Bulletin d’avalanche

Stevens Pass

Jan 18 – 19, 2020Northwest Avalanche Center

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Danger d’avalanche

Mon

Alpin

3 · Marqué

Limite forestière

3 · Marqué

Sous la limite

2 · Modéré

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Points saillants

Rising temperatures with light rain and snow will maintain dangerous avalanche conditions. You can trigger a large avalanche in new snow on slopes 35 degrees and steeper. The potential for wet avalanches will increase throughout the day. Reduce your risk by seeking out lower-angled and supported terrain.

Problèmes avalancheux

Storm Slabs
N
NE
E
SE
S
SW
W
NW
Alpin
Limite forestière
Sous la limite forestière
Probabilité
Likely
Taille
1
Shallow, dense slabs rest atop light, cold storm snow. This up-side-down layering configuration is a good recipe for avalanches. Although snowfall is tapering off, the storm snow needs more time to settle and stabilize. Winds will continue to transport snow at upper elevations creating thicker slabs and deep drifts. Fresh slabs are primed for human triggers at upper elevations, especially on convex rollovers and in leeward areas just below ridgelines. Watch for obvious signs of unstable snow like shooting cracks, collapses, and recent avalanches. Ease into terrain slowly, and avoid slopes 35 degrees and steeper.
Loose Wet
Alpin
Limite forestière
Sous la limite forestière
Probabilité
Possible
Taille
1
The potential for loose wet avalanches will increase throughout the day as rising temperatures push snow levels up to 4000ft by the afternoon. Loose wet avalanches could gouge into recent cold storm layers and entrain significant snow, creating larger debris piles that you might expect. Loose wet slides could trigger slab avalanches as they run downslope as well. Rollerballs and pinwheels are a good indicator wet snow avalanches will follow. Steer away from steep slopes by the time snow surfaces becomes wet and weak.

Discussion

Over the past 24hrs, Stevens pass experienced a warming trend as 4-6in of new snow accumulated. NWAC staff traveled in the Smith Brook drainage on Sunday and observed widespread, natural storm slab activity (observation). Fresh slabs were highly reactive with remotely triggered avalanches and wide propagation observed. Avalanches were small (D1) and shallow however, failing on a density break within the new snow.

The barrage of storms since the New Year began brought impressive storm totals, and the current snow depth at Stevens Pass is over 120% of normal for mid-January (Snow Depth Climatology).

Very small, shallow natural storm slab avalanche in the Smith Brook drainage on a south aspect at 4000ft. 01/18/20. Photo: Matt Primomo

Snowpack Discussion

January 16th, 2020 (The regional synopsis is updated every Thursday @ 6 pm)

In the past week and a half, there have been five avalanche fatalities in three separate accidents in the US. One occurred near Kellog, ID and another outside of Baker City, OR. Local avalanche centers will perform accident investigations including final reports. You can find preliminary accident information at avalanche.org.

From January 9th to 16th the Pacific Northwest slid into deep winter. A cold and snowy regime brought a nearly continuous barrage of storms through the area. Temperatures bottomed out as modified arctic air made its way south from interior Canada, and many stations recorded the lowest temperatures of the season so far. A snowpack has been growing at lower elevations due to some lowland snow on both sides of the Cascades. NWAC’s snow depth climatology report shows most stations have surpassed average depths on the ground for this time of year. Quite the comeback from two weeks ago, when most were at 25-64% of normal.

Location

Total Snow Depth (in) 1/8/20

Total Snow Depth (in) 1/16/20

Hurricane Ridge

Heather Meadows Mt Baker

126

Stevens Pass

Snoqualmie Pass

Mission Ridge Mid Mtn

Crystal Mt Green Valley

Paradise Mt Rainier

105

138

White Pass Upper

110

Timberline

118

Mt Hood Meadows

Snow depths continued to rise. Total snow depths doubled in some locations.

The mountains went through a period of prolonged dangerous to very dangerous conditions as the snow kept coming. Many locations picked up over a foot of new snow per day for a number of days in a row, and storm slab instability was widely experienced across the region. At times, instabilities within new snow layers were very reactive, and you didn’t have to do much to provoke an avalanche. Many people triggered small to large soft slab avalanches, even well below treeline. The cold temperatures tended to preserve these instabilities longer than usual during this time.

Small ski triggered storm slab near Mt Hood Meadows. January 11, 2020. Scott Norton photo.

This cold, low density snow was also susceptible to wind drifting as westerly winds buffeted the alpine zone from the 8th to the 15th. On the 15th the mean winds shifted, and a south and east wind event disturbed the powder on open, exposed terrain near the passes and at upper elevations throughout the region. This created wind slab problems in some unusual locations.

Wind slabs formed over the low density powder snow. Mt Baker Backcountry. January 15, 2020. Zack McGill photo.

Trailbreaking in undisturbed snow was often very deep and difficult. In most places at any point in the week you could step off your skis or machine and sink in up to your chest in deep powder snow. The deep snow presented hazards of its own such as tree wells, and made it very easy to get stuck on a machine or lose a ski. Many folks experienced excellent, deep powder conditions and stuck to conservative terrain choices.

-MP

A cold winter’s day over the Chiwaukum Range, from Stevens Pass. Matt Primomo photo.

Régions couvertes

Stevens Pass