Avalanche Forecast
Regions: Cascades - South East.
A mix of dangerous winter and spring avalanche conditions is likely to linger mainly in the above treeline along the east slopes following weather and snowfall on Sunday.Your ability to identify and avoid avalanche problems in the above treeline will be essential on Monday.
Detailed Forecast
An upper ridge and surface high pressure should begin to strongly build west of the US west all the way to the Gulf of Alaska on Monday. Light snow showers on both sides of the crest should decrease and end on Monday. New snow along the east slopes from Sunday should be mostly in the 4-8 range in the above treeline by Monday morning.
A mix of likely dangerous winter and spring avalanche conditions is likely persist in the above treeline along the east slopes on Monday. The sun is getting much stronger and new snow will be reactive to solar effects.
Loose wet avalanches should be likely in the above treeline on solar slopes on Monday. Avoid avalanche terrain if you start seeing pinwheels or initial small natural loose wet avalanches. Be aware of terrain traps where even a small loose wet avalanche could have unintended consequences.
New wind slab of a foot or more is likely on lee slopes mainly in the above treeline on Monday. This is most likely on north to east slopes but possible on other aspects. Watch cracking and firmer or chalky wind transported snow. Remember that snow pits may not be helpful due to variability in the snowpack.
New storm slab layers may linger into Monday where there was rapidly accumulating snow for more than a few hours on Sunday. Pay special attention to slope convexities where storm slab is mostly likely to be triggered.
Cornices have grown large recently. Cornices can break much further back on ridges than expected and releases can be unpredictable during the spring.
Snowpack Discussion
Weather and Snowpack
Snow accumulations Wednesday to Friday following the last front varied based on proximity to the Cascade crest and higher terrain with about 12 inches at the NRCS Lyman Lake Snotel, 6 inches at Harts Pass and next to nothing at Blewett, Mission Ridge and Berne weather stations. Crest level westerly winds were strong and sustained at the Mission Ridge station through Thursday afternoon.
A strong front and upper short wave crossed the Northwest Sunday morning. This is being followed by a large upper trough and cool unstable air mass Sunday afternoon and evening.
Frequent March storms have built large cornices along ridges in many areas.
A spotty layer of surface hoar was buried on about 3/12 mainly in the northeast Cascades with the terrain most suspect being N through E facing slopes. The last known human triggered avalanche on this layer was in the Washington Pass area on 3/19.Â
No avalanches have been noted for quite some time on a persistent surface hoar layer buried February 27th in the Washington Pass area and to a lesser extent the central-east zone. Avalanches releasing on or down to this layer are unlikely. Â
Recent Observations
The North Cascades Heli Skiing operation skied extensively on a non-solar slope in Cedar Creek on Monday 3/21 and did not observe any avalanche activity in nearby terrain.
On Wednesday 3/23 NWAC pro-observer Tom Curtis was in the Blewett Pass/Table Mountain area and found dust on a mostly supportive crust prior to the incoming frontal system. Also of note, many windward and solar slopes were nearly or completely melted out up to about 6000 feet.Â
Mission Ridge pro-patrol reported strong west winds Thursday morning but little in the way of avalanche concerns in area due to the lack of new snow. On Saturday they reported minor small point release loose wet avalanches and minor rollerballs.
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.
Elevations: Alpine, Treeline.
Likelihood: Possible
Expected Size: 1 - 1
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: Alpine, Treeline.
Likelihood: Possible
Expected Size: 1 - 1