Avalanche Forecast
Regions: Cascades - South East.
Further building or new wind slab should be the main avalanche problem on Wednesday and should be sensitive to human triggering. Be sure to read the forecast since wind slab may build on aspects where you don't expect it.
Detailed Forecast
Forecast updated for east slopes of the Cascades and Mt. Hood Wednesday morning to extend Considerable rating into the near treeline band due to moderate to strong E-NE winds overnight and ample low density snow available for transport to build new and sensitive wind slab. Â
Previous discussion: Fair cold weather is generally expected to continue over the Olympics and Cascades on Wednesday. Low clouds will probably linger from the Columbia Basin to the Cascade east slopes and the east side of Mt Hood.
But expect locally strong east winds to develop over the Olympics and Washington Cascades Tuesday night and Wednesday. Therefore new or further building wind slab should be the main avalanche problem on Wednesday. This new wind slab should be found mainly on NW to SE aspects on Wednesday especially at exposed locations.There should be quite a bit of recent snow available for transport. Watch for firmer wind transported snow and remember that stiffer wind slab has the potential to propagate to larger avalanches.
Although NW to SE aspects will be indicated in the wind slab avalanche problem diagram remember to watch for firmer wind transported snow on all slope aspects or cross loaded slopes especially in areas with varied terrain and modified wind directions. Older wind slab may also linger on other slope aspects.
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.
The Mission Ridge pro-patrol also reports that there were a couple of snow immersion close calls at the ski area on Monday. Always ride of ski with partner when there has been recent heavy snowfall and keep you partner in sight at all times.
Snowpack Discussion
Weather and Snowpack
A low pressure system moved south over western Washington on Saturday night followed by cold Arctic air. NWAC stations along the east slopes had WÂ winds Saturday and up to about 4 inches of snowfall by Sunday morning.
An unusual moist reverse orographic east flow caused snow especially along the east slopes of 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 Ridge.
Fair cold weather with east winds has been seen on Monday and Tuesday with clouds lingering from the Columbia Basin to the Cascade east slopes and the east side of Mt Hood.
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 PWLÂ 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 basal facets. While no avalanche occurred, the terrain where the collapse occurred was connected to a large avalanche path that was NE facing near treeline.
A report from Delancey Ridge east of Washington Pass was received via the NWAC Observations tab of a loud snow pack collapse on a southwest aspect at about 4700 feet.
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 the hairpin 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.
The NCMG was out again at the hairpin at Washington Pass on Monday and found 20-25 cm of storm snow and no significant signs of instability.
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
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