Avalanche Forecast
Regions: Mt Hood.
Dangerous avalanche conditions are expected above treeline on Sunday. Recent storm snow will become wet and unstable with direct sun and warming, late morning and afternoon. Small loose wet slides may entrain significant snow in steeper sun exposed terrain. Wind slabs will stabilize, but due to potentially large recent wind slabs, extra caution is advised.
Detailed Forecast
A break in storms is expected Sunday, allowing for sunshine and gradual warming. Light winds and sunshine should allow for recent storm snow to quickly become wet and weak, especially on steep slopes facing the sun during the late morning and afternoon. Loose wet avalanches should become increasingly likely in this terrain.
Watch for wet surface snow deeper than a few inches, pinwheels and initial natural releases that indicate an increasing loose wet avalanche danger. Be wary of steep slopes with terrain traps such as cliffs or if heavily treed, where even a small loose wet avalanche could have big consequences. Initial small loose wet avalanches may entrain significant wet snow and become dangerous quickly.
Recent potentially deep wind slabs should persist on various aspects above treeline due to the strong and shifting recent winds. Expect wind slab formation in potentially unusual locations, possibly well below ridges and to lower elevations near treeline.
Recent cornices are very large and have likely been weakened during this most recent storm cycle. Natural cornice releases and resulting slab avalanches are dangerous and unpredictable. Give cornices a wide berth if traveling along ridge-lines and avoid slopes below large cornices. See a blog post regarding cornices here.
Other avalanche problems not listed that may be encountered on Sunday include:
Storm slab in more sheltered areas above treeline that experienced rapidly accumulating snowfall for more than a few hours.
Remember that solar effects can rapidly increase the touchiness of various types of avalanches at this time of year, loose-wet, cornices, glide and wind slab to name a few.
Snowpack Discussion
Weather and Snowpack
March was a wet and wild month for weather and avalanches in the Cascades.Â
Fair but cool weather has been seen in the Olympics and Cascades Monday and Tuesday. Reports generally indicate strong surface or near surface crust layers and shallow recent snow.Â
A frontal boundary on Wednesday to Thursday morning brought heavy rain and snow to Mt Baker with light rain or snow for the rest of the Cascades. Snow levels were generally between 6500-7500 feet on Wednesday on Mt. Hood, lowering to 5500-6500 on Thursday. For the 3 days ending Friday morning NWAC stations at Mt Hood had about .8 inches of WE which will have fallen as snow above the snow levels.
A deep low pressure system moved north just off the Washington coast on Friday and Friday night. This storm deposited about 5-10 inches in the Mt Hood area by early Saturday with very strong SE-SW winds Friday afternoon and evening. Winds averaged 35-50 with gusts above 70 at NWAC stations on Mt Hood late Friday! These winds were so strong that wind slabs were less widespread and formed lower on leeward slopes than typical. Â
Cooling, diminishing showers and winds Saturday allowed for the storm snow to begin settling and stabilizing.Â
Recent Observations
The Meadows pro-patrol on Saturday and Sunday reported triggered loose wet avalanches becoming possible on solar slopes. Otherwise the recent crust was the predominate snow surface with small shallow areas of wind transported snow soaked by rain and frozen in place near and above treeline.
On Thursday, the Meadows pro-patrol checked in reporting no recent avalanche concerns in their area with the bomber rain crust the dominant feature.
On Friday NWAC pro-observer Laura Green was at Mt Hood Meadows and above treeline indicated a continuing 1 inch plus surface crust over wet snow. Wind transport had created 6-8 inch pocket of snow, but she reported no avalanches and no signs of instability.
Saturday, the Mt Hood Meadows patrol reported that fresh wind slabs were stubborn and isolated. No ski cut avalanches were released. There were about 6 inches of new storm snow at the 6600 ft elevation band Saturday morning.Â
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
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, North West.
Elevations: Alpine.
Likelihood: Possible
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, North West.
Elevations: Alpine.
Likelihood: Unlikely
Expected Size: 1 - 2