Avalanche Forecast
Issued: Feb 19th, 2019 10:13AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Persistent Slabs.
NWAC,
Northwest Avalanche Center
A storm overnight Tuesday will bring low-density snow, strong westerly winds, and will build fresh wind slabs in leeward terrain. The new snow may not bond well to weak and variable old snow surfaces. Avoid steep, open, wind-loaded slopes, and carefully evaluate the lingering persistent weak layer buried in the snowpack.
Summary
Discussion
Natural and triggered wind slab avalanches (up to D2) were reported in the Mt. Baker area over the past few days. Two recent natural avalanches, on Lake Ann Peak (Feb. 16) and Table Mountain (Feb. 18), appear to have been triggered by cornice fall - both in upper elevation northerly terrain. One of these avalanches looks to have stepped down onto the persistent weak layer we have been tracking throughout the zone:
Natural cornice fall triggered avalanche (D2) on a north facing slope at 5400ft on Table Mountain. 02/18/19 Zach McGill Photo
New snow and wind are expected over the next 24 hours. The most snow should accumulate overnight on Tuesday, and conditions should dry out throughout the day Wednesday. The new snow will bury weak old snow surfaces (surface hoar, facets, crusts) that observers have reported recently throughout the Hwy 542 corridor. Track the old/new snow interface carefully - this may become a lasting problem.
Feathery surface hoar (6-8mm) found in the Twin Lakes/Hwy 542 area. 02/17/19 Anjin Herndon Photo
Snowpack Discussion
February 19th, 2019
Recap
We’re now over a week out from a major winter storm and avalanche cycle, February 9-13th, 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 we’re getting from the peak of the cycle, the more variation in avalanche conditions we’re seeing between different zones and even within the individual zone. 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.
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. In the days after the natural cycle all observations, including rumbling collapses, remote triggering, and snowpack tests screamed, “avalanche!”
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 and Mixed Messages
As the facets and surrounding snowpack structure changes, we’re seeing the potential for triggering avalanches change, as well. Now, the likelihood of triggering persistent slab avalanches is decreasing. In some regions, the problem is trending to unlikely. Meanwhile, the consequences (size and destructive potential) remain significant, if not the same.
Time has helped round the February 8th facets. No longer will every clue give a resounding answer as to whether or not you can trigger an avalanche. To complicate things, observations like snowpack tests can be notoriously difficult to interpret, requiring a lot of time practicing good snow-craft. Snowpack tests often don’t give us a clear “go or no-go” answer, if such a thing exists.
Q: How do we interpret observations that are contradictory, when some point at the potential to trigger avalanches and others indicate better stability?
A: Focus on the observations that show the potential to trigger avalanches. Look for obvious clues, like recent avalanches, shooting cracks, or collapses. Prioritize observations that indicate triggering (initiation) and propagation.
A natural persistent slab avalanche (D2), likely occured on 2/12 on southwest through southeast aspects of Windy Mountain at 5,400ft in the Tye River drainage. Photo: Dan Veenhuizen.
Case Study
In a recent profile, east of Stevens Pass, I found the February 8th facets (0.5-1.5mm) rounding and buried 59cm from the surface. The results of the profile were:
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 pointed towards triggering an avalanche was likely, but some did not. Confusing, right?
With all of this data in my head, it was the collapse that stuck out. That was enough evidence for me to avoid slopes steeper than 35 degrees. That was a more obvious answer than all the other pieces of data I gathered and it’s the easiest to interpret. Without the collapse, I would have prioritized the test results that indicated I could have triggered a slide. If only snowpack tests would give you the sinking feeling of almost triggering an avalanche that you get from a rumbling collapse...
Problems
Wind Slabs
Winds will easily redistribute the new snow and form fresh winds slabs in leeward terrain near and above treeline. Wind speeds should peak Tuesday night and slowly decrease throughout the day Wednesday. Previously formed wind slabs will be masked by the new snow. The thickest slabs will exist above treeline. The new snow may remain unconsolidated in wind sheltered and lower elevation areas - watch for loose dry avalanches. Carefully evaluate the new/old snow interface. 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. Stick to sheltered or wind scoured areas to avoid triggering a wind slab.
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
Persistent Slabs
The slab/weak layer combination required for these persistent slabs does not exist everywhere in the West-North zone. It is this spotty distribution that makes our current situation tricky. We have reports of avalanches and/or snow profiles highlighting this problematic layer on all aspects and in each elevation band. The most recent avalanche possibly involving this layer occurred on February 18th (Table Mountain Picture above). Observations targeting our buried persistent weak layer have been highly variable.
This layer is buried 2-3 feet below the snow surface. On colder shaded slopes, the culprit seems to be buried surface hoar. On sunnier aspects, we are finding small facets over a crust. Obvious signs of instability such as collapses or shooting cracks have been uncommon. Digging in the snow is the best way to define where this layer may be problematic. 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
Expected Size
Valid until: Feb 20th, 2019 10:13AM
