Avalanche Forecast
Regions: Mt Hood.
We are in the midst of the first extended spring warm up and avalanche cycle. The regional focus on Friday will continue on loose wet snow avalanches, cornice failures and glide avalanches. There is probably as much or more danger from loose wet avalanches in the lower terrain bands as the upper bands.
Detailed Forecast
Another day of light winds, sunny weather and very warm temperatures is expected in the Olympics and Cascades on Friday. Freezing levels will probably be near or above 10,000 feet. This means the regional focus will continue on loose wet snow, cornice failures and glide avalanches.
The regional cycle of extensive, potentially large natural or triggered loose wet avalanches is expected to continue on Friday. Melt-freeze crusts can quickly break down with intense spring sunshine. In areas that have received more recent snowfall, small loose wet avalanches have the potential to entrain deeper layers. Be aware of terrain traps where even a small loose wet avalanche could have unintended consequences. Plan to avoid steep solar slopes by late morning/mid-day to minimize the problem. There is probably as much or more danger from loose wet avalanches in the lower terrain bands as the upper bands.
Many areas have massive cornices along ridge-lines so avoid slopes below these overhead hazards. Unlike the loose wet problem you will likely not get advance notice that a cornice is about to fail. Cornices can break much further back on ridges than expected and releases can be unpredictable during the spring.
The likelihood of triggering older wind slabs continues to diminish, but in isolated areas above treeline, a mix of old wind slab may still maintain an unlikely wind slab potential on a variety of previous lee slopes. Watch for cracking and firmer or chalky wind transported snow.
Snowpack Discussion
Weather and Snowpack
Snow accumulations Wednesday 3/23 to Friday 3/25 varied based on elevation, with only a few inches below 5000 feet and about 8-10 inches at the Mt. Hood Meadows and Timberline stations. This snow was generally well-bonded to a moist crust buried March 22nd.
A strong front followed by a large upper trough and a cool unstable air mass crossed the Northwest Sunday and Monday. Mt. Hood stations accumulated 10-12" of new snow through Sunday evening.Â
Skies were mostly sunny along the lower slopes Tuesday, but a warming trend was more muted at Mt. Hood due to cool and persistent NE winds.
A large upper ridge and warm air mass is now over the Northwest causing light winds, sunny weather and the warmest temperatures by far so far this spring. Temperatures on Thursday are well into the 50's F at most NWAC sites and even the 60's F at some of the lower elevation sites on both sides of the Cascade crest.
Frequent March storms have built large cornices along ridges in many areas.
The mid and lower snowpack along the west slopes should generally be a stable mix of crusts and layers of moist and rounded snow crystals.
Recent Observations
NWAC pro-observer Laura Green was out in Newton Canyon on Friday 3/25 and found that winds had plastered the latest snow down with conviction, with moderate compression tests in the new snow and hard compression tests at the 3/22 interface. Limited rollerballs were seen on solar slopes. A size 2 natural avalanche from a potential cornice release was seen in White River Canyon.
The Mt Hood Meadows pro-patrol reported limited avalanche control results on Monday. NW winds Sunday night had transported snow to unusual aspects and into the below treeline band, but the new snow was generally bonded well in the ski area. Another large natural avalanche was observed in White River Canyon above 8000 feet likely releasing Sunday night.
A nice example of a reactive wind slab on a small slope was found in the Timberline area Monday. A 10 inch wind slab of denser snow released with a ski cut during warming Monday (photo below).Â
On Tuesday the Meadows pro-patrol reported that northeast winds transported snow above treeline again to non-traditional aspects. Unlike areas further north, the warming trend on Mt. Hood was muted Tuesday and the snow surface remained firm near and above treeline with a mix of hard wind slab, sastrugi and wind scoured surfaces. In the below treeline terrain softening on solar aspects with increasing loose wet potential was seen in the afternoon.  Â
Photo 3-28-16 by Alex Laudon, Timberline 6200 ft SE aspect.
On Thursday the Meadows pro-patrol reported small ski triggered shallow loose wet avalanches on steep solar terrain Thursday morning then on all aspects Thursday afternoon.
Avalanche Problems
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.
Aspects: East, South East, South, South West, West.
Elevations: All elevations.
Likelihood: Likely
Expected Size: 1 - 1
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind lips of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
Cornices can never be trusted and avoiding them is necessary for safe backcountry travel. Stay well back from ridgeline areas with cornices. They often overhang the ridge edge can be triggered remotely. Avoid areas underneath cornices. Even small Cornice Fall can trigger a larger avalanche and large Cornice Fall can easily crush a human. Periods of significant temperature warm-up are times to be particularly aware.
A corniced ridgeline. A large cornice has formed at the top of the ridge. A smaller cornice has formed to the left of the trees from crossloading.
Cornices are easy to identify and are confined to lee and cross-loaded ridges, sub-ridges, and sharp convexities. They are easiest to trigger during periods of rapid growth (new snow and wind), rapid warming, and during rain-on-snow events. Cornices often catch people by surprise when they break farther back onto flatter areas than expected.
Aspects: North, North East, East, South East.
Elevations: Alpine, Treeline.
Likelihood: Possible
Expected Size: 1 - 1
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: North, North East, East, South East.
Elevations: Alpine.
Likelihood: Unlikely
Expected Size: 1 - 1