Avalanche Forecast
Regions: Cascades - West.
Dangerous and complex avalanche conditions exist throughout the region. We have high uncertainty regarding the current conditions in the West Central zone. Avoid fresh wind drifts and use extra caution near steep unsupported slopes at all elevations.
Discussion
Snow and Avalanche Discussion
The Mountain Loop area received more than three feet of snow over the weekend, and 2-3 feet of new snow since Monday. The recent snow has fallen at cold temperatures and may be poorly bonded to variable old snow surfaces including surface hoar, firm crusts, and low-density storm snow layers. The most recent snow has fallen with warmer temperatures making a “strong over weak†layering configuration. Avoid wind loaded terrain, especially near ridges.
In the neighboring Stevens Pass and Baker zones, persistent slab problems have developed. Observations from the West Central zone are limited, but we expect similar snowpack structure and buried weak layers may exist in this zone as well. Carefully evaluate the snowpack and choose conservative terrain during this time of elevated danger.
We have received a lot of snow over the past few days. These are the conditions when tree well and snow immersion suffocation accidents occur. Don't travel alone, and keep eyes on your partners. Check out https://www.deepsnowsafety.org/ for more information.
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
Wind Slabs
Plenty of snow is available for transport, and variable winds will continue to build wind slabs near and above treeline. Avoid fresh drifts and steer around areas where the wind has stiffened the snow on leeward slopes. Approach terrain near steep unsupported slopes with wind-drifted snow very cautiously, feeling for firm or hollow sounding snow as a sign that wind slabs may be present. You can stay safe by traveling on ridges, wind-scoured areas and lower angled and supported slopes.
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: Likely
Expected Size: 1 - 1
Persistent Slabs
These avalanches are difficult to predict, and a lack of recent observations from the West Central zone creates high uncertainty regarding the distribution and sensitivity of this avalanche problem. We do know that the snowpack throughout the zone has seen a major change over the past several days and needs time to adjust. Persistent slabs can break widely across terrain features, come down on top of you, and can be triggered from a long distance away. Continue to stick to slopes under 30 degrees and smaller terrain features. Observers in the neighboring Stevens Pass zone have consistently reported; 1) Remotely triggered avalanches. 2) Widespread collapsing of the snow underfoot or a "whumphing" sound. 3) A layer of weak, sugary facets and surface hoar resting on a crust 4-5 feet below the snow surface. Watch for these signs indicating that you can trigger a Persistent Slab avalanche.
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