Avalanche Forecast
Regions: Cascades - North East.
You can still trigger a large and dangerous avalanche on a buried persistent weak layer. Northerly winds on Wednesday night may be strong enough to transport snow and create fresh wind slabs at upper elevations. If you experience cracking in the snow, collapsing, or snowpack tests indicating that you can trigger avalanches, avoid slopes steeper than 35 degrees.
Discussion
Snow and Avalanche Discussion
The recent storm brought a dusting of new snow to the Washington Pass area. Westerly winds have been strong at upper elevations over the past few days. No new avalanches have been reported since this past weekend. Snowpack observations in upper elevation terrain have been limited, but fresh wind slab formation is suspected. Weak snowpack structure lingers throughout the zone. The last major loading event and subsequent natural avalanche cycle occurred February 12th and 13th. Avalanches failed on facets over a crust with the most significant avalanche activity, and widest propagation, occurring on east and south aspects.
Significant snowpack variability can be found throughout the East-North zone. In outlying shallow snowpack areas further east of Washington Pass such as Goat Mountain and Twisp River, observers have reported particularly poor snowpack structure recently, even below treeline. Facets over a crust are the culprit. Be especially careful if you venture into these areas.
Snowpack Discussion
February 19th, 2019
Recap
Weâre now over a week out from a major winter storm and avalanche cycle that left a string of school cancellations and avalanche near misses in its wake. As with snowfall amounts, the avalanche cycles have been similar, but not identical in all regions. The further weâre getting from the peak of the cycle, the more variation in avalanche conditions weâre seeing between regions and even within individual zones. Variable snow totals from storms this week are further adding to the range of conditions you will encounter. In some places, these storms may add stress to existing weak layers.
In the days after the natural cycle it was obvious that you could trigger an avalanche. Large crowns were visible and you could feel and hear collapses in many zones. Managing your risk was easy. Avoid avalanche terrain. Since the natural avalanche cycle of the 11-12th quieted down, the main concern for avalanches has focussed on the February 8th facets in regions where the weak layer is problematic.
A natural persistent slab (D2) on a north aspect at 4200ft low in Glacier Creek drainage (Hwy 542). 02/13/19 Lee Lazzara Photo
Variability, Complexity, and Manag
As the time moves on and the snowpack structure changes, weâre seeing the potential for triggering avalanches change as well. The February 8th layer is rounding (strengthening) and the likelihood of triggering an avalanche on it is decreasing. so much so that the problem is trending to unlikely in some regions. Unfortunately, the consequences (size and destructive potential) remain the same if you do trigger an avalanche on this layer.
These conditions are commonly described as "low probability - high consequence" scenarios. Under these circumstances, common clues may paint a conflicting picture and snowpack tests become even more difficult to interpret (snowpack tests often donât give us a clear âgo or no-goâ answer, if such a thing exists).
Q: How do we manage our risk when observations are contradictory and difficult to interpret?
A: When avalanche conditions are complicated, defer to less consequential and simpler terrain to manage your risk. Prioritize obvious clues, like recent avalanches, shooting cracks, or collapses. Focus on other observations that indicate a potential to trigger avalanches. Snowpack tests are just one piece of the decision-making puzzle. Lean on them as reasons to reduce your group's exposure to avalanche terrain. Donât use them to justify traveling in more consequential terrain.
A natural persistent slab avalanche (D2), likely occurred on 2/12Â on southwest through southeast aspects of Windy Mountain at 5,400ft in the Tye River drainage. Photo: Dan Veenhuizen.
Case Study
On the 17th I dug a profile, east of Stevens Pass on a north-northeast aspect at 4,127ft. I found the February 8th facets (0.5-1.5mm) rounding and buried 59cm from the surface. After much investigation, I found the following results at the February 8th interface: CTH (SP), ECTN28, PST 45/100 (END), 5 yellow flags (structural indicators). Later that day, about 2000 linear feet away from the profile site at the same elevation and slightly different aspect, we experienced a massive rumbling collapse.
All this crypto snow-speak means that some of the observations I made indicated that triggering an avalanche was likely, but some did not. Depending on your interpretation, some results could be argued either way. Confusing, right?
With all of this data in my filed book, it was the collapse that stuck out. It was enough evidence for me to avoid slopes steeper than 35 degrees. That was a more obvious answer than all the other data I gathered and itâs the easiest to interpret. Without the collapse, I would have prioritized the test results that indicated I could have triggered a slide.
Avalanche Problems
Persistent Slabs
The snowpack structure is problematic throughout the zone, and it will take time to heal. Although we are a few days out from any natural avalanche activity, triggering a large persistent slab avalanche remains possible. One to two feet of recent snow rests on weak faceted grains which, in some areas, can be found over a crust. This is a great recipe for avalanches. It appears that the shallower snowpack zones further east, or closer to the Columbia River, may have a far worse snowpack structure. A large amount of uncertainty remains for upper elevation terrain - be very cautious of recently wind loaded areas. Continue to watch for recent avalanches, shooting cracks, and collapses. This could be a good time to simply stay out of large, open slopes greater than 35 degrees where you could trigger one of these lingering persistent slabs.
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: All aspects.
Elevations: All elevations.
Likelihood: Possible
Expected Size: 1 - 1
Wind Slabs
Plenty of low-density snow is available for transport. Winds may blow the strongest overnight on Wednesday and form fresh slabs in leeward terrain near and above treeline - be careful, these fresh slabs could form on weak old snow surfaces. Old wind slabs still exist but are becoming stubborn to trigger. Look for signs of wind transported snow such as textured snow surfaces, drifting, or fresh cornices. If you see these features, expect wind slabs on nearby slopes.
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