Bulletin d’avalanche

Stevens Pass

Feb 29 – Mar 1, 2020Northwest Avalanche Center
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Danger d’avalanche

Mon
Alpin
2 · Modéré
Limite forestière
2 · Modéré
Sous la limite
2 · Modéré
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Points saillants

On Sunday, the sun may warm recently fallen snow enough to form small wet avalanches on sun-exposed slopes. You can trigger avalanches on leeward slopes near and above treeline where the wind drifted slabs starting Friday night. Steer around fresh drifts and deeper pillows of snow on slopes 35 degrees and steeper. 

Problèmes avalancheux

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1

    Avalanches will be larger above treeline. Use caution on leeward slopes where slabs may be thickly loaded and drifted. You may even find wind-affected areas below treeline. Check the bond of the recent snow to the older surface. A slick crust at the Feb 28th interface may keep triggering avalanches possible. If you see signs of instability, like cracks shooting through the snow avoid slopes 35 degrees and steeper. 

  • Loose Wet

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1

    There may be enough sun to cause wet loose avalanches on steep sunny slopes on Sunday. Roller-balls and moist surface snow are indicators that you could see wet avalanches. Don't linger on or under slopes over 35 degrees if the snow is becoming moist and sticky. The sun will start warming east aspects early in the day and will move to west aspects by afternoon. Areas near exposed rocks may melt quicker than other locations. A good rule of thumb for sunny slopes is to avoid them once wet snow is unsupportive enough to up to your boot tops.

Discussion

A storm delivered 10-15" of snow with strong westerly wind Friday night into Saturday. The snow fell on a crust or moist melt forms on most slopes. On Saturday, observers and avalanche workers reported reactive but small slab avalanches at the new/old snow interface. 

About 2-3 feet below the snow surface, the February 22nd interface holds rounding facets and surface hoar and can still be found in snow profiles. Otherwise, there re no interfaces to note in the upper snowpack.

Snowpack Discussion

February 27, 2020 (The regional synopsis is updated every Thursday @ 6 pm)

In the lead up to last weekend, dry, clear and cold conditions produced surface hoar in many areas as well as faceted snow (check out the National Avalanche Center’s encyclopedia for more details). These weak grain types were not difficult to find and recreationists and professionals across the region began to wonder what role these weak snow surfaces would play when the next storm arrived.

Surface Hoar was not difficult to observe during this period of high pressure. This picture is from 2/21 on an SSE aspect, 5,000’ at Stevens Pass just a day before it may have been buried. Photo: Josh Hirshberg

We didn’t have to wait too long, as a major storm arrived on Sunday 2/23, creating dangerous avalanche conditions for most zones. This system dropped close to 2’ of snow at Mt. Baker while areas to the south along the west slopes of the Cascades saw closer to 1 foot. Areas along the east slopes of the Cascades also saw significant new snow amounts with Blewett Pass and Mission Ridge receiving around 6-8” of fresh snow. 

The wind was also a major factor with this storm from about Stevens Pass southward. Multiple stations recorded winds over 80mph and some went over 100mph. This wind was enough to strip surfaces down to old crusts on windward aspects, leaving leeward aspects with wind slabs and touchy cornices. In the case of Mt. Hood, relentless winds this season have stripped the snow down to blue ice from one of our previous atmospheric river events, creating slide for life conditions on certain slopes. 

The old snow interface, termed 2/22 for the day it was buried, has us thinking hard about the snowpack moving forward. In the northern part of the region, less wind allowed for more storm than wind slabs, with avalanches that released on both interstorm layers and on the buried 2/22 interface for a day or two following the storm. As you moved further southward or eastward, reactivity on the 2/22 interface was trending downward or non-existent after a natural avalanche cycle that immediately followed the storm. Was the storm potent enough to wipe out the surface hoar and facets? There was a surprising natural avalanche on Stevens Pass during the evening of Monday 2/24, which is suspected to have run on small facets. This avalanche and continued snowpack observations around the region proves that in specific areas, reactive weak snow still existed. 

This natural avalanche failed 2ft deep on the 2/22 interface on a SE aspect near 5700ft on Cowboy Mountain. 02/25/20. Photo: Stevens Pass Ski Patrol

On Tuesday, the sun started to shine in some locations, bringing back the threat of wet avalanches and again making us wonder about how warming would affect the 2/22 interface. We never reached the tipping point for widespread wet avalanche activity, but rollerballs and snow shedding off trees and rocks were certainly present in many zones. Drizzle or light snow made an appearance on Wednesday along many west-side zones. Thursday saw a return to warm temperatures and sunny skies all the way from Mt. Hood to Stevens Pass with cooler and cloudier conditions for the North Cascades. All in all, it was a quiet week avalanche-wise and the 2/22 wasn't activated. 

We have one more dry mild day before we move into a stormy Saturday. During this stretch, we’ll continue to monitor buried weak snow grains for distribution and reactivity. Check your local forecast zone for the latest. 

Régions couvertes