Avalanche Forecast

Issued: Jan 16th, 2020 10:18AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

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Mt. Hood has received a massive storm cycle over the past week, giving rise to very large and dangerous wind slabs at upper elevations. These slabs formed through Tuesday night, but with fresh wind transport Thursday night and Friday, you can still trigger them and the consequences of a slide would be severe. Avoid all wind-loaded slopes above treeline, recognizing that wind slabs may be difficult to identify under lower density snow, and limit time in drainages connected to large avalanche slopes on the upper mountain. In lower-elevation wind-sheltered terrain you might trigger a small or large avalanche on a steep rollover where you can feel denser snow layers just below the snow surface.

Summary

Discussion

On Mt. Hood, evidence of active avalanche activity this week is abundant, although some of it may be slightly masked by new snowfall. With unusually cold temperatures, weak layers are slow to heal, maintaining the avalanche hazard elevated. Crowns from naturally triggered storm slabs have been up to 18” and loose dry avalanches are abundant and many can even be seen from Highway 26 and 35. Keep this activity in mind as you plan your tour and note avalanche activity in the areas you travel. Consider making travel plans that minimize time spent in canyons or gullies below large avalanche paths that have seen 5-6” of water in the form of snow over the last 6 days along with strong to extreme westerly winds.

With all this new snow, deep snow hazards exist at lower elevations and less wind-affected areas. Before heading out, learn about tree well or snow immersion suffocation risks and travel with a partner (www.deepsnowsafety.org). 

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

Wind Slabs

An icon showing Wind Slabs

Mt. Hood Meadows Pro Patrol 2 very large artillery-triggered wind slabs (D2.5) above 7000 ft on NE aspects, with avalanches running up to 650 ft (far, but not full path). Mt Hood Patrol also triggered also a very large (D3) avalanche with artillery on Wednesday. These slabs formed with strong to extreme winds which finally relented Tuesday night. Moderate winds Thursday night and Friday should transport more low-density snow and further destabilize some of these poorly healing slabs. The wind slabs at upper elevations are slow to heal in the cold upper-elevation temperatures and may be reactive on Friday, with high consequences. Give larger terrain at upper elevations on Mt. Hood some healthy respect, limiting exposure to this terrain, especially during times of active wind-loading. Near treeline, avoid steep open slopes where you see signs of wind transported snow such as cornices, and pillow-like drifts of snow.

 

Release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.

 

Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..

 

Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.

 

Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

1 - 2

Storm Slabs

An icon showing Storm Slabs

Light snow falling at gradually warming temperatures should add more dense snow to the surface of the snowpack, further destabilizing a 6" slab of denser snow that produced avalanches on Wednesday at Mt. Hood Meadows. Use small, steep test slopes to test for this unstable snow structure. Steer around steep convexities where you are most likely to trigger an avalanche on a steep and unsupported rollover approaching 40 degrees or steeper. Watch for cracking in the snow as a sign that you can trigger an avalanche on a nearby slope. With such deep snow, even a small avalanche can be difficult to escape.

Given the cold temperatures and lots of new snow, you will find wind-sheltered areas with deep, low-density snow. On very steep slopes loose snow avalanches or “sluffs” can be large enough to push you over or knock you into tree wells or open creeks. Be aware of stopping below steep slopes or above hazards like gullies, creeks, and cliffs.

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.

Elevations: Treeline, Below Treeline.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Jan 17th, 2020 10:18AM