Avalanche Forecast
Regions: Cascades - East.
With the slightly higher snow levels forecast through Tuesday, a new round of wet snow avalanches are likely in the upper portion of the below treeline band and the near treeline band while storm snow concerns dominate above treeline. Realize that wet snow avalanches, specifically wet slab avalanches, are very hard to predict and demand an extra amount of caution when engaging with avalanche terrain.
Detailed Forecast
The relatively high snow levels will continue with rain forecast below 5500-7000 feet through Tuesday morning along the east slopes of the Cascades. We should see a relative break in the steady precipitation Tuesday afternoon before another wave of moisture returns Tuesday night. Periods of moderate W-SW winds above treeline will continue to transport new and recent snowfall only for the upper portion of the above treeline band Monday night through Tuesday.Â
We have already experienced a region-wide avalanche cycle such that the overall likelihood of wet snow avalanches along the east slopes has locally diminished depending on elevation. With the slightly higher snow levels forecast through Tuesday, a new round of wet snow avalanches are likely in the upper portion of the below treeline band and the near treeline band. Realize that wet snow avalanches, specifically wet slab avalanches, are very hard to predict and demand an extra amount of caution when engaging with avalanche terrain.
Loose wet avalanches are likely on steeper slopes for areas and elevations that see rain. While most loose wet avalanches will be shallow, a few may gouge down to deeper layers and become more powerful than you expect.Â
Cornices will be weakened and prone to failure due to rain and mild temperatures. Very large cornices have been reported in many areas and have been involved in recent accidents and close calls. A cornice failure could trigger a large and destructive wet slab avalanche.Â
Wet slab avalanches are highly unpredictable and not necessarily tied to peak warming or rainfall. Glide cracks can indicate areas where wet slab avalanches are more likely to occur. Besides their destructive potential, wet slab avalanches often surprise backcountry travelers when they run further than expected.Â
Unstable storm slabs may build near and above treeline during periods of rapid snowfall accumulation and will be larger near the Cascade crest.Â
Only storm slab will be listed but if you manage to venture into the above treeline zone Tuesday, expect new or previous wind slab mainly on NW-SE aspects due to recent SW-W winds. Continue to watch for firmer wind transported snow on all aspects especially in areas of complex terrain.
The persistent slab problem previously listed in the NE zone has taken a temporary hiatus until investigation post-storm cycle. However be aware that avalanches stepping down to the Valentine's Day or 2/17 crust would be large and very dangerous.
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. This gave the snowpack a test and caused an avalanche cycle of several types of avalanches in the Cacades.
A strong frontal system brought increasing precipitation and winds along with a warming trend to the Cascades on Thursday. On Thursday night rain may have pushed up to about 4000 feet in the northeast Cascades and about 5000 feet in the southeast Cascades. By Friday morning NWAC stations along the Cascade east slopes had about 0.35- 0.90 inches of WE (water equivalent) with only 0-5 inches of snow.
Another front crossed the Cascades on Saturday causing more SW-W winds, snow at higher elevations, and rain at low elevations. It looks like there was up to a few inches of new snow east of the crest on Sunday morning.
A mild frontal system slowly sagged south across the Cascades on Monday with light to moderate rain and snow. There was a strong downslope drying effect with much less precipitation east of the crest with this system. Precipitation totals through 5 PM PST (or 6 PM PDT).
Recent Observations
North
The NCMG were out on Thursday near Washington Pass and reported the 2/17 crust and the Valentine's Day layer at about 80-130 cm down. Deep tap tests of the Valentine's Day crust at 95 cm gave inconsistent results. PSTs gave results that ranged from no result to PST End, illustrating variation in the reactivity of the Valentine's Day layer.
On Saturday a report for Delancy Ridge via the NWAC Observations page indicated many natural large loose wet avalanches on solar slopes.
The NCH on Saturday reported significant recent warming and settlement. Recent natural size D1.5 - D2 wind and storm slab were seen in the Silver Star area. Pits in the upper Willow drainage had generally right side up layers with the Valentine's Day crust seen at 100-128Â cm.
The NCH were in the Varden and Silver Star area on Sunday and reported many natural size D1-2 storm slabs on steep north slopes in the 5200-8400 ft range. Some cracking and propagation was observed. Shallow storm slabs were reactive about 10 cm down.Â
Central
No recent observations.Â
South
No recent observations.Â
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.
Elevations: Treeline, Below Treeline.
Likelihood: Likely
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, North West.
Elevations: Alpine, Treeline.
Likelihood: Likely
Expected Size: 1 - 2
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