Avalanche Forecast
Regions: Cascades - East.
Along the east slopes, the avalanche danger will decrease sharply away from the Cascade crest and at lower elevations. In areas that have received more recent snowfall, fresh wind slab will continue to be the primarily hazard near and above treeline. Loose snow avalanches will be likely on steeper slopes.
Detailed Forecast
Orographic showers along the Cascade crest in cool NW flow aloft Thursday night will wind down Friday morning followed by clearing skies on Friday. Moderate W-NW crest level transport winds will also ease off late Thursday night and Friday morning. Along the east slopes, the avalanche danger will decrease sharply away from the Cascade crest and at lower elevations.
In areas that have received more recent snowfall, fresh wind slab will continue to be a primarily hazard on Friday near and above treeline. Firm wind transported snow and snowpack cracking can be good signs of wind slab layers.
We will flip which loose snow avalanche we identify as the primary problem with more sunshine expected Friday versus Thursday. Loose wet avalanches will be likely on steeper solar slopes by mid-day. Loose dry avalanches should be small but likely on steeper non-solar slopes. Be aware of terrain traps where even a small loose avalanche could have unintended consequences in the wrong terrain.   Â
Shallow human triggered storm slabs are possible as new snow becomes more cohesive with daytime warming on Friday. Watch for graupel layers received Thursday night to become potential bed surfaces for storm slabs.Â
Recently formed cornices have grown large. If traveling along ridgelines, be aware that cornices break much further back than expected and you don't want to go for a ride with a chunk of cornice. Despite the cool weather, cornice releases can be unpredictable during the spring.
Due to recent sensitive storm slab releases on a suspected spotty 3/12 buried surface hoar layer, we are currently listing it as a persistent slab. Though it may be spotty and not widespread it should nonetheless get attention from backcountry travelers in the NE zone.
Snowpack Discussion
Weather and Snowpack
A spotty layer of surface hoar likely formed and was buried on about 3/12 mainly in the northeast Cascades with the terrain most suspect being N through E facing slopes.
A low pressure system crossed the Northwest on Monday and Tuesday. NWAC stations along the east slopes accumulated a few inches of snow above about 4000 feet through Wednesday morning. A strong front crossed the Cascades Wednesday night. Post-frontal snow accumulations varied based on proximity to the Cascade crest and higher terrain with about 6 inches at the NRCS Lyman Lake and Harts Pass Snotel sites and next to nothing at Blewett, Mission and Berne weather stations. Crest level westerly winds were strong and sustained at the Mission Ridge station through Thursday afternoon.Â
Frequent March storms have built large cornices along ridges in many areas.
No avalanches have been noted for quite some time on a persistent buried surface hoar layer from 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 last known human triggered avalanche on the 3/12 PWL in the Washington Pass area was from 3/19.Â
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 mostly dust on a mostly supportive crust prior to the incoming frontal system. Also of note, many windward and solar slopes were either 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.Â
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Avalanche 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: North, North East, East, South East.
Elevations: Alpine, Treeline.
Likelihood: Possible
Expected Size: 1 - 1
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
Persistent Slabs
Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
Aspects: North, North East, East.
Elevations: All elevations.
Likelihood: Unlikely
Expected Size: 1 - 1