Avalanche Forecast
Issued: Apr 2nd, 2018 11:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Persistent Slabs.
NWAC,
Northwest Avalanche Center
Several weak old persistent layers exist in the snowpack. Limited information from our eastern cascade zones has resulted in a higher degree of uncertainty. You can avoid triggering a persistent slab avalanche by avoiding large open slopes greater than 35 degrees. You are most likely to trigger these old weak layers on NW-N-E aspects, near and above treeline, in locations closer to the Cascade crest.
Summary
Detailed Forecast
Avalanche danger will not significantly change Tuesday in the Cascade East zones.
Observations indicate that persistent weak layers exist within the snowpack, can fail, and may produce avalanches. You are most likely to trigger a persistent slab on shaded aspects near and above treeline in locations where significant snow overlays these layers such as closer to the Cascade crest. Snowpack tests can help identify these persistent weak layers; however, they cannot demonstrate a slope is safe. You can avoid triggering a persistent slab by avoiding large steep open slopes in locations where these persistent slabs are thought to occur.
In locations close to the Cascade crest small shallow wind slabs may have formed during the most recent storm. If you find new surface snow deeper than 6 inches, take time to assess the upper layers of the snowpack. If you find strong snow over weaker snow, avoid startzones where you may trigger and avalanche.
Expect new rollerballs, pinwheels, and small loose wet avalanches to occur during periods of prolonged sunshine. The April sun is very strong and snow conditions can change quickly. Be cautious on steep slopes where the consequences of even a small loose wet avalanche can be high.
Snowpack Discussion
In general 1-3 inches (2-8cm) of new snow has fallen around the eastern zones in the Cascades. Locally higher snow totals may be found, particularly close to the Cascade crest. The new snow fell on a variety of snow surfaces including melt-freeze crusts, thin rain crusts, old wind textured snow and settled soft snow.
Older layers of weak snow can be found in the snowpack. We have very limited information about the distribution and reactivity of these layers leading to a higher level of uncertainty. If you have any information or observations on layers within the snowpack please consider submitting them to NWAC via our public observations page. The exact weak layer and depth depends on your location. Several common weak layers are:
- 3/25: Buried surface hoar layer found on shaded aspects near and above treeline.
- 3/22: Buried surface hoar layer found on shaded aspects near and above treeline.
- 2/13 Facets above a firm and thick melt-freeze crust can be found in many locations near and above treeline. This layer is thought to be dormant at this time but may re-awaken with a substantial weather input.
Other weak snowpacks may be found in locations further east of the Cascade crest or in areas where shallow snow exists.
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 3rd, 2018 11:00AM
