Avalanche Forecast
Issued: Mar 24th, 2018 10:28PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Storm Slabs and Wind Slabs.
NWAC,
Northwest Avalanche Center
Human-triggered avalanches continue to be likely on Sunday, particularly in above treeline terrain on Sunday. Expect this problem to be widespread through the exposed terrain. You are most likely to trigger a soft wind slab on convex rollovers or steep unsupported slopes. Sun-affected snow may create (or have created) a storm slab problem. Wind-sheltered norttherly terrain below treeline is likely to have the least hazard and the best conditions for skiing and riding on Sunday.
Summary
Detailed Forecast
The recent storm/wind instabilities will persist another day and you will be able to trigger an avalanche, particularly above treeline.
The soft Wind Slabs observed should be reactive another day and with 12-18" of recent snow, slabs are likely to be large. Watch for shooting cracks, snow that settles as you move in the terrain, or signs that wind has transported snow onto lee slopes and mitigate your risk by sticking to supported slopes less than 35 degrees that are not exposed from above. This problem is most likely above treeline, but may extend into the treeline terrain.
Storm slab danger will decrease on Saturday, but watch out for the potential of large avalanches, particularly where the strong march sun warms slopes on the non-polar side of the compass, forming a slab. This is not a classic storm slab scenario, but if a slab structure exists, it will be hazardous on Tuesday.
Wind-sheltered norttherly terrain below treeline is likely to have the least hazard and the best conditions for skiing and riding on Sunday.
Snowpack Discussion
Around 12" of new snow fell in the Mt Hood area Friday through Saturday at temperatures at temperatures in the low 10's to low 20's generally and snowfall during the day Saturday was generally light. Moderate winds during this period blew low-density snow on top of lower density snow to create a widespread very soft wind slabs, particularly above treeline and were quite reactive to trigger and in some cases remote trigger. These were identified within the ski area and on south-facing back-country terrain.
Wednesday and Thursday witnessed lighter amounts of snow falling on a cooling trend. Weather stations indicate that rain reached around 6000’ on Wednesday night before lowering freezing levels brought a transition to snow. Cooling temperatures throughout the storm should create a generally good bond 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 Saturday, MHM pro patrol reported many small to large skier-triggered and explosive-triggered soft slab avalanches near and above treeline. Some were remote-triggered. The problem described was a soft wind slab.
On Saturday, snow professionals traveled in the Newton drainage and found 1 foot of high quality new snow. Winds had created loose, but not reactive wind deposits. Near and above treeline, back-country travelers reported south-facing slopes were forming slabs and becoming more reactive.
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
Storm Slabs
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-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.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
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 25th, 2018 10:28PM
