Avalanche Forecast
Regions: Olympics.
You will need to keep a sharper than usual eye on weather conditions if you take a trip into the backcountry on Tuesday. Be prepared to ratchet back your plans if winds become stronger and snow or rain becomes heavier than you expect and you see signs of instability such as cracking of new snow layers or loose wet avalanches before Tuesday night.
Detailed Forecast
Warm front moisture should begin moving into the Northwest on Tuesday especially to the Olympics and north to central Cascades. The avalanche danger levels will be a moving target as they rise on Tuesday especially in the Olympics and north to central Cascades.
SW alpine winds are likely to begin to build new wind slab on mainly N-SE slopes on Tuesday. Alpine winds have been predominately from the SSE at Hurricane the past few days. Hence W to SE slopes will be indicated. But watch for firmer wind transported snow that indicates wind slab on all aspects in areas of more complex terrain.
New storm slab is likely in areas that rapidly accumulate more than a few inches of new snow. The warming trend will also help build upside down denser snow over initial lower density snow.
Recent cornices are very large. Make sure to avoid areas on ridges where there may be an overhanging cornice as well as travel on steep slopes below cornices! Natural cornice releases and resulting slab avalanches are unpredictable. See a blog post regarding cornices here.
Loose wet avalanches will not be listed as an avalanche problem but will be possible in the below treeline where there is significant rain and warming. Watch for wet surface snow deeper than a few inches, pinwheels or initial but growing natural releases.
You will need to keep a sharper than usual eye on weather conditions if you take a trip into the backcountry on Tuesday. Be prepared to ratchet back your plans if winds become stronger and snow or rain becomes heavier than you expect and you see signs of instability such as cracking of new snow layers or loose wet avalanches before Tuesday night.
The avalanche danger should continue to significantly increase on Tuesday night.
Snowpack Discussion
Weather and SnowpackÂ
The first week of March was very cool and snowy. This was followed by periods of heavy rain in the second week of March. This caused significant avalanche cycles in most areas March 9-10. Significant snowpack consolidation occurred over this period due to rainfall and warmer temperatures.Â
Another strong low pressure system brought several inches of rain to the west slopes of the Olympics and Cascades on Friday, 3/17 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. The rain event 3/18-19 has formed a very strong crust layer, now buried by this past weeks storm snow.
Since Wednesday, daily weather systems crossed the Northwest. In the Hurricane Ridge area, about 2 feet of snow has fallen over the past five days as of Monday morning.
Similar to most of the recent wind patterns, the latest front passing the area Sunday had moderate sustained S-SE winds. This has transported available snow to build fresh wind slabs in the Hurricane Ridge area.
Daily early spring warming temperatures has allowed surface snow melt and consolidation, at nearly the same rate as accumulations. With about 2 feet of snowfall received in the past five days, the total snowdepth has increased only about 10 inches.Â
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. The new cornice formation was occurring along ridgelines.Â
Matt was back in the Hurricane Ridge area on Friday. Hurricane Ridge continues to live up to its name because the wind was again the main story with fresh wind slab becoming deeper and more sensitive on lee slopes which included some WÂ 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 the cloud cover increased late morning. 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
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: All elevations.
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, South East, West, North West.
Elevations: Alpine, Treeline.
Likelihood: Possible
Expected Size: 1 - 2