Avalanche Forecast
Issued: Mar 6th, 2018 11:01AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Persistent Slabs, Deep Persistent Slabs and Wind Slabs.
NWAC,
Northwest Avalanche Center
Persistent Slab avalanches claimed lives along the east slopes of the Cascades over the weekend. Similar Persistent Slab potential exists along the west slopes of the Cascades. Avoid complex terrain and sit out this low likelihood - high consequence problem; ensure a wide buffer between where you travel and open slopes over 35 degrees as well as large avalanche paths. Also watch for lingering wind slabs at higher elevations Wednesday.
Summary
Detailed Forecast
Fatal avalanche incidents along the east slopes of the Cascades occurred this weekend in the Teanaways near Long's Pass (Saturday) and north of the Methow Valley at Setting Sun Mountain (Sunday). Persistent Slab avalanches were reported both the Long's Pass and Setting Sun incidents. Recent Persistent Slab avalanche activity has been confined to the Stevens Pass area along the west slopes of the Cascades, but a similar snowpack structure exists in other areas along the west slopes.
You may be able to trigger Persistent Slab avalanches in the upper snowpack on sun-exposed slopes (generally southerly aspects) greater than 35 degrees. These avalanches are difficult to manage. To reduce your risk of being caught, avoid steep, open, sunny slopes, and large avalanche paths. Also watch out for unsupported slopes that end in rocks, cliffs or steep rolls. If you experience collapsing or audible whumphs, avoid any nearby avalanche terrain. This interface was involved in two fatal avalanche incidents in the Snoqualmie Pass area 2/25.
Deep Persistent Slab avalanches are unlikely and difficult to trigger, but are also very difficult to predict. You would likely not survive a Deep Persistent Slab avalanche that releases to the 2/13 layer. Stay safe by avoiding triggering smaller avalanches in the surface snow and staying off the previously mentioned steep slopes.
Wind Slabs are trending toward unlikely, but may still exist in isolated areas especially above treeline where there have been few recent observations. Generally avoid areas of recently drifted snow, deeply pillowed features, and fresh cornices on slopes 35 degrees and steeper. These areas may exist well below ridge-lines.
Small loose wet avalanches are possible on Wednesday on steep solar slopes and below treeline but will not be listed due to more dangerous avalanche problems. Avoid steep sunny slopes near terrain traps if the surface snow becomes moist and watch for signs of natural pinwheeling and rollerballing as a clear signal to change aspects.
Snowpack Discussion
Only small loose wet avalanches were reported Tuesday on steep sunny aspects. The last snowfall was 3-6" of low density snow that accumulated Sunday through early Monday morning, with an anomaly of around 12" in the Snoquamlie Pass area. The previous snow surface consisted of sun crusts on solar aspects and several feet of settled powder from last week in sheltered terrain. Several sun crusts can be found on solar slopes within the upper snowpack. Widespread surface hoar was reported in the Stevens Pass area early in the weekend. The most recent storm with significant snow and wind ended Thursday morning 3/1. The most recent human triggered wind slab avalanches were observed on Mt. Baker Saturday.
On E-S-W aspects, a thin breakable sun crust was buried on 2/23. Very small weak facets have been reported surrounding the crust. This was the weak layer found or suspected in several avalanches. This layer has not yet had significant time to heal. It is found 1-3 feet below the surface on steeper slopes that have received direct sun during the past week. Snow profiles and snowpack tests can confirm the presence of this layer; however they are not good for proving its absences. That said, the crust has not been found in near treeline terrain and is more likely to be found at lower elevations in the below treeline band. Several other crusts exist within the upper snowpack on slopes that received direct sunshine.
Some observations from last week suggest other persistent grains at this same 2/23 interface on shaded slopes. Buried surface hoar and large preserved stellars were reported in avalanches and snowpack tests at this interface about one week ago.
Avalanche and snowpack observations continue to indicate that avalanches are possible on a layer of weak sugary facets buried on 2/13. This weak layer is generally 3 to 6 feet below the snow surface and just above a very firm melt-freeze crust (2/8). In the southern Cascades, recent observations suggest it may be easier to trigger avalanches on the 2/13 facets near the Crystal Mountain area compared to terrain near the Paradise side of Mount Rainier where the layer is considerably deeper.
There are no significant layers of concern below the 2/8 crust.
Observations
North
On Saturday, an NWAC professional observed two large wind slab avalanches several feet deep triggered by snowmobilers on the Easton Glacier around 6000'.
On Saturday, an avalanche professional in the Bagley Lakes area noted recent wind transported snow in the near treeline zone, but no skier triggered avalanches on that wind-affected snow. On east aspects, the 2/8 crust was down 60 inches or more and facet crystals above this layer were rounding. No other significant layers were present on this aspect.
Central
An avalanche professional in the Skyline area of Stevens Pass Saturday through Monday found the 2/23 facet/crust interface on south aspects becoming less reactive in snowpack tests. However, the 2/13 layer continued to show the potential for an avalanche to fail and propagate on this layer on most aspects.
Several pertinent observations were reported Saturday on our observations page, including an avalanche professional that observed a loose wet avalanche step down and trigger a deeper avalanche on an east aspect of Jim Hill near Stevens Pass that potentially ran on the 2/23 facet/crust interface.
On Saturday, NWAC professional Observer Jeremy Allyn was in the Mt. Snoqualmie area where he observed a right-side-up density profile to the 2/8 crust down 5' (150 cm) on a SW aspect at 5100 feet. No 2/23 crust was observed at this location. No new or recent avalanche activity was observed.
On Friday, a guide and avalanche professional reported a large avalanche near Highland Bowl on a SSE aspect near treeline on Stevens Pass. This slope had seen recent wind loading and likely ran on the 2/23 facet/crust interface about 2 feet below the surface.
Stevens DOT reported two avalanches Wednesday morning 2/28. One failed on the 2/23 interface on a NE aspect. The weak layer appeared to be buried surface hoar. An avalanche from earlier in the week was larger and suspected of failing on the 2/13 facet/crust combination. This is the most recent avalanche report we have on the 2/13 PWL from the West Slopes of the Cascades.
South
On Thursday 3/1, Forecaster Dallas Glass reported natural slab avalanches in wind-loaded terrain near treeline in the Crystal backcountry. Dallas reported snowpack tests showing potential for triggering avalanches on crust/facet combinations in the upper snowpack as well as up to 3 feet down on the older faceted 2/13 layer.
Problems
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.
Elevations: Treeline, Below Treeline.
Likelihood
Expected Size
Deep Persistent Slabs
Release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer, deep in the snowpack or near the ground. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage. They commonly develop when Persistent Slabs become more deeply buried over time.
Deep Persistent Slabs avalanches can be destructive and deadly events that can take months to stabilize. You can trigger them from well down in the avalanche path, and after dozens of tracks have crossed the slope.
A snowboarder triggered this Deep Persistent Slab near treeline, well down in the path.
Deep, persistent slabs are destructive and deadly events that can take months to stabilize. You can triggered them from well down in the avalanche path, and after dozens of tracks have crossed the slope. Give yourself a wide safety buffer to handle the uncertainty, potentially for the remainder of the season.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
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.
Likelihood
Expected Size
Valid until: Mar 7th, 2018 11:01AM
