Stevens Pass Avalanche Forecast

Issued: Mar 20th, 2020 11:00AM

Fri Mar 20th Current Conditions
Alpine Moderate Treeline Moderate Below Treeline Low
Sat Mar 21st 2 Day Outlook
Alpine Moderate Treeline Moderate Below Treeline Low

The alpine rating is moderate, the treeline rating is moderate, and the below treeline rating is low. Known problems include Loose Wet and Cornices.

Warm sunny weather will weaken cornices and drive another round of wet loose avalanche activity Saturday. Use caution on sun-exposed slopes, and expect increasing avalanche danger as the snowpack heats up throughout the day. 

Summary

Snowpack Discussion

March 19, 2020 (The regional synopsis is updated every Thursday @ 6 pm)

This week brought a whirlwind of change to our community, the mountains, and the snowpack. It’s mind-boggling to think of how everything changed in just a few short days. Last weekend, a strong winter storm brought frigid temperatures and fluffy powder to many mountain locations. While the calendar read March, conditions felt more like mid-winter. The hands-down winner during this storm was Mission Ridge and the surrounding mountains. As the skies cleared, sunshine and warm temperatures settled into the northwest. Now we’re in the midst of a prolonged spring-like pattern as the snowpack slowly transitions. The biggest change this week may be the impact of the Coronavirus on our community, local mountain operations, and backcountry travel. We continue to work through and adapt to all of these changes in our daily lives as we mark the first official day of spring. Read more about NWAC Operations under Covid-19

Wenatchees for the Win

We all know the Wenatchee mountains can receive serious dumps of snow, but this season conditions had been fairly dry. Prior to this past week, Snotel sites and weather stations in the Wenatchees reported only about 65% of average snow depth for the season. A strong low-pressure system began impacting the Pacific Northwest last Friday. However this storm did not follow the usual storm track, it meandered south off the coast, spinning moisture around its center, and sending snow into the Cascades from the SE. When moisture wraps around and approaches our mountains from this direction, it can pack quite a punch along the eastern slopes of the Cascades and in particular the Wenatchees. Early Friday morning, Mission Ridge entered a near 48hr stretch of continuous snowfall. A secondary impact of this low-pressure system was an influx of cold Canadian air which dropped snow levels to near sea level for nearly all locations. The cold air mass combined with the unusual storm track led to substantial accumulations of light fluffy snow; Mission Ridge quickly stacked up over 2ft of very light powder. This new snow and strong winds produced some of the spiciest avalanche conditions for this season in the East Central zone. Numerous natural and human triggered avalanches occurred during and following the storm, including some remotely triggered slides.

Skier triggered avalanche on the Diamond Head in the Wenatchee Mountains. NW, 5200’ 3/14 Photo: Adam Butterfield. 

Other locations in the region only added a few inches to their snow totals for the season. The position of the storm only brought dribs and drabs to the mountains from about Hwy 2 and south. A few standouts further south managed to ring out around a foot (Mt St Helens, White Pass, and Paradise). Strong easterly winds in most areas significantly redistributed the new snow and resulted in reports of numerous natural and human triggered slab avalanches Saturday and Sunday. 

Natural wind slab avalanche on Mt St Helens. W 5500’ 3/14: Photo Nate Berry 

Spring has Sprung

Thursday the 19th ushered in the first official day of spring. All winter, we’ve seen storms creating sometimes unique and occasionally similar snowpacks and avalanche conditions for each of our forecast zones. Following the storm last weekend, a sunny and warm springtime pattern took hold of the region. As it did so, it brought with it a prolonged gradual warm-up and loose wet avalanche cycle. Consequently, differences formerly found in each region slowly resolved as the mountains transitioned into spring. 

A natural loose wet avalanche near Washington Pass, Cutthroat Peak. 3/17. Photo: Gus Goldman

Two items still stand out about this spring transition. 1: the snowpack has not fully moved into a spring-like state. You can still find pronounced cold dry layers and firm icy crust in many areas. 2: We have not seen a spring “shed” cycle yet, where several large natural avalanches occur as the snowpack adjusts to percolating water and warmer temperatures. Forecasting spring shed cycles can be difficult, and it's still unclear when or even if a larger natural wet cycle will occur. 

You can continue to support your community-based avalanche center by submitting observations. 

Stay safe, stay healthy, and thank you for all your support. 

Dallas Glass

Problems

Loose Wet

An icon showing Loose Wet

Likelihood

possible

Expected Size

1 - 1

The sunny springtime weather pattern and prolonged wet loose avalanche cycle will continue Saturday. Each evening, cold clear nights help the snow surface freeze and limit the avalanche hazard. During the day, rapidly warming temperatures and strong sun weaken surfaces and drive the potential for more wet loose avalanches (observation). This diurnal cycle will repeat itself again, and Saturday looks to be the warmest day yet with freezing levels rising to 7000ft. 

Time your travel carefully, and keep return routes in mind. The most dangerous avalanche conditions will exist in the afternoon, especially on south and west aspects. When you find wet, unconsolidated surfaces or see roller balls and pinwheels, it’s time to seek out firmer slopes on other aspects, or simply call it a day and head home. Our dry winter snowpack is in transition. As it continues to thaw and adjust, surprising avalanche events in isolated areas are possible. Be leery of large complex sunny terrain during the heat of the day where you’re most likely to see a deeper slab or glide avalanche.

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:

North East, East, South East, South, South West, West, North West.

Elevations:

All elevations.

Cornices

An icon showing Cornices

Likelihood

possible

Expected Size

1 - 1

Large cornices loom atop many alpine slopes. Several are beginning to droop, and you could see some of them fall. Choose routes that minimize the amount of time you spend under these unpredictable features. A cornice collapse or other avalanche could trigger slides that break into deeper layers of the snowpack. 

Cold dry snow continues to hang on at higher elevations and in shaded terrain. Pay attention if you cross back into these winter-like areas. A handful of human triggered avalanches have occurred recently on wind loaded slopes at upper elevations. If you’re traveling in the alpine, take time to evaluate terrain for signs of wind loading before committing to steeper terrain.

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.

Elevations:

Alpine, Treeline.

Valid until: Mar 21st, 2020 11:00AM

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