Avalanche Forecast

Archived

Jan 19th, 2020–Jan 20th, 2020

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Regions

Olympics.

The avalanche danger is decreasing following Sunday's rain on snow and we think the upper snowpack is handling the water well. You can still trigger small storm slabs on steep, convex slopes above the rain/snow line or loose wet avalanches on any slopes steeper than 35 degrees below this transition. Avoid exposure to large, recently formed cornices that are sagging, failing, and may entrain wet or dry snow on the slopes below.

Discussion

The Olympics saw a prolonged period of cold snow that nearly doubled their snowpack, followed by a period of heavy snowfall, wind, and moderating temperatures on Saturday. Then, on Sunday, the temperatures rose above freezing (with snow levels estimated around 6000 ft). The light drizzly rainfall combined with moderate winds wet the top meter of the snowpack on a SSW aspect near the Hurricane Ridge visitor center. We don't know whether more northerly aspects were protected more by the wind. Rollerball activity was observed in some locations, indicating that the snow surfaces had the potential to produce loose wet avalanches on steep slopes. Generally, the snowpack seemed to be handling the modest additions of water nicely.

Forecast Schedule

For the 2019-20 winter season, avalanche danger ratings will be issued for the Olympics every Friday through Sunday and during mid-week holidays. During the week, No Rating will be issued but forecasts will include expected conditions and relevant travel advice. If you are out in the Olympics, share your backcountry observations with us and the greater community.

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

51

91

Heather Meadows Mt Baker

95

126

Stevens Pass

63

85

Snoqualmie Pass

33

77

Mission Ridge Mid Mtn

18

28

Crystal Mt Green Valley

66

92

Paradise Mt Rainier

105

138

White Pass Upper

69

110

Timberline

57

118

Mt Hood Meadows

53

98

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.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Loose Wet

Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.