Avalanche Forecast
Regions: Cascades - East.
The recent shift to NE winds makes new wind slab most likely on NW to SE aspects on Tuesday along the east slopes especially in the central and south part. Remember that stiffer wind slab has the potential to propagate to larger avalanches.
Detailed Forecast
A weakening upper trough should linger over the Northwest on Tuesday. Meanwhile a cold Arctic air mass will slowly push further south over the BC-Montana area causing cold offshore surface flow over the Northwest. Alpine NE winds should ease a bit but not go away on Tuesday. Low clouds from the Columbia Basin will still probably affect the south Cascades on Tuesday.
The recent shift to NE winds makes new wind slab most likely on NW to SE aspects on Tuesday. This should be most likely along the east slopes in the central and south part. Watch for firmer wind transported snow mainly on lee NW to SE slopes at exposed alpine locations such as at Mission Ridge. Remember that stiffer wind slab has the potential to propagate to larger avalanches. But since winds have shifted the past couple days don't let your guard down and watch for wind transported snow on other slopes as well.
New storm slab is also likely along the east slopes in the central and south part due to the snowfall there ending Monday morning.
Along all of the east slopes the 12/17 PWL persistent slab problem is gaining strength, becoming more deeply buried and less sensitive in snowpack tests. However, continue to avoid large open terrain in areas where an overall shallow snowpack exists, or where you find this layer in snow pits, especially if you experience direct observations such as whumpfing or shooting cracks.Â
Snowpack Discussion
Weather and Snowpack
A low pressure system moved south over western Washington on Saturday night followed by an influx of cold Arctic air. NWAC stations along the east slopes had WÂ winds Saturday and up to about 4 inches of snowfall by Sunday morning. A colder Arctic air mass began to move into the Northwest.
High surface pressure and moisture east of the crest caused east flow and snow mainly in the central to south Cascades Sunday afternoon and night. NWAC stations along the east slopes had a shift to NE winds and another 0-34 inches of snow on Monday morning with the most snow in the central to south part such as at Mission and further cooling as the Arctic air mass further moved into the Northwest.
Recent Observations
Last week Mission Ridge ski patrol produced 1.5 to 3 ft hard slab avalanches during control work. These avalanches were releasing on basal facets about 15 cm from the ground.
Tom Curtis was out at Mt. Lillian Friday and found reactive wind slabs along ridges with shooting cracks and whumpfing on north aspects near 5900 feet. Â The wind slab was likely collapsing down to the 12/17 Persistent Weak Layer (buried surface hoar) about 25-35 cm down. Wind slabs were found on NW-E-SE aspects with some wind loading apparent well below treeline.
On Saturday 12/24, a backcountry skier in the Lake Clara area near Mission Ridge reported a huge whumpfing noise, likely indicating a collapse of the basal facets. While no avalanche occurred, the terrain where the collapse occurred connecting to a large avalanche path that was NE facing near treeline.
Ski tourers in the Washington Pass area Saturday reported no results from hand shears in the near and below treeline bands and no avalanches while avoiding possible wind loaded terrain above treeline. Nice, low cohesion surface snow conditions were reported below treeline.Â
On Sunday reports from multiple snow pits by Mission Ridge patrol testing the basal facet layer, now indicate a much stronger more settled snowpack from a week ago. The basal facets remain intact, but show signs of rounding and overall consolidation and bonding.
The NCMG at Washington Pass on Sunday found that new snow was causing loose dry avalanches on steep terrain. North winds were seen to be quickly forming touchy new wind and storm slab.
The Mission Ridge ski patrol had their hands full with about 34 inches of new snow and strong northeast winds on Monday morning. It was not possible for them to visit lee S-W slopes were likely new, deep wind slab was expected.
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: All aspects.
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
Persistent Slabs
Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Unlikely
Expected Size: 1 - 1