Avalanche Forecast
Regions: Stevens Pass.
A hefty slab of storm snow rests on a weak layer over a crust. The snowpack remains unstable on Stevens Pass. If you venture into the backcountry, be certain of your ability to avoid slopes steeper than 30 degrees and put plenty of space between yourself and large avalanche paths.
Discussion
Snow and Avalanche Discussion
Stevens Pass has received over 52 inches of snow since February 8th. The calmer weather has allowed natural avalanche activity to settle down. However, It will still be easy to trigger large and dangerous avalanches. In addition to avalanches, the deep snow has hazards of its own such as Snow Immersion Suffocation, or tree well hazards, and roof avalanches. Don't linger beneath roofs, and travel in the mountains with partners and keep them in sight.Â
See the Regional Synopsis for an overview of recent storm and avalanche activity. An extended period of instability began on the 11th with natural avalanches reported through the 13th. Local ski patrols, highway workers, and backcountry travelers all reported extensive slides. Common notable characteristics of these avalanches are very widely propagating crowns about 3 feet deep. While some avalanches ran in storm layers in the upper snowpack, the largest and most concerning avalanches appear to be running on a weak layer of facets (and in some places surface hoar) buried on February 8th. Visibility, deep snow, and dangerous conditions have limited observations. A few notable natural (unless otherwise noted) slab avalanches suspected to have run on the February 8th facets are: Tye Peak, SE, 5200ft up to 300ft wide. The entire Highlands Bowl on Big Chief Mtn was triggered remotely. Skyline Ridge, E, 5250ft. Arrowhead Mtn, Zephir paths in the Weyhauser clearcut, N, ~3500ft. Nason Ridge S, 5,100ft.
February 15, 2019: Lots of instability has been observed, even at elevations. Near Coles Corner, Hwy 2. Photo: Matt Primomo
Snowpack Discussion
Since February 8th, the mountains (and low elevation cities) of the Pacific Northwest have experienced cold and very stormy 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 13thÂ
5 day totals ending morning of Feb 13th
Water Equivalent (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
Â
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).
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.
Avalanche Problems
Persistent Slabs
It is not worth trying to outsmart this problem. We know enough to know its there, a uniform slab and weak layer over aspects and elevations have created a dangerous setup for slab avalanches. The slab rests on facets over a crust, a weak layer that makes me cringe. You won't find me on anything steep this weekend. If you decide to go into avalanche terrain this weekend, be sure of your ability to 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. These persistent slabs have been breaking incredibly widely across terrain features, even in the trees. They may be triggered from a distance, may come down on top of you, and may kill you.
Professional guides and avalanche workers are avoiding steep slopes like the plague. Even small, steep slopes below treeline that I rarely think much about may be exactly the type of place to avoid this weekend. Why? Numerous observations of:
1. Lots of avalanches with crowns well over 200ft wide, some that were triggered remotely.
2. Widespread collapsing, which makes a "whumphing" sound as air escapes from the collapsed weak layer, sometimes shaking trees a long distance away.
3. A layer of weak, sugary facets and surface hoar resting on a crust 3-4 feet below the snow surface.
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