Avalanche Forecast
Regions: Snoqualmie Pass.
Watch for sensitive shallow new storm slab formed Saturday night and new wind slab in the alpine on lee easterly aspects. Give recently formed cornices a wide berth and minimize travel under slopes with large overhanging cornices.
Detailed Forecast
A system passing through in Northwest flow Saturday night and Sunday morning should transition to generally light snow showers by mid-day Sunday. A warm front approaching from the west should hold off on producing another round of moderate snow until Sunday evening. Significant W to NW wind transport on Sunday should be confined to the above treeline zone...so continue to expect new wind slab on traditional easterly aspects at higher elevations.Â
Generally shallow, new storm slab formed Saturday night and Sunday should be the deepest in the central Cascades...and maybe the most sensitive near and south of Snoqualmie Pass where slight warming Sunday may allow for denser snow to layer on top of lower density snow Saturday night.Â
Overall the snow received over the last 2 weeks continues to settle and stabilize. Human triggered large or very large avalanches are becoming unlikely but are still high consequence if able to step down to poorly bonded previous storm or wind layers. Cornices have grown large and are still reported to be sensitive. A cornice failure could provide a large enough natural trigger to trigger a large and destructive avalanche. While the old Jan crust and any remaining weak layers around or just above it are likely 1.5 m to 2 m down in wind sheltered terrain and becoming very unlikely to trigger...it's important to remember where this layer is and where an avalanche could release to under the right circumstances.  Â
The sun is starting to get stronger as we head toward March. Watch for any sun effects Sunday morning over the south Cascades to quickly produce roller balls and pinwheels on solar slopes that could trigger a loose wet avalanche or a wind or storm slab layer on a steep slope.
There's plenty of new snow to go around...continue your careful snow pack evaluation and cautious terrain selections on Sunday.
Snowpack Discussion
Recent Northwest Weather
An extended mild and dry period occurred the latter half of January generally causing a crust to form in all areas. Very cold weather followed in early February which generally produced low density snowfall and faceting near and just above the crust.
Saturday was the first relative break in a seemingly endless storm cycle over the last two weeks. Constant frontal systems produced periods of strong winds and heavy snowfall. The water equivalent over the past 12 days is about 9-15 inches with snowfall about 7-12 feet at NWAC weather stations near and west of the crest.
Observations for Washington Cascades near and west of the crest
Several avalanche cycles have been seen in the last 2 weeks. Â The forecast attention near and west of the crest will be focused on recent storm and wind layers. Note that persistent slab is a concern east of the Cascade crest. Be sure to read the forecast for that area if you venture even slightly east of the crest.
The late January crust layer and associated weak layers are now deeply buried but continue to produce big results with big explosives by area ski patrol on wind loaded slopes near and above treeline. Perhaps of less relevancy to a backcountry forecast but still plenty interesting is the average 6' crown with a maximum of over 15' Alpental ski patrol was able to produce with large explosives Friday in their back bowls. Of greater concern to backcountry skiers and riders should be a natural wind slab avalanche that occurred near White Pass on a easterly aspect near treeline Friday morning. This natural avalanche produced an 8' crown and underscored NWAC observer Tom Curtis's report from the area Friday of heavy wind loading on lee slopes near 7000 feet.Â
Pictures of wind slab avalanche and large cornice growth near Gunsight Peak, White Pass, Photo by Chris Talbot, Pro Patrol 2-21-14Â
Here is a video observation from NWAC observer Dallas Glass in the Snoqualmie Pass area Wednesday identifying midweek storm snow instabilities. Dallas re-visited the Snoqualmie Pass area Friday and reported a settling and stabilizing snow pack. He saw possible 30-50 cm wind slab layers on lee slopes and storm slab that was becoming less reactive except on isolated terrain features.
The mid and base pack should still consist of mostly stable crusts and melt form layers from periods of warm weather earlier this winter.
Keep an eye on your riding partner in the deep snow until the new snow settles and the risk of SIS subsides.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.
Elevations: Alpine, Treeline.
Likelihood: Possible
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.
Elevations: Alpine, Treeline.
Likelihood: Possible
Expected Size: 1 - 2