Avalanche Forecast
Regions: Cascades - South East.
Alpine winds are expected to increase and create fresh wind slab primarily above treeline Friday night. Treat wind loaded slopes with caution. The potential for loose wet avalanches on steeper solar aspects/lower elevations will increase during sunny periods. Cornices have recently proven dangerous and unpredictable and capable of triggering very large avalanches.
Detailed Forecast
Showers should quickly decrease Saturday morning. Partial clearing is expected in the afternoon, with increasing sunshine further east of the Cascade crest. The air mass will be cooler, but it's spring-time and solar effects should push temperatures near to above freezing at lower and mid-elevations in the afternoon. Â
Alpine winds are expected to increase and create fresh wind slab primarily above treeline Friday night. Treat wind loaded slopes with caution. The potential for loose wet avalanches on steeper solar aspects/lower elevations will increase during sunny periods. Cornices have recently proven dangerous and unpredictable and capable of triggering very large avalanches.Â
Recent winds have been mostly S-SW, so firmer wind slab should be found mainly on NW-SE slopes near and above treeline. However due to periods of recent E-SE winds, watch for wind slab on all aspects, especially in areas of complex terrain.
Subtle daytime warming and sunny periods Saturday afternoon will increase the potential for loose wet avalanches on steeper solar slopes and at lower elevations. Watch for surface wet snow deeper than a few inches, pinwheels and increasing natural releases. Avoid steeper slopes with terrain traps where even small loose wet avalanches could have unintended consequences.Â
Shallow new storm snow is expected to bond well due to a cooling trend Friday night but storm slab may still become locally sensitive in areas that receive rapid accumulations Friday night or Saturday morning.Â
You need to avoid areas on ridges where there may be a cornice and slopes below cornices! Natural cornice releases and resulting slab avalanches are unpredictable and as seen above in the special note, cornices have triggered major slab avalanches as of late. See a blog post regarding cornices here.
Snowpack Discussion
Special Note: For more information on the massive natural cornice triggered avalanche on the north side of Ruby Mountain on Sunday 3/19 and general thoughts about low-likelihood/high consequence avalanches, please read NWAC's upcoming blog post that will be issued this weekend.Â
Weather and Snowpack
The first week or so of March was very cool and snowy. NWAC and NRCS stations indicate about 2 feet of snow in the northeast Cascades with less elsewhere along the Cascade east slopes.Â
The 2nd week of March was equally active with non-stop Pacific frontal systems pummeling the PNW. Unfortunately, these systems delivered more rain than snow to lower/mid elevations along the east slopes of the Cascades. At least two regional avalanche cycles occurred during the stretch. The most recent cycle from mid-March had many massive slides that covered Hwy 20 near Washington Pass up to 40' deep in places! Significant snowpack consolidation occurred over this period due to rainfall and warmer temperatures. Far less precipitation was seen further east of the crest during this period for areas like Blewett/Mission Ridge.Â
A strong low pressure system brought 6-12 inches of snow to the northeast and central-east slopes of the Cascades Friday 3/17 and mostly rain for the southeast Cascades. This was followed by snow levels rising to 6000-6500 ft in the northeast and central-east Cascades and likely 7000 ft in the southeast Cascades by Saturday morning 3/18.Â
During this past week, weaker fronts crossed the Northwest on Tuesday and Wednesday with light amounts of new snow along east slopes of the Cascades, mainly near and above treeline and near the Cascade crest. The Barron Yurt near Hart's Pass reported 10-15 cm (4-6 inches) of recent storm snow through Thursday morning. A frontal system passing through Thursday night and Friday morning produced only a few inches in the northeast Cascades but 5 inches in the Mission Ridge area. Â Â
Recent Observations
North
Guides near Washington Pass Tuesday, 3/21 reported hearing many natural avalanches near midday following a prolonged sunbreak and subsequent brief warm up. Numerous avalanches were likely small, however, some avalanches sounded much larger, likely involving deeper layers below the relatively shallow recent storm snow. Â
NCH reported no significant new avalanche activity observed outside of one small natural storm slab on a north aspect above treeline Thursday.
Jeff Ward was at the Barron Yurt on Friday. Recent wind transport of shallow new snow was minimal in the immediate area. Wind loaded slopes were not reactive to snowpack tests or ski cuts. One small skier triggered storm slab release was observed on a steep north aspect, releasing about 35 cm deep on the mid-March melt-freeze crust.   Â
Central
On Friday, Mission Ridge pro-patrol reported very sensitive storm and/or wind slab up to 6-8 inches deep on lee aspects near and above treeline. NW aspects were especially sensitive.
NWAC pro-observer Tom Curtis was on the lower slopes of Mt. Cashmere Friday below treeline. About 5 cm (2 in.) of new snow had accumulated in this area. On steep test slopes, Tom easily triggered loose wet avalanches entraining moist underlying snow with the potential to become large. Although not directly observed, Tom heard several large natural avalanches release up the Trout Cr drainage along ridge-crest in the morning. It is possible these avalanches were cornice triggered. Â
South
No recent observations.Â
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, West, North West.
Elevations: Alpine, Treeline.
Likelihood: Likely
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: 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, Treeline.
Likelihood: Unlikely
Expected Size: 1 - 2