Avalanche Forecast
Regions: Olympics.
There should be just enough new snow and definitely enough recent wind to elevate storm related hazards on Friday and create locally dangerous conditions primarily near and above treeline. Watch for loose wet avalanches below treeline as snow levels creep up during the day. Cornices have recently proven dangerous and unpredictable and capable of triggering very large avalanches.
Detailed Forecast
A frontal system will slowly move across the Olympics Thursday night with light to moderate new snow accumulations expected. Skies should remain cloudy on Friday with showers increasing in the afternoon. After a subtle cooling trend Thursday night, snow levels should rise slightly during the day on Friday. Â
There should be just enough new snow and definitely enough recent wind to elevate storm related hazards on Friday and create locally dangerous conditions primarily near and above treeline. Watch for loose wet avalanches below treeline as snow levels creep up during the day.Â
Recent winds have been mostly S-SE, so firmer wind slab should be found mainly on W-N-SE slopes near and above treeline. Human triggered wind slab avalanches are considered likely on Friday, so avoid steeper wind loaded slopes.Â
New storm snow is expected to bond well to the existing snow surface Thursday night but storm slab may still become locally sensitive in areas that receive rapid accumulation Thursday night or Friday. Â
Snow levels will creep up to around 4500 feet in the Olympics Friday. Loose wet avalanches won't make the cut to be listed as an avalanche problem but are still possible in the below treeline band. Watch for surface wet snow deeper than a few inches, pinwheels and increasing natural releases. Avoid steeper slopes with terrain traps where small loose wet avalanches could have unintended consequences.Â
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.
Snowpack Discussion
Weather and SnowpackÂ
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 2nd 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.Â
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.
Weaker fronts crossed the Northwest on Tuesday and Wednesday at moderate snow levels. Sustained moderate winds out of the S-SEÂ on Thursday transported shallow and recent fresh snowfall in the Hurricane Ridge area.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. New cornice formation was occurring along ridgelines.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: Possible
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, North West.
Elevations: Alpine, Treeline.
Likelihood: Unlikely
Expected Size: 1 - 2