Avalanche Forecast
Issued: Mar 23rd, 2018 11:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs.
NWAC,
Northwest Avalanche Center
New wind slabs will slowly gain strength Saturday. You are most likely to trigger a wind slab on convex rollovers, steep unsupported slopes, and below fresh cornices. Use visual clues such as snow drifts, cornices, and frim snow with cracking to identify and avoid wind loaded areas. If the sun comes out, be ready to get off of and avoid steep sunny slopes where you see new rollerballs and pinwheels.
Summary
Detailed Forecast
Avalanche danger will slowly decrease Saturday as new wind slabs gain strength. Even though the hazard is decreasing, you may still trigger a wind slab on steep slopes near ridgeline and cross-loaded mid-slope features. You are more likely to trigger a wind slab on convex rollovers, steep unsupported slopes, or below fresh cornices. You can use visual clues to identify and avoid wind loaded slopes greater than 35 degrees. Snow drifts, cornices, and firm snow surfaces with cracking all indicate that nearby slopes received wind deposited snow.
The strong March sun can quickly change conditions on slopes receiving direct sunshine. If the sun comes out, expect small loose wet avalanches, rollerballs, and pinwheels. Be ready to move off of and avoid steep sunny slopes during periods of intense sunshine.
Snowpack Discussion
Four to eight inches of new snow fell in the Mt Hood area Wednesday through Friday with more snow at higher elevations. Weather stations indicate that rain reached around 6000’ before lowering freezing levels brought a transition to snow. Cooling temperatures throughout the storm should create a generally to old snow surfaces.
Moderate to strong winds redistributed the snow forming new wind slabs and fresh cornices. Mt Hood Meadows patrol reported wind slabs triggered during avalanche control work both Thursday and Friday.
The new snow and wind slabs fell on a variety of snow surfaces. On southerly aspects a firm and thick melt-freeze crust can be found that formed during sunny weather last week. On more shaded slopes a thinner and weaker crust can be located below the new snow.
Several melt-freeze cycles the weekend of March 10-11 helped to consolidate and strengthen the snowpack. While we are tracking several layers deeper in the snowpack, there are currently no significant layers of concern.
Observations
On Friday NWAC professional observer Laura Green traveled in the Mt Hood backcountry. Laura reported small soft slabs in the near and above treeline band.
On Thursday, MHM pro patrol reported 8-12” (20-30cm) wind slabs avalanches during avalanche control work.
On Wednesday, MHM pro patrol reported a thin crust on sunny aspects with 3-4” of old cold snow on shaded slopes.
Problems
Wind Slabs
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
Expected Size
Valid until: Mar 24th, 2018 11:00AM
