Avalanche Forecast
Regions: Stevens Pass.
A thick slab of storm snow rests on a weak layer over a crust. The likelihood of triggering large avalanches may be slowly decreasing, but overwhelming evidence indicates that 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 big, steep terrain.
Discussion
Snow and Avalanche Discussion
The calmer weather has allowed natural avalanche activity to settle down. However, if you are a gamblin' man, you may still trigger large and dangerous avalanches. In addition to avalanches, the deep snow has hazards of its own such as Snow Immersion Suffocation, tree well hazards, and roof avalanches. Don't linger beneath roofs, travel in the mountains with partners and keep them in sight.Â
On Saturday, three separate professional parties got test results that indicated propagation on the layer of concern on Stevens Pass. Check out one of the profiles here. On Friday the 15th, a skier remotely triggered a slab from approximately 50ft away at 4,800ft in an open area between old growth trees near Lanham Lakes. The slide took out the entire clearing, edge to edge, and ran on facets over a crust. Many steep slopes ran during the cycle on the 12th and 13th. 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. Many areas may have been reloaded by additional snow and wind. Common notable characteristics of these avalanches are very widely propagating crowns up to 3 feet deep. 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 Weyerhauser clearcut, N, ~3500ft. Nason Ridge S, 5,100ft.
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 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
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
It is not worth trying to outsmart this problem. We know enough to know it is there, and well pronounced near and below treeline. More uncertainty exists in the alpine elevation band. It is healing, but more time is needed before I'd trust any steep slopes. A uniform slab 2 to 3 feet deep rests on a few inches of facets over a crust, a weak layer that makes me cringe. If you decide to go into avalanche terrain this weekend, be sure of your ability to avoid steep slopes. This is not the time to go into big, complex terrain. Measure your slope angles. Stay far away from steep slopes that may be connected to where you are. These persistent slabs have been breaking incredibly widely across terrain features, even in open areas in the trees. They may be triggered from a distance.
Professional guides and astute backcountry travelers continue to avoid steep slopes and complex terrain. 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 recent avalanches with crowns well over 200ft wide, with recent remotely triggered, large activity.
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.
4. Snowpack tests indicating propagation.
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