Avalanche Forecast
Issued: Apr 4th, 2018 10:36AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Persistent Slabs.
NWAC,
Northwest Avalanche Center
Several old weak snow layers exist in the snowpack east of the Cascade crest. We currently have limited information concerning these persistent weak layers. You can avoid triggering a persistent slab avalanche by staying off of large open slopes greater than 35 degrees near and above treeline. If you are traveling near the Cascade crest, expect small shallow wind slab avalanches to have formed. Use visual clues such as fresh cornices, blowing snow, and snow drifts to identify and avoid wind loaded slopes.
Summary
Detailed Forecast
Limited new snow totals expected Wednesday will not significantly increase avalanche danger in the Cascade East zones.
Several old weak layers of snow have been reported in the snowpack over the last several weeks. Recent weather conditions likely have not caused significant changes in these persistent weak layers. You are most likely to trigger a persistent slab on shaded aspects near and above treeline. While snow profiles and snowpack tests can demonstrate the presence of a persistent weak layer they cannot prove its absence. The best way to avoid triggering a persistent slab avalanche is to stay off of large steep opens slopes in locations where persistent slabs are thought to occur.
In some locations near the Cascade crest you may find small shallow wind slabs. If you see more than 6 inches of soft snow on the ground, expect wind slabs in nearby terrain. Use visual clues such as blowing snow, new cornices, and snow drifts to identify and avoid wind loaded slopes.
A lower elevations rain showers are likely to occur. Expect new rollerballs, pinwheels, and small loose wet avalanches if you experience rain while traveling. Stay off of slopes greater than 35 degrees if you observe prolonged or intense rain showers.
Snowpack Discussion
Around 2-4 inches (5-10cm) 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. An observation on NWAC’s public observation page reported 6 inches (15cm) near Washington Pass. 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 the 3/8 buried surface hoar was reported as 1.5 feet (45cm) below the snow surface.
Central
On Friday 3/30, 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 5th, 2018 10:36AM
