Avalanche Forecast
Issued: Jan 22nd, 2018 1:20PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Storm Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
Increasing winds, precipitation, and very strong warming during the day will create increasing and very dangerous avalanche conditions. Ratchet back your travel plans as the day progresses and avoid above-treeline terrain or terrain connected to higher-elevation start zones.
Summary
Detailed Forecast
An incoming system will bring increasingly stormy weather with strong winds and moderate precipitation intensities along with rapid warming during the daylight hours.
This pattern will lead to a variety of avalanche problems, with the avalanche danger for each problem increasing as the day progresses. Travel in above treeline terrain is not recommended on Tuesday.
Plenty of new snow is available for transport by increasing moderate to strong winds re-distributing snow into the below-treeline terrain. Increasingly large and sensitive wind slabs will form and are likely to be very sensitive to triggering by late in the day on Sunday. Older wind slabs are still in the process of healing and may be more stubborn to trigger, but could yield avalanches that are larger than you expect. Identify and avoid wind loaded features such as snow drifts, slopes below cornices, and crossloaded slopes.
Storm slabs will become increasingly likely as warming and higher density snowfall create ideal conditions for slab formation. Expect slabs to be shallow, but avalanches may entrain significant recent lower density snowfall to form large avalanches.
A loose wet avalanche problem will develop as a problem during the afternoon hours as snowfall changes to rain at lower elevations where less snowfall has accumulated since the 01/17 crust. Natural loose wet avalanches are likely on steep, unsupported, or convex terrain up to 5000 ft, where enough recent snowfall has accumulated to create a problem. Avoid avalanche terrain as temperatures climb Tuesday afternoon. Be aware of the consequences of a small avalanche, particularly where cliffs, rocks, gullies, and creeks magnify the consequences.
Conditions will continue to deteriorate Tuesday night as further warming and heavy rain and higher elevation snow create very dangerous avalanche conditions anywhere significant recent snowfall has accumulated.
Snowpack Discussion
An active weather pattern from 1/18 onwards and continuing through Monday has deposited 1.5-2.5' of snow falling at temperatures generally in the upper 20's over the most recent 1/17 crust. Observations indicate the recent snow bonded well to the crust.
Recent light to moderate winds transported snow significantly in exposed terrain, particularly. West winds, seen earlier in the storm period, shifted to the South Sunday and became lighter and more variable.
Poor visibility and dangerous travel conditions have made observations in above treeline terrain difficult to obtain. A high degree of uncertainty exists regarding the extent and sensitivity of wind slabs above treeline.
Below treeline lower snow totals have limited the development of avalanche problems.
Observations
On Monday morning, Mt Hood Meadows Pro Patrol reported 6" of new snow at 5300' and 13" at the top of the mountain that fell on Sunday night. Wind slabs up to 1' were found into near treeline terrain, with the larger releases triggerd by explosive.
On Sunday, Mt Hood Meadows Pro Patrol reported about 12" of snow over the 1/17 crust. Winds were transporting snow at all elevations.
NWAC Pro Observer Laura Green was in the near treeline area of Heather Canyon on Friday. Recent storm snow had been redistributed by strong winds. Sensitive wind slabs were found on exposed slopes. Mostly shallow storm snow was found well bonded and not reactive to tests below treeline.
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.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Loose Wet
Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Several loose wet avalanches, and lots of pinwheels and roller balls.
Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.
Elevations: Below Treeline.
Likelihood
Expected Size
Valid until: Jan 23rd, 2018 1:20PM
