Avalanche Forecast
Issued: Mar 30th, 2018 11:40AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Persistent Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
Heightened avalanche danger exists and you can trigger persistent slab avalanches, primarily near and above treeline on polar aspects. Watch out for steep roll-overs (convexities), unsupported features, and larger terrain of consequence. Warmer temperatures and sun are making it easier to trigger wet avalanches on steep, rocky, and sun exposed slopes. If you see recent roller balls, Loose Wet avalanches, or experience wet surface snow deeper than your ankle, move to shaded and drier slopes.
Summary
Detailed Forecast
Heightened avalanche danger exists due to the potential for human-triggered large avalanches on Saturday.
You can trigger persistent slab avalanches on Saturday at upper elevations and on shaded aspects. Persistent Slabs are difficult to manage and can break in surprising ways. To stay safe, avoid where the problem exists or if you go there stick to lower angle slopes and limit your exposure to large avalanche paths. 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 terrain.
The strong March sun is expected to be out in force on Saturday. Watch out for areas of unsupportive wet snow where Loose wet avalanches may occur. Plan your travel to minimize exposure to steeper slopes receiving significant sunshine. New rollerballs, new loose wet avalanches, or wet surface snow deeper than your ankle are all signs that a given slope may be unstable.
Snowpack Discussion
Warm sunny weather has created unsupportive wet snow at lower elevations and has allowed melt-freeze cycles to continue.
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. 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.
A large amount of uncertainty exists regarding the distribution of two reactive buried surface hoar layers recently identified along the east slopes of the Cascades. These layers are possible to trigger and deserve careful snowpack evaluation. The weak layers were identified on N and N-NW aspects at 5900 ft on Dirtyface Mountain. If you gather additional information regarding these layers, please consider submitting an observation to NWAC.
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 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
On Friday, Matt Promomo was on Dirtyface Mountain where he found two buried surface hoar layers on shaded aspects at variable depths, depending on location, within the top meter of the snowpack. Test results indicated the potential for propagation on both weak layers. Snow pits indicate a weakening 2/8 crust.
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.
Aspects: All aspects.
Elevations: All elevations.
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
Valid until: Mar 31st, 2018 11:40AM
