Avalanche Forecast
Issued: Mar 25th, 2018 12:14PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Storm Slabs.
NWAC,
Northwest Avalanche Center
Increasing wind and new snow will create dangerous avalanche conditions at upper elevations. Use visual clues to help you navigate around convex rollovers or steep unsupported slopes where you can trigger a Wind Slab avalanche. As temperatures warm, Storm Slabs may develop. Be prepared to evaluate conditions and dial back your terrain selection as the danger increases.
Summary
Detailed Forecast
You will be able to trigger a large Wind Slab on Monday with increasing danger as the day progresses.
Light snow will combine with moderate winds to build on top of older and still healing soft wind slabs that formed Friday through Saturday, likely adding to their instability and potentially rejuvenating the problem. These slabs are most likely to be found at higher elevations on Monday, where cold temperatures likely slowed the healing process of these slabs. Navigate around larger terrain features and steeper slopes near and above treeline that are either actively loading or may have been loaded earlier in the week. And dial back your terrain selection as winds increase on Saturday.
Storm Slabs will be possible to trigger as higher density snow falls on the recent lower-density snow, creating a slab. Watch for this problem to develop later in the day and observe visual clues such as cracking of the surface snow to detect slab formation. With 1.5-2' of recent dry snow to work with on northerly aspects, even a shallow slab could entrain significant snow and run farther than you expect.
Snowpack Discussion
Mid-to-upper elevation stations at Mt. Hood have seen 2-4" of snowfall on Sunday and increasing winds were likely transporting the low density snow.
Around 12-14" of new snow fell in the Mt Hood area Friday through Saturday 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 both Friday and Saturday. 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. Temperatures have stayed cold since last Wednesday night, which was the last time temperatures were above freezing at Mt. Hood Meadows base.
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 created a generally good bond to old snow surfaces.
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 March 10-12, which helped to consolidate and strengthen the snowpack. On more shaded slopes a thinner and weaker crust can be located below the new snow.
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.
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
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
Valid until: Mar 26th, 2018 12:14PM
