Avalanche Forecast
Issued: Mar 5th, 2018 11:59AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Persistent Slabs, Loose Wet and Wind Slabs.
NWAC,
Northwest Avalanche Center
Persistent Slab avalanches claimed lives along the east slopes of the Cascades over the weekend. 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. Slopes on the south half of the compass are of particular concern. Also watch for lingering wind slabs at higher elevations and small loose wet avalanches on sunny slopes Tuesday.
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. 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. Soft non-wind-effected snow may cover new wind slabs making them harder to identify.
Small loose wet avalanches are likely on Tuesday on steep solar slopes. They have the potential to be slightly larger in the Snoquamlie Pass area. Avoid steep sunny slopes near terrain traps as the surface snow becomes moist and watch for signs of natural pinwheeling and rollerballing as a clear signal to change aspects.
Snowpack Discussion
3-6" of low density snow accumulated through early Monday morning, with an anomaly of around 12" in the Snoquamlie Pass area. Temperatures remained cool Monday for early March with limited sun effects across the west slopes of the Cascades. The previous snow surface consisted of sun crusts on solar aspects and several feet of settled powder from last week in sheltered terrain. Widespread surface hoar was reported in the Stevens Pass area early in the weekend. Sunshine on Saturday triggered small wet loose avalanches on steep sunny slopes throughout the Cascades. The most recent storm with signficanct 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. Small loose avalanches were triggered by sunshine and the sun influenced the release of slab avalanches that approached large (D2) in specific terrain features on the south side of Mt. Herman. 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
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.
Snowpack tests from around the west slopes of the Cascades and Passes continue to indicate that propagation of an avalanche on the 2/13 facets is possible. This layer has been found 2.5 to 4 feet below the snow surface.
South
On Thursday, 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
Loose Wet
Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Several loose wet avalanches, and lots of pinwheels and roller balls.
Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.
Aspects: East, South East, South, South West, West.
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, Treeline.
Likelihood
Expected Size
Valid until: Mar 6th, 2018 11:59AM
