Avalanche Forecast
Issued: Jan 16th, 2020 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Storm Slabs.
NWAC,
Northwest Avalanche Center
Be suspicious of any slope where you anticipate the wind drifted snow into deeper and thicker slabs, find fresh cornices, or encounter steeper convex rollovers. A few inches of new snow may hide clues that potential avalanches exist below. Finding and avoid unstable snow could pose a real challenge and result in a surprise avalanche for an unwary traveler.
Summary
Discussion
The avalanche danger should trend down a bit on Friday, but that’s not a green light. Three clear points stick out for Friday’s forecast: 1. The West-South just experienced a significant east/southeast wind event creating unstable snow in some unusual locations. 2. Decent snowfall accompanied the wind event in most areas. Several sites received 10-12” of snow as of Thursday morning. 3. Natural avalanches occurred in the last 48hrs.
On Thursday in the Crystal backcountry, NWAC staff found evidence of 6 natural avalanches that likely occurred Wednesday night. Four of these events crossed a common snowshoe and skin track into Bullion Basin. (SW, 5800’, SS-N-D1.5-R2-S) The group also triggered a large rumbling collapse and whumpf in open trees on a NE aspect at 5800'. These are all clear signs of 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
Wind Slabs
You’ll need to look and feel for wind slabs on Friday. Many of the visual clues we often use could be hidden from view by a few inches of soft snow. Instead, look for larger observations such as fresh cornices and drifts to point out potentially loaded slopes. Feel the snow underneath you. When it changes, becomes stiff, or even feels hollow, you likely found a wind-loaded feature. Steer around any steep slope where you suspect the wind created new slabs. You are most likely to find wind loading on SW-NW-NE aspects but could encounter them anywhere.
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
Expected Size
Storm Slabs
You may notice the subtle up-side-down nature of the recent snow at lower elevations. It feels heavier than the soft snow below. Steer around slopes where you find convex rollovers that exceed 40 degrees. On Thursday, these features produced cracking and small avalanches.
The West South received quite a bit of snow over the past week. Several travel concerns should be on your mind including deep snow immersion and treewells. Get informed and travel with a partner (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.
Elevations: Below Treeline.
Likelihood
Expected Size
Valid until: Jan 17th, 2020 10:00AM
