Avalanche Forecast
Regions: Cascades - North East.
Moderate to locally strong E-NE winds Tuesday night and Wednesday have loaded unusual aspects and built wind slab in all elevation bands. Allow fresh wind slabs time to stabilize and avoid terrain where even a small slab avalanche could have unintended consequences.
Detailed Forecast
Becoming cloudy overnight Thursday through Friday with occasional very light snow, mainly in the north part. Very light amounts of new snow are expected through Friday, if any. Winds should be mostly westerly and light to moderate.Â
Moderate to locally strong E-NE winds Tuesday night and Wednesday have loaded unusual aspects and built wind slab in all elevation bands.
Allow fresh wind slabs time to stabilize and avoid terrain where even a small slab avalanche could have unintended consequences. Avoid travelling on any terrain if you encounter firmer wind transported snow, which may be on all slope aspects or cross loaded slopes, especially in areas with varied terrain and modified wind directions.Â
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 like the Mission Ridge/Blewett Pass area or where you find this layer in snow pits, especially if you experience direct observations such as whumpfing or shooting cracks.
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 West 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 were seen on Monday and Tuesday. Fair and cold weather continued Wednesday with easterlies moderate at Pass level and strong NE winds near ridgetops and above treeline seen at the Washington Pass and Mission Ridge stations.
Sunny weather continued Thursday with slightly warmer temperatures. Crest level winds shifted from moderate easterly Thursday morning to light to moderate westerly by Thursday evening.Â
Recent Observations
Tom Curtis was out at Mt. Lillian Friday, 12/30 and found reactive wind slabs along ridges with shooting cracks and whumpfing on north aspects near 5900 feet. 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.
Ski tourers in the Washington Pass area Saturday 12/31 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 1/1 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 New Years Day found that new snow was causing loose dry avalanches on steep terrain. While north winds were quickly forming touchy wind slabs.
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.
There was a solo skier who triggered, was caught and killed in a slide in the Crystal Mountain backcountry. The avalanche likely occurred Wednesday, 1/4 and was discovered Thursday. Initial report here: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: All aspects.
Elevations: All elevations.
Likelihood: Possible
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