Avalanche Forecast
Regions: Cascades - North East.
Recently buried weak snow has created dangerous conditions. If you venture into the mountains, avoid slopes steeper than 30 degrees and put plenty of space between yourself and steep slopes. Avalanches may be triggered even well below treeline, and from a distance.
Discussion
Snow and Avalanche Discussion
Recent avalanches have failed within new snow and on buried faceted layers. Avalanches with the widest propagation occurred on southerly slopes in the area. We continue to receive reports of dangerous avalanche conditions in the northeast Cascades including recent avalanches, collapses, shooting cracks, and unstable results in snowpack tests (observation, observation). Incremental loading has slowly added a slab over layers of facets found above and below crusts in the upper snowpack. Warmer temperatures may actually increase the chance of triggering a slab as the snow gains cohesion above the weak layers.Â
Snowpack Discussion
February 15, 2019
Since February 8th, the mountains (and low elevation cities) of the Pacific Northwest have experience cold and very storm weather. Significant snowfall has added up in all forecast zones. Records from Snoqualmie Pass DOT avalanche workers back to 1973 show that February 11-12th set a record for the most snow recorded in a 24hr period at that location. The table below shows storm totals starting February 8th through the morning of the 13th
5 day totals ending morning of Feb 13th
Water Equivilent (inches)
24hr storm totals
(inches)
Difference in Height of Snow (inches)
Hurricane Ridge
1.97
N/A
+ 30
Mt. Baker
1.94
44
Â
Washington Pass
1.66
NA
+ 16
Stevens Pass
2.71
49
Â
Snoqualmie Pass
3.91
80
Â
Mission Ridge
1.86
38
Â
Crystal
2.91
59
Â
Paradise
4.55
N/A
Â
White Pass
N/A
57 (4400ft)
+ 26 (5800ft)
Mt. Hood Meadows
4.70
43
Â
Heavy precipitation brought many mountain regions to their tipping point. Avalanches ran readily with a peak of snowfall intensity. For Stevens Pass, Snoqualmie Pass, East Central, West South, Mt Hood, and possibly West Central zones we have good confirmation that this cycle happened from the night of February 11th through the 12th. In other zones, snow totals haven’t been significant enough for widespread avalanche cycles, or we lack data (like in the East South zone).
A natural persistent slab avalanche (D2.5) on a southeast aspect at 6,600ft. Grindstone Mtn in Icicle Canyon. Likely ran 2/12. Photo: Matt Primomo
The high rates of precipitation drove avalanches in the storm snow. Notably, a persistent weak layer of facets and surface hoar was buried in most zones on February 8th. Storms produced a widespread and prolonged cycle of avalanches on the February 8th interface, involving a variety of aspects and elevations. Local ski patrols, highway workers, and backcountry travelers reported extensive avalanching with widely propagating crowns and very sensitive conditions. With less stormy weather, observers have just begun to get a sense of the extent of the avalanche activity. Triggering persistent slab avalanches will be a concern for backcountry travelers in zones where the February 8th weak layer is active for at least the near, if not distant future. Stay tuned for more updates.
Large surface hoar near Snow Lake Divide on February 7, 2019 just before it was buried on the 8th. Photo: Jeremy Allyn
Avalanche Problems
Persistent Slabs
Near Washington Pass and Mazama, a layer of recently buried facets may be found underneath the new storm snow. The instability associated with this layer may last some time. As the new snow settles into a cohesive slab, instability may continue. Look out for slab structure, especially on southerly aspects and in recently wind loaded areas. If you find facets underneath the recent storm layers, you found the problematic structure. If there is enough snow to act as a cohesive slab on top, you have a recipe for dangerous avalanches. Watch for cracking, listen for collapses, or dig down to look for these layers of concern. It is not worth trying to outsmart this problem. If you decide to go into avalanche terrain this weekend, keep it simple, and avoid slopes 30 degrees and steeper. Measure your slope angles. Stay far away from underneath big avalanche paths, and don't go near steep slopes that may be connected to where you are. Observers report extensive collapsing at lower elevations as well. Be very careful of small steep slopes in the trees, and watch out for potential terrain traps such as gullies, tree strainers, etc.
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: 1 - 1