Avalanche Forecast
Issued: Apr 1st, 2018 12:23PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Persistent Slabs.
NWAC,
Northwest Avalanche Center
You are most likely to trigger Persistent Slab avalanches near and above treeline on shaded aspects. Watch out for steep roll-overs (convexities), unsupported features, and larger terrain of consequence. If you find yourself in terrain with dry, soft surface snow, take time to carefully evaluate layers in the top 1-2 feet of the snowpack. Watch for shallow avalanches in any area with more than 6" of new snow.
Summary
Detailed Forecast
Light snow accumulations aren't enough to change today's avalanche danger. You may be able to trigger Persistent Slab avalanches 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. There are some lingering layers in the top two feet of the snowpack including surface hoar on shaded northwest through northeast aspects. Digging below the surface and using snowpack tests can help you identify these layers. Old, weak snow layers exist 3-5 feet below the surface. It's been weeks since any reported avalanches involved these deeper weak layers.
If you do find areas with fresh slabs deeper than 6 inches, consider that you could trigger an avalanche due to new snow and wind. Slick, frozen snow surfaces is one of the days hazards. A fall on steep, firm slopes could be difficult to stop and could have serious consequences.
Snowpack Discussion
Light snow has added a dusting to old surface crusts. Cold temperatures and a weak storm have frozen slopes that were wet and in some areas unsupportive at the end of last week. A warm storm last week brought rain between 4500 and 5000 on the east side of the Cascade crest. At higher elevations dry soft snow can still be found. Moderate winds formed wind slabs as recently as Wednesday near and above treeline.
Weak layers of surface hoar were buried on the 22nd and 25th of March. Uncertainty exists regarding the distribution of these two weak layers along the east slopes of the Cascades. They are probably not widespread, but likely can be found from 5,000ft up to almost ridge tops on shaded northwest through northeast aspects. 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.
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 last month. Many of these weak snow grain types have gained strength through rounded or melting. 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.
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. If you have any more recent observations from the Washington pass area, feel free to submit them.
Central
On Friday, Matt Promomo was on Dirtyface Mountain where he found two buried surface hoar layers (3/22) on shaded aspects at variable depths within the two feet 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: North, North East, East, West, North West.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Apr 2nd, 2018 12:23PM
