Avalanche Forecast
Regions: Cascades - North East.
Bottom Line: At upper elevations, avalanches may be deep and unsurvivable. Dangerous conditions exist after another significant storm has added more snow over a very weak layer. Stay off of, and out from underneath slopes steeper than 30 degrees if you see signs of danger like recent avalanches, and shooting cracks.
Discussion
Snow and Avalanche Discussion:
When digging around for information today, the most common response was “I thought about going out, but the snow quality seemed low, and the avalanche danger high.†We are in the dark regarding what happened during the storm. We do know that we picked up around an inch of water equivalent on Washington Pass, but the total snowpack height did not increase. On Harts Pass we picked up 1.4" and 5" of snow on Harts Pass. The high amount of water to snow mean it is an elevation game at this point. Conditions are likely very dangerous as one ascends in elevation where more precipitation fell as snow.Â
Another freezing rain crust formed up to about 4500ft across the eastern slopes, with a rain crust likely up to at least 5,500ft. Above 5,500ft the new snow is likely sitting on the recent crop of surface hoar and/or facets, which could make for poor bonding and potentially wide avalanches.
Snowpack Discussion
20190104 Regional Synopsis
The first few days of 2019 were active here in the Northwest. A strong weather system impacted the region bringing warm temperatures, heavy precipitation, and strong winds. This weather system did not impact the forecast areas equally. Even within the same forecast zone we can see wide discrepancies in precipitation numbers. The snowpack you encounter this weekend will be largely dependent on where you go and the elevation at which you travel
Storm Precipitation Totals as of Friday Afternoon
Hurricane Ridge: 2.41â€
Mt Baker: 6.52â€
Stevens Pass: 2.58â€
Snoqualmie Pass: 2.27â€
Crystal Mountain: 0.52â€
Paradise: 2.23â€
White Pass: 0.55â€
Washington Pass: 1.05â€
Mission Ridge: 0.31â€
Mt Hood Meadows: 0.51â€
A few big stories stand out in the current snowpack: recent avalanche warnings in the northern zones, persistent slabs in the western areas, and a complex and weak snowpack in the eastern zones.
The northern zone experienced the brunt of this latest weather system. This led to two days of avalanche warnings and at least one large natural avalanche cycle. It's tough to say what the snowpack looks like in areas near and above treeline, but we know those areas received substantial new snow.
Photo: Large natural avalanche at Mt Baker Ski Area during the recent storm. -Mt Baker Ski Patrol
Earlier in the week we began forecasting a new persistent slab in our west-slope zones. A layer of buried surface hoar produced avalanches last Saturday, Sunday, and Monday. How did that layer fair after this recent round of weather? In locations like Mt Baker and Paradise, it was well tested with heavy precipitation. In other locations, less water may not have adequately stressed the weak layer. As visibility improves and more observation come-in the picture may become more clear.
Photo: Large remotely triggered persistent slab avalanche in the Crystal backcountry: Jeremy Allyn
In the eastern zones a complicated and weak snowpack exists. Several persistent weaklayers have plagued these regions most of the winter. Don’t expect this to change anytime soon. Snow profiles and snowpack test can give you a glimpse into the persistent layer. Remember, snow profiles cannot prove the absence of a weak layer or that a layer has “healed.â€
Photo: Large remotely triggered slide on buried surface hoar from Christmas above Leavenworth on 12/31: Matt Primomo
Avalanche Problems
Persistent Slabs
On the 31st, I found a highly reactive layer of buried surface hoar down about 18” near Leavenworth. The instability on this layer was found starting at 5,600ft and became ‘in your face’ at 6,400ft. Here, I triggered a number of very wide avalanches while walking on a ridgeline from a distance. A recent avalanche cycle on this layer was observed, but not everything had slid. Activity appeared to be mainly on North through Southeast aspects. These avalanches were surprising in how widely they broke across a slope and the fact that they were triggered from up to 200ft away. Observers in the Washington Pass area were also triggering avalanches on this layer the previous day. I would be wary of any steep, open slopes above 6,000ft right now, because I would expect these persistent grains to be buried about 2 feet down. From recent experience we know that slides on this layer can be very wide. Snowpack tests can help identify this layer, but cannot prove its absence. On January 2nd, an observer noted that skiers were getting into aggressive terrain near Washington Pass with no results. This may be because of the lack of a slab over this weak layer. We likely have a slab now. Be observant of shooting cracks or collapsing to check for this layer. Use slopes of less than 30 degrees for travel, and avoid getting underneath any steep slopes at upper elevations. At lower elevations, where water has percolated through the snowpack and refrozen to create thick crusts the snowpack is likely bonded much better.
The further east you get, the less snow is on the ground. Recent observations from Mission Ridge and above Mazama speak to a shallow snowpack with weak snow near the ground. This poor snowpack structure warrants respect, because if the layer near the ground failed, the avalanche could be well over 4 feet deep. Places to be cautious of in particular are where stiff slabs sit over weak snow, slopes where grass and rocks poke through with deep drifts nearby, and any steep slopes on the far eastern side of the range.
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: Likely
Expected Size: 2 - 2