Avalanche Forecast
Regions: Cascades - East.
Avalanche concerns increase as you gain elevation and enter wind exposed slopes. Use visual clues to help you look for and avoid typical places where these slabs form, such as slopes below ridges. Breakable crust may create challenging travel conditions at lower elevations.
Discussion
An inch or two of new snow may be found in the mountains, and ridgetop winds were strong enough to drift the very low density snow around. Previously, the cold temperatures substantially weakened snow surfaces, and widespread surface hoar was observed in the area. It has been a number of days now since the big storm event last weekend, and the last reported avalanche and instabilities on a weak layer near the ground were on the 23rd. A near surface crust can be found up to around 5,500ft throughout much of the zone, making travel difficult at these lower elevations. It is also very unlikely you'll trigger avalanches at these lower elevations because of refrozen conditions. Check the new Regional Synopsis for a look back at our recent wet and wild past.Â
More remnants of last weekends avalanche cycle: East Face of Mt. Mastiff, E at 6,600ft from Hwy 2. December 26, 2019. Photo: Matt Primomo
Snowpack Discussion
December 26, 2019 (The regional synopsis is updated every Thursday @ 6 pm)
No matter where you are in the northwest, the snowpack looks a world different than it did a week ago. A major storm, widespread avalanches, developing weak layers, and an initially thin snowpack made for a hectic week. Let’s take a look at a few large scale trends for our holiday season.Â
A very large natural avalanche (D3) on the Shuksan Arm near Mt Baker that occurred during the storm cycle. Photo: Adam U.Â
Wet and Wild
“Some of the highest precipitation rates I’ve ever seen.†“Absolutely puking!†“Rivers running in the streets.†“Snowing snow hard I can’t see my hand in front of my face.†These were all statements we heard describing the wet and wild storm that impacted the Northwest from about Thursday (12/19) through Sunday night (12/22). Water totals were staggering in many locations (Table 1). For Hurricane Ridge, Mt Baker, Washington Pass, and high elevation terrain this deluge translated into significant snowfall. However, warm air in the central and southern parts of the region brought rain well into the near treeline bands. Whether your favorite spot saw rain or snow, four things are clear. 1. This was a huge loading event. 2. A widespread natural and triggered avalanche cycle (up to D3) occurred. 3. Avalanche danger spiked during and just after the storm before trending down through the week. 4. The snowpack changed dramatically.Â
Location
Precipitation 12/19-22
Hurricane Ridge
4.97â€
Mt Baker Ski Area
7.35â€
Washington Pass
3.53â€
Stevens Pass
6.28â€
Leavenworth
2.94â€
Snoqualmie Pass
7.89â€
Crystal Mt Ski Area
7.45â€
Paradise, Mt Rainier
6.57â€
Mt Hood Meadows
2.18â€
Table 1: Precipitation totals for select weather station locations December 19-22. Â Â
The Emergence of Persistent Slab Avalanche Problems
Coming out of this huge loading event, there was hope that many of the early season weak layers had been destroyed. Unfortunately, this was not the case in some locations. Old weak snow layers reared their heads in areas east of the Cascade Crest and near Crystal Mountain. While the exact extent and character of these layers can differ slightly, most locations are finding a layer of weak sugary facets associated with a crust about a foot above the ground. Persistent slabs are tricky to assess. Do not solely rely on snowpits and snowpack tests to help you choose terrain. How will these layers change going forward? Only time and observations will tell.Â
You may find weak sugary facets near a crust about 12 inches above the ground similar to what you see here. Photo: Jesse Charles
Low Tide Snowpacks During the Holidays
The storm this past week definitely helped the meager early-season snowpacks in all locations. Areas like Hurricane Ridge, Mt Baker, and Washington Pass experienced a jump of 20 or more inches in their snow depths over the past week. However, even with these increases, an early season snowpack still describes most locations. Expect numerous obstacles as you travel including open creeks, rocks, and trees. With this thin snowpack, limited access, and difficult travel we still have limited observations in some regions.Â
Open, deeply incised creeks in the Alpental Valley. Photo: Dallas Glass
Lack of information leads to a higher than normal degree of uncertainty. If you travel to higher elevations or more remote trailheads, recognize you could experience different conditions than the forecast suggested. You can help us fill in the gaps by submitting your observation here.Â
Happy Holidays!Â
-Dallas Glass
Avalanche Problems
Wind Slabs
You may find recently drifted snow at upper elevations. Wind slabs may be easy to initiate, as they likely rest over weak snow surfaces. You are most likely to find freshly drifted slabs just below ridgetops, and on the edges of exposed gully features. Use surface textures to help identify areas that may hold these recently formed wind slabs. Steer around them, or default to lower angled terrain if you find lots of slabby snow at upper elevations.
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: Possible
Expected Size: 1 - 1
Persistent Slabs
A layer of faceted snow from late November may be found near the ground, sometimes resting on top of a firm crust. This layer can be found 1-3ft beneath the surface, or about 1ft above the ground. Although signs of instability on this layer have tapered off, we continue to find the weak structure. You don't want to get caught in a slide that breaks down near the ground, it would be a rough ride. I would especially avoid slopes that appear highly variable because of recent wind drifting, steep convex roll overs, and unsupported slopes that face the north half of the compass.
Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
Aspects: North, North East, East, West, North West.
Elevations: Alpine, Treeline.
Likelihood: Unlikely
Expected Size: 1 - 1