Avalanche Forecast
Regions: Cascades - East.
Above treeline, heightened avalanche conditions should develop through the day near ridges. Wind slabs may become touchy by afternoon in areas receiving heavier precipitation. At lower elevations, shallow wet snow conditions should maintain the possibility of mostly small loose-wet avalanches. Avoid overhead hazard, such as the recently formed large cornices.
Detailed Forecast
A weak disturbance will spread increasing light rain and snow at gradually rising freezing levels overnight Monday. Only light amounts of precipitation are expected overnight. This should begin to wet and weaken shallow surface snow layers.
A stronger band of moisture Tuesday should bring periods of light to moderate rain and snow at gradually rising freezing levels. Winds should be moderate along the ridges above treeline Tuesday.
These conditions should build shallow fresh wind slabs at higher terrain along ridges, mainly on NW-NE-E facing terrain. The degree of increase in the avalanche danger Tuesday should depend on the amount of precipitation received through the day. In areas receiving heavier precipitation the wind slab problem may become increasingly sensitive to human trigger, especially by Tuesday afternoon. Watch for firmer wind transported snow on all aspects, especially in areas of complex terrain.Â
At lower elevations, light rain should maintain shallow wet snow conditions and make small loose-wet snow avalanches possible.
Avoid areas with terrain traps where a small loose wet avalanche could have unintended consequences.Â
It is always a good plan to travel well back from ridges, suspected of cornice formation, or on steep slopes below cornices.
Snowpack Discussion
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. 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
After a short respite from the active weather pattern on Thursday, another strong low pressure system brought 6-12+ inches of snow to the northeast and central-east slopes of the Cascades Friday night and mostly rain for the southeast Cascades. This was followed by snow levels rising to 6000-6500 feet in the northeast and central-east Cascades and likely 7000 feet in the southeast Cascades by early Saturday morning. Rapid cooling began mid-day Saturday. Strong W-SW winds were likely transporting new snow above treeline by mid-day Saturday.Â
Sunday through Monday was cool with light winds with a general clearing trend seen throughout the Cascades through early Monday with high clouds late. Â
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.
More observations from Friday in this zone indicated a significant distinction in the recent storm snow with elevation, with up to 4 more inches of storm snow above about 6500 feet. The several recent warm precipitation events this week appear to have limited the upper elevations of rain to about 6500 feet. At elevations up to 8000 feet, the recent precipitation has fallen as snow. In these higher elevations, some shallow triggered storm slab avalanches have been noted over the past few days.
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. 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.
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.
Elevations: Treeline, Below Treeline.
Likelihood: Possible
Expected Size: 1 - 1