Avalanche Forecast
Regions: Mt Hood.
The sunny skies and fluffy snow may tempt you onto larger terrain, but conditions are dangerous. Winds blowing snow near ridgelines have created large, reactive soft slabs. Stick to lower angle sheltered terrain where you’ll find the best riding.
Discussion
Snowpack and Avalanche Discussion
Storm-related avalanche concerns are trending down headed into Christmas Day, but we still don't have a great deal of information about avalanche conditions above 7000 feet where more snow and wind recently impacted avalanche terrain.As of 12/26, Meadows pro-patrol is still finding weak snow (facets) buried 12/7 in backcountry snowpits between 5500 to 6600 ft on NW through SE aspects, in sheltered terrain roughly 2-3 ft down in the snowpack. The layer is trending toward unreactive. No natural or explosive avalanches have released down to this layer, but we'll continue to track this layer as it evolves over time.
Snowpack Discussion
Regional Synopsis: December 24, 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. Â 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. Â
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
Avalanche Problems
Wind Slabs
Large wind slabs have formed in the terrain as moderate winds with strong gusts transported 4” of lower density snow on Wednesday with additional snow and wind continuing Wednesday night. You can trigger these slabs on wind-loaded slopes greater than 35 degrees near ridge-lines. Sheltered terrain at lower elevations will have better skiing and riding without the wind slabs.
While not marginal amounts of new snow fell to create a classic storm slab problem, keep in mind that sun changes the slab properties of the upper snowpack and may create this problem on south-facing aspects. When the sun comes out, expect small loose avalanches to occur on steep, rocky, sunny slopes. Don’t let them catch you off guard, especially if you are on slopes that could have dangerous consequences.
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