Avalanche Forecast

Archived

Mar 24th, 2017–Mar 25th, 2017

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Regions

Olympics.

Human triggered wind slab avalanches are considered very likely on Saturday, so avoid steeper wind loaded slopes near and above treeline! Keep terrain selection simple and conservative. Cornices have recently proven dangerous and unpredictable and capable of triggering very large avalanches.

Detailed Forecast

Showers should decrease late Saturday morning and become light and scattered in the afternoon. Sunbreaks are likely in the afternoon. 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.

Human triggered wind slab avalanches are considered very likely on Saturday, so avoid steeper wind loaded slopes! Keep terrain selection simple and conservative. Recent winds have been mostly S-SE, so firmer wind slab should be found mainly on W-N-SE slopes near and above treeline.

Make sure to avoid areas on ridges where there may be a cornice and slopes below cornices! Natural cornice releases and resulting slab avalanches are unpredictable. See a blog post regarding cornices here.

Subtle daytime warming and/or sunbreaks 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.

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.

Snowpack Discussion

Weather and Snowpack

The first week or so of March was very cool and snowy. NWAC stations in the Olympics and along the west slopes of the Cascades piled up about 3 to 8 ft of snow with the most at Mt Baker.

The second week of March was equally active with non-stop Pacific frontal systems pummeling the PNW. Unfortunately, these systems delivered far more rain than snow. At least two regional avalanche cycles occurred during the stretch, including a very active avalanche cycle in the Hurricane Ridge area 3/9 through 3/10 with large or very large natural and human-triggered avalanches reported. Significant snowpack consolidation occurred over this period due to rainfall and warmer temperatures.

Another strong low pressure system brought several inches of rain to the west slopes of the Olympics and Cascades on Friday 3/17. Several inches of snow accumulated Friday night before continued warming pushed snow levels above Hurricane Ridge with moderate rain seen through early Saturday morning 3/18. Rapid cooling later Saturday morning was followed by generally light snow showers with little in the way of new snow accumulation.

During this past week, weaker fronts crossed the Northwest on Tuesday and Wednesday at moderate snow levels. Sustained moderate winds out of the S-SE on Thursday and Friday continuously transported shallow and recent fresh snowfall in the Hurricane Ridge area.

Recent Observations

NWAC pro-observer Matt Schonwald was in the Hurricane Ridge area Thursday afternoon. Moderate S-SE winds were quickly building fresh 10-12" wind slab on lee aspects and scouring windward aspects to the most recent rain crust. Wind slabs near treeline were becoming increasingly sensitive by the end of the day. New cornice formation was occurring along ridgelines.

Matt was back in the Hurricane Ridge area on Friday. Hurricane Ridge continues to live up to it's name because wind was again the main story with fresh wind slab becoming deeper and more sensitive on lee slopes which included some westerly slopes near treeline. Fresh and large cornices were also building and deemed likely to fail. All wind loaded avalanche terrain was avoided. Generally shallow loose wet avalanches occurred below treeline on solar aspects until cloud cover increased late morning.

Problems

Wind Slabs

Wind Slab avalanches are the 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.

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 drifts 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.

Loose Wet

Loose Wet avalanches are the 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. 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.