and precipitation. Expect wind loading farther downslope than usual due to strong westerly winds on a variety of aspects. Slabs may fail on weak, persistent grain types formed prior to this storm cycle. Travel in avalanche terrain is not recommended Tuesday.”

To find out more about conditions in the Pacific Northwest, visit www.nwac.us. (Note: as of Wednesday morning, the danger rating has been reduced to considerable, but many of the same issue still exists in the snowpack.)

Utah Avalanche Center (UAC)

In Utah, avalanche conditions are a bit more favorable, with a few zones rated as considerable in the northern part of the region and danger ratings reducing farther south. In the Logan area, windslab is the primary concern and the reason danger is rated as considerable. This instability is primarily near and above treeline. Persistent deeps slab avalanches remain a threat in this region as well.

“The avalanche danger is considerable today for triggering a hard windslab on upper elevation slopes facing northwest through southeast,” explains the UAC on Tuesday. “These hard windslabs can be triggered along the ridgelines or in open bowls, and the danger is moderate at the mid elevations where the wind has also drifted the snow. There is considerable danger of triggering a deep slide of all the storm snow on an upper elevation slope.”

In the Uintas and Salt Lake area, windslab near and above treeline is a concern, with terrain below treeline generally rated at low avalanche danger.

“Hard windslabs can be triggered along the ridgelines or in open bowls and long running sluffs can be triggered on steep slopes. There is an isolated chance of triggering a deep slide of all the storm snow on an upper elevation, rocky northerly facing slope,” the UAC explains for these central regions.

To the south, most terrain is rated as low with a few localized pockets of instability where drifts could be triggered up high.

To find out more about conditions in Utah, visit utahavalanchecenter.org.

Gallatin National Forest Avalanche Center (GNFAC)

In Montana, persistent slab is continuing to create dangers, with new snow accumulating on these known instabilities. Fresh snow combined with wind has created windslab across the area under the purview of the GNFAC that is reactive, and all areas are rated as considerable for most aspects and elevations as of Tuesday.

“Weak snow near the ground is found throughout the southern ranges. Over the last five days it’s been responsible for natural and human triggered avalanches, cracking and collapsing under a skier’s weight and poor stability test scores. Fresh windslabs will be easy to trigger today. Given the poor snowpack structure, more weight from new snow and wind-loading, human triggered avalanches are likely,” reports the GNFAC on Tuesday, December 20.

To find out more about conditions in Montana, visit www.mtavalanche.com.

Bridger Teton National Forest Avalanche Center (BTAC)

In Wyoming, avalanche danger is rated as Considerable for all aspects and elevations, with the exception of the Teton Area, which will see a rise in danger to High as Tuesday progresses. New snow and strong winds have created a reactive surface slab across the region, especially on leeward-facing slopes.

The BTAC warns, “These destructive slabs could be human triggered with depths to five feet. Natural releases are possible.”

And for the Teton region, as instabilities worsen, the BTAC cautions people to be aware of changing conditions in the field.

“Monitor conditions and adjust plans accordingly,” says the BTAC. “Travel in avalanche terrain is not recommended during periods of high avalanche hazard. Very large deep slab avalanches continue to be a life-threatening hazard at the upper elevations. New windslabs that will be forming during the day and older windslabs that were formed during the previous storm cycle will become increasingly sensitive to human triggers as the day progresses.”

To find out more about conditions in Wyoming, visit www.jhavalanche.org.

—

As the winter progresses, it’s important to keep an eye on snowpack trends and instabilities. This is not a complete guide to avalanche forecasts, so stay informed and educated by visiting local avalanche forecasting websites and get educated about avalanche safety through programs like Know Before You Go, the Mountain Academy hosted by Solomon and Atomic and through courses offered by the American Institute for Avalanche Research and Education (AIARE) offered nationwide.