Avalanche Forecast
Issued: Feb 16th, 2018 11:10AM
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
Travel in avalanche terrain is not recommended. Heavy snowfall and very strong wind will create very dangerous avalanche conditions. Stay far away from slopes steeper than 30 degrees or terrain where avalanches could come from above.
Summary
Detailed Forecast
Travel in avalanche terrain is not recommended Saturday. If you go into the backcountry today, you should be certain of your ability to avoid all avalanche terrain. Put a wide buffer of terrain between where you travel and any slopes over 30 degrees. Use extreme caution in areas where avalanches could come down from above. Avalanches may be surprisingly large and run farther than you expect.
Intense snowfall and very strong winds will create large to very large wind slabs at all elevations with large storm slabs rapidly developing near and below treeline. You can easily trigger Wind Slabs on steep slopes. The upper snowpack will become very unstable as high snowfall rates of denser storm snow fall on lower density and faceted snow, sitting above a good sliding surface (a crust). Natural avalanches are likely and human-triggered avalanches are very likely on Saturday.
As the storm totals grow, you will be more likely to trigger storm slab avalanches. Storm Slabs exist on all aspects. Avoid open slopes greater than 35 degrees where you will be likely to trigger a storm slab.
Snowpack Discussion
On Friday, light snow fell at Mt. Hood on a warming trend, depositing 3-5" of snow in the Mt. Hood region. Moderate to strong westerly winds re-distributed this snow and formed small to potentially large wind slabs on lee slopes. Scoured surfaces surfaces are expected on windward slopes, while lee sloped hold 2-4 feet or more of wind deposited snow. In sheltered areas 10-12 inches of storm snow fell Wednesday onto a strong firm crust layer. Soft subtle storm slabs formed with settlement of the new snow, but time has healed them.
The recent snow sits on top of a firm crust layer formed on Feb 8th. The warm wet weather from the beginning of February created a strong consolidated snowpack. There are no significant layers of concern in the mid and lower snowpack.
Observations
NWAC professional observer Laura Green and the Mt Hood Meadows ski patrol Thursday reported wind transported snow near and above treeline. Wind scoured slopes were reported on windward aspects. Deep wind slabs up to 6 feet were observed during avalanche mitigation work Thursday morning.
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: All elevations.
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.
Elevations: Treeline, Below Treeline.
Likelihood
Expected Size
Valid until: Feb 17th, 2018 11:10AM
