Avalanche Forecast

Forecast Discussion

Unseasonably cold and very active weather this past week has brought a massive change to the region with a doubling or near doubling of the snowpack in many areas near and above treeline and suddenly enough snowpack below treeline for recreation (and avalanches!). Green Lake and Lost Horse Snotel sites picked up nearly 4” and 2.5” of snow water equivalent, respectively, with snow depths increasing by around 20” in both cases. The most recent bout of intense precipitation came on Wednesday into Wednesday night with likely higher density snow adding 5-6” to the snowpack. We likely formed a storm slab over the lower-density snow from earlier in the week. Use small, steep test slopes and frequent hand shears to identify strong over weak layering and avoid slopes steeper than 35 degrees where this problem exists. Additional snowfall arriving late Friday may exacerbate this problem.

Near and above treeline, strong to extreme W winds earlier this week through Tuesday night were present at White Pass and were likely equally as strong in this zone. We think this may have created large (potentially even isolated very large) slabs at upper elevation. These may now be hidden by less wind-affected layers. Move cautiously as you ascend in elevation, constantly probing the snow for more cohesive or supportable layers. With winds switching directions over the course of the storm cycle, it may not be easy to locate wind slabs, so consider sticking to wind-sheltered terrain.

In some locations, you may find deep, unconsolidated snow. Deep snow hazards are real. Before heading out, learn about tree well or snow immersion suffocation risks and travel with a partner (www.deepsnowsafety.org).

Forecast Schedule and No Rating definition

We do not have enough specific snowpack information to issue an avalanche hazard rating for the East South zone. However, even when No Rating is applied, applicable avalanche conditions and backcountry travel advice will be provided throughout the season - this forecast is updated every day, all winter long!  When weather systems produce very dangerous avalanche conditions in adjacent zones, NWAC will issue an avalanche warning for this zone as well.

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.