Avalanche Forecast
Issued: Mar 28th, 2018 1:27PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Loose Wet and Persistent Slabs.
NWAC,
Northwest Avalanche Center
You may trigger wind slab avalanches Thursday on wind loaded slopes especially near convex rollovers, steep unsupported slopes, and beneath cornices. Use visual clues such as wind textured snow, firm snow surfaces, and cornices to identify and avoid steep wind loaded slopes. If the sun comes out or the clouds thin, expect snow conditions to change quickly becoming wet and loose. If you see signs of new rollerballs, loose wet avalanches, or experience wet snow deeper than your ankle, stay off of nearby slopes greater than 35 degrees.
Summary
Detailed Forecast
You may still trigger lingering wind slab avalanches Thursday on wind loaded slopes and cross-loaded feature especially closer to the Cascade crest. You are most likely to trigger these avalanches on NW-NE aspects, steep convex rollovers, unsupported slopes, and below cornice. Use visual clues such as snow drifts, cornices, wind textured snow, and firm snow surface conditions that produce cracks to identify and avoid wind loaded areas.
If the sun comes out or the clouds thin, expect the strong March sun to quickly effect the surface snow. This can occur very quickly this time of year and does not take full bright sunshine to occur. If you see new rollerballs, observe new loose wet avalanches, or experience wet surface snow deeper than your ankle, it’s time to change slopes. Avoid slopes greater than 35 degrees on aspects where you see any of these pertinent observations.
It has been more than three weeks since the last reported avalanche involving older persistent weak layers. While many of these layers have not gone away, it is now difficult to trigger a Persistent Slab avalanche. We do not have any recent information or observations regarding the current state of these older layers. Snowpack tests can provide useful information, however they are not a decision making tool. If you see any cracking, experience collapses, or hear whumphs, stay off of nearby avalanche startzones.
Snowpack Discussion
Rain and snow showers this week have been generally confined to areas near the Cascade crest. Snow levels hovered between 4500 and 5000 feet through much of the precipitation. Above these elevations a few inches of new snow may have accumulated. Drier weather has prevailed in other areas of the Easter Cascade regions. Moderate winds transported soft snow at higher elevations likely forming wind slabs on lee slopes.
On slopes receiving direct sunshine expect new surface crust to have formed. The thickness of these new melt-freeze crusts depends on aspect and location within the region.
Below the surface snow, a high degree of variability can be found in the snowpacks east of the Cascade crest. Various melt-freeze crusts, strong rounded snow, and older weak snow layers may be observed.
Old persistent weak layers were involved in 3 avalanche fatalities over the past month. Many of these weak snow grain types have gained strength and rounded. The remaining weak layers are generally inactive or dormant. Drastic changes to the snowpack, such as significant amounts of new snow or a major rain event could make these layers a threat once again. Currently, the persistent weak layers that you may find include:
- Surface hoar and facets were buried on 3/8. This layer is typically found 18-24” (45-60 cm) below the snow surface on shaded aspects near and below treeline.
- Weak sugar-like facets (2/13 or 2/16) are 3-5 feet (90-150cm) below the snow surface. This is the most widespread persistent weak layer. It can be found just above a firm crust buried on 2/5.
Other shallow and very weak areas of snowpack may be found further east of the crest or in areas with less snow
Observations
North
On Saturday 3/23, professionals near Washington Pass reported a small skier triggered avalanche in the new snow on a steep east aspect at 6500 ft. The 3/8 buried surface hoar was reported as 1.5 feet (45cm) below the snow surface.
Central
No recent observations from this region.
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
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.
Elevations: Treeline, Below Treeline.
Likelihood
Expected Size
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
Expected Size
Valid until: Mar 29th, 2018 1:27PM
