Avalanche Forecast
Regions: Cascades - North East.
Be aware that conditions and avalanche problems will change with elevation and aspect. Solar effects can occur rapidly at this time of year. Cornices have recently proven dangerous and unpredictable and capable of triggering very large avalanches.
Detailed Forecast
A break between weather systems should be seen on Thursday. Sun is most likely in the Olympics, the south Cascades and east of the crest but watch for solar effects in all areas.
Recent winds have been mostly S-SW so firmer wind slab should be found mainly on NW-SE slopes. But there have been winds from other directions lately such as local east winds on Monday so watch for firmer wind transported snow on all aspects, especially in areas of complex terrain.
Watch for solar effects in all areas especially in areas where there has been more than a few inches of snowfall the past couple days. Solar effects should make loose wet avalanches possible mainly on solar slopes in all the terrain bands but keep an eye out on all aspects. Watch for surface wet snow deeper than a few inches, pinwheels and increasing natural releases.
You need to avoid 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 lately. This avalanche in the special note above narrowly missed multiple parties and easily could have been a major avalanche accident!
See a blog post regarding cornices here.
 Although not listed as an avalanche problem watch for storm slab if you are in an area that has had more than a few inches of rapidly accumulating snowfall.
Increasing alpine winds, increasing clouds, and increasing rain or snow should be seen Thursday evening as the next front begins to move into the Northwest.
Snowpack Discussion
Weather and Snowpack
Special note:
A major load has been placed on our snowpack over the past two weeks especially in the north Cascades. This has resulted in many very large avalanches and a few very large to historic avalanches. For example, from March 9-19, Mt Baker received nearly 15 in of water equivalent! About 5 in of water equivalent was received at the NRCS Hart's Pass and Lyman Lake station during the same period. In large terrain, above 7000 feet, much or all of this fell as snow and with very strong winds. Be aware we have a problem of low probability but very high consequence avalanches, which cannot be predicted, which may be with us for some time this spring. Most recently:
 Deep hard slab triggered by a massive natural cornice failure on Ruby Mountain, Sunday afternoon, 3-19. Photo via the NWAC Observations page.
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. At least two regional avalanche cycles occurred during the stretch. The most recent cycle from earlier in the week 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 Pass and Mission Ridge.Â
Another strong low pressure system brought 6-12 in or more 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. Rapid cooling began mid-day Saturday. Strong W-SW winds were likely transporting new snow above treeline by Saturday mid-day.
Weaker fronts crossed the Northwest on Tuesday and Wednesday. This will be continuing to cause snow mainly in the above treeline areas. 2 day snowfall ending Thursday morning could be up to about 5 inches in the above treeline especially in the northeast Cascades.
Recent Observations
North
The North Cascades Heli Guides were near Silver Star on Thursday 3/16 reported many previous natural storm slab avalanches up to size 3, likely releasing midweek. They also reported several small triggered storm slab avalanches. This activity was on N-NW slopes in the 8000 ft range.
On Sunday, the NCH operation was in the Silver Star and Varden Creek drainages and found evidence of recent storm slab releases in unsupported steeper terrain mainly above treeline, but that recent storm slabs were becoming less sensitive to human triggering.
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
Central
NWAC observer Jeff Ward was in the Cle Elum drainage on Mt Hawkins and Red Mountain up to about 6000 ft on Thursday 3/16 and noted evidence of an impressive avalanche cycle the past couple days. The current conditions were 3 cm of new snow over 3-7 cm thick, firm crust with recent rain penetration to about 40 cm in the snowpack. The Valentine's Day crust was found at 90 cm but tests did not give results.
NWAC observer Tom Curtis was at Blewett on Diamond Head up to about 5500 ft on on Thursday 3/16 where there were no recent avalanches just consolidation. SW-W-NW slopes were scoured, with a breakable or supportive crust elsewhere over moist 1F or P rounded crystals down to the ground. He found some reactivity in a layer at about 50 cm down, which should not be a current problem, but might have contributed to avalanches a week ago.
Mission Ridge pro-patrol reported a well-behaved springlike snowpack in their area with no avalanche problems noted over the weekend.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: South East, South, South 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: Possible
Expected Size: 2 - 3