Bulletin d’avalanche

Stevens Pass

Dec 25 – 26, 2018Northwest Avalanche Center
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Archivé · expiré 8 years ago

Danger d’avalanche

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

New snow on Wednesday will drive the avalanche danger. As more snow accumulates, the danger will increase. You are most likely to trigger an avalanche in areas where the wind has drifted snow into firmer, deeper slabs, or later in the day if we receive more than 6 inches of new snow.

Problèmes avalancheux

  • Storm Slabs

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

    Snowfall intensities should be light during the day, though may increase in the evening hours. As a result, storm slab instabilities may increase in the afternoon/evening. Storm slabs typically become reactive once more than 6” or 8” of snow has accumulated over the old surface, and during periods of heavy snowfall. Check for how the new snow is bonding to the old snow surface by using tests on small, inconsequential slopes. If snowfall rates increase and more than 6” has accumulated, step back and re-evaluate travel on steep slopes.

  • Deep Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Unlikely
    Taille
    2

    The persistent weak layer that we have been tracking since December 9th has been observed to be rounding, gaining strength, and sintering on Stevens Pass. This layer remains weak in areas further east and in places with a shallow snowpack. It remains near the top of our minds and will continue to dictate how we travel in the eastern zones.

    You may be able to trigger deep slabs in old snow near and above treeline in the eastern portion of the zone where the snowpack is more shallow. These are very difficult to predict. To reduce the risk of this low likelihood, but high consequence situation, avoid trusting particular types of steep slopes.  Be mindful of old, hard wind slabs and where they may overly shallow, rocky areas. Near treeline, avoid steep unsupported slopes in open stands of trees, as you may still trigger an avalanche on buried surface hoar in these areas. If you hit the wrong place in the snowpack, it could result in a very large and dangerous avalanche. Don’t underestimate how far and wide these could run when identifying safer areas to stop and regroup. Be intentional about putting a significant distance in between yourself, and where avalanches can start, run, and stop.

Discussion

Snow and Avalanche Discussion:

On Christmas Day the mountains were quiet, not a creature was stirring, not even a mouse. The skies were mostly clear, with some low level clouds building in the afternoon. The ski quality looked excellent. A number of small loose wet avalanches were observed to have run the past few days on steep southerly slopes. Surface hoar was found growing in open areas with the cold temperatures and clear skies. With light snowfall on the way, these may get covered and preserved. If getting out tomorrow, look for a thin layer of buried surface hoar underneath the new snow.

Be sure to check out the Regional Synopsis for a big picture view to the start of our snow and avalanche season.

Snowpack Discussion

Regional Synopsis December 25, 2018

In most parts of the state, a stout melt freeze crust was formed when it rained to high elevations around Thanksgiving. The one exception to this event was in the East North Zone, where the precipitation from the Thanksgiving storm was all snow. A quick storm at the end of November put a small amount of snow above the melt-freeze crust, and preserved the older basal facets in the northeastern areas.

Cold and clear weather dominated the first week in December, with valley fog and very cold temperatures east of the crest. The surface snow sat around and decomposed. Surface hoar grew large on top of this.

The jet stream took aim at the Pacific Northwest in the 2nd week of December.  Most notably, light storms buried and preserved a widespread layer of surface hoar and/or near surface facets on december 9th. From December 9th to December 23rd, storms kept coming. Freezing levels fluctuated, but never moved much above 5000ft throughout the Cascades (although the southernmost volcanoes and Mt. Hood saw rain well above 6000).

Initially, the storm track favored the northern zones. The accompanying avalanche cycle began on December 11th. Most of these slides were soft slabs, but some propagated widely on the December 9th layer. Higher snowfall totals in the West North resulted in very large (D3+) avalanches in the mountains along Hwy 542.

A second, and larger avalanche cycle occurred during heavy snowfall and strong wind events between December 18th and 20th. Although these cycles were once again most prevalent in the northern and eastern zones, big storm totals around Mt. Rainier tipped the balance down south as well. This 2nd cycle was impressive, with very large and destructive avalanches (some D4) reported. The culprit was once again the December 9th surface hoar/facets (and/or the basal facets in the northern and eastern zones).

Today we have a large difference in snowpack depths between the Pacific Crest and the Eastern Slope. This is nothing unusual, as more often than not the west side of the Cascades and the passes get more snow than areas further east. Moving forward, places with a deep snowpack (say greater than 5ft) and warmer temperatures may continue to gain strength. Areas with a shallow snowpack (say less than 3.5ft) may take much longer. In a general and applied sense, this means the avalanche danger/conditions may begin to diverge between the western and eastern zones.  

As the skies clear and we move into high pressure, take note as to which avalanche paths have run large on deep, weak layers, and those which haven’t. Be sure to track surface conditions, as this next period of cold, clear weather may create the next weak layer when the storm track does turn back toward us. As always, please share your photos and experiences with us!

Happy Holidays

Régions couvertes