Avalanche Forecast

Issued: Jan 17th, 2020 10:00AM

The alpine rating is considerable, the treeline rating is considerable, and the below treeline rating is considerable. Known problems include Storm Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

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Warming, strong winds, and heavy precipitation should combine to create dangerous avalanche conditions Saturday in most areas of the West-South. Steer away from all open slopes greater than 35 degrees when you encounter more than 8 inches of new snow.

Summary

Discussion

A significant winter storm will impact the West-South region Friday night and most of the day Saturday causing avalanche danger to increase throughout that time period. Avalanche danger should peak mid to late morning as high winds and heavy precipitation combine with warmer air. Mt St Helens and Mt Adams could see the most snow, warmest temperatures, and the highest avalanche danger. Other locations in the West South could see less snow accumulate causing avalanche danger to be lower in those areas. 

Look for avalanche concerns to be limited to the new snow and how it bonds to the old snow surface. It’s hard to say how the heavier snow will react to the soft snow surface from Friday, but all the pieces are in place (warming, winds, and precipitation) to create unstable snow.

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

An icon showing Storm Slabs

Warming temperatures Saturday should create stronger heavy snow over weaker lighter snow. This upside-down slab structure could lead to avalanches on any open slope greater than 35 degrees. You can find storm slabs by looking for areas with more than 8 inches of new snow and digging with your hand or shovel to feel the snow. If you see cracking or experience your feet getting stuck under the surface, you’ve likely found slab conditions. Because storm slabs are often widespread, if you find them in one location, they are likely in all locations. 

If you travel in an area with less new snow, turn your attention to features where the wind drifted the snow deeper. Drifts, textured snow surfaces, and cornices can help you identify and avoid wind loaded terrain. With all the recent snow, treewells and deep snow immersion should be on your mind. Get informed and travel with a partner. https://www.deepsnowsafety.org/

Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. 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.

 

You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.

 

Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely

Expected Size

1 - 1

Valid until: Jan 18th, 2020 10:00AM