Avalanche Forecast
Regions: Mt Hood.
A wind storm overnight should form widespread and dangerous slabs near and above treeline. Hard slabs can be difficult to assess. They may lure you out onto a slope before breaking above your head. In wind-sheltered locations below treeline, storm slabs will become more reactive as the day wears on.
Discussion
Snowpack and Avalanche Discussion
Recent professional reports indicate the bottom 50†of the snowpack to comprised of well-bonded dense snow and crust layers. The most recent crust was buried January 4th and should extend to the top of the near treeline terrain. New snow of varying depth sits on top of the crust.
Snowpack Discussion
January 4, 2019
The first few days of 2019 were active here in the Northwest. A strong weather system impacted the region bringing warm temperatures, heavy precipitation, and strong winds. This weather system did not impact the forecast areas equally. Even within the same forecast zone we can see wide discrepancies in precipitation numbers. The snowpack you encounter this weekend will be largely dependent on where you go and the elevation at which you travel
Storm Precipitation Totals as of Friday Afternoon
Hurricane Ridge: 2.41â€
Mt Baker: 6.52â€
Stevens Pass: 2.58â€
Snoqualmie Pass: 2.27â€
Crystal Mountain: 0.52â€
Paradise: 2.23â€
White Pass: 0.55â€
Washington Pass: 1.05â€
Mission Ridge: 0.31â€
Mt Hood Meadows: 0.51â€
A few big stories stand out in the current snowpack: recent avalanche warnings in the northern zones, persistent slabs in the western areas, and a complex and weak snowpack in the eastern zones.
The northern zone experienced the brunt of this latest weather system. This led to two days of avalanche warnings and at least one large natural avalanche cycle. It's tough to say what the snowpack looks like in areas near and above treeline, but we know those areas received substantial new snow.
Photo: Large natural avalanche at Mt Baker Ski Area during the recent storm. -Mt Baker Ski Patrol
Earlier in the week we began forecasting a new persistent slab in our west-slope zones. A layer of buried surface hoar produced avalanches last Saturday, Sunday, and Monday. How did that layer fair after this recent round of weather? In locations like Mt Baker and Paradise, it was well tested with heavy precipitation. In other locations, less water may not have adequately stressed the weak layer. As visibility improves and more observation come-in the picture may become more clear.
Photo: Large remotely triggered persistent slab avalanche in the Crystal backcountry: Jeremy Allyn
In the eastern zones a complicated and weak snowpack exists. Several persistent weaklayers have plagued these regions most of the winter. Don’t expect this to change anytime soon. Snow profiles and snowpack test can give you a glimpse into the persistent layer. Remember, snow profiles cannot prove the absence of a weak layer or that a layer has “healed.â€
Photo: Large remotely triggered slide on buried surface hoar from Christmas above Leavenworth on 12/31: Matt Primomo
Avalanche Problems
Wind Slabs
Moderate winds were gusting strong Thursday night into Friday, creating wind slabs. Mt. Hood Meadows Pro Patrol reported generally good bonding of shallow wind slabs to the refreezing crust near treeline on Friday. However, above treeline where temperatures stayed below freezing throughout the storm, expect wind slabs to be more reactive on wind-loaded features. Navigate around convex rolls and unsupported terrain features steeper than 35 degrees. When in doubt, minimize your exposure to avalanche terrain by traveling on wind scoured ridges and slopes under 35 degrees.
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
Storm Slabs
Storm slabs will form where significant new snow accumulates without significant wind effect. These slabs are likely to form later in the day Sunday as snow continues to accumulate. You can use simple observations such as measuring the depth of the new snow (>8”), small slope tests, and simple hand pits to find strong snow (slab), over weaker snow (weaklayer). These observations can be used to check whether storm slabs are present in nearby terrain. If present, avoid unsupported slopes and slopes steeper than 35 degrees.
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.
Elevations: Below Treeline.
Likelihood: Possible
Expected Size: 1 - 1