Avalanche Forecast
Issued: Apr 6th, 2018 1:05PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wet Slabs and Wind Slabs.
NWAC,
Northwest Avalanche Center
Saturday will be a dangerous day to venture into avalanche terrain along the East Slopes of the Cascades. Large and destructive Wet Slabs could run naturally, especially in areas with existing weak layers. Minimize your exposure to large avalanche terrain from above. Expect fresh Wind Slabs to build quickly near and above treeline in the afternoon as the snow level lowers.
Summary
Detailed Forecast
Saturday will be a dangerous day to venture into avalanche terrain along the East Slopes of the Cascades.
Large and destructive Wet Slabs could run naturally, especially in areas with existing weak layers. You can trigger a dangerous Wet Slab avalanche at most elevations through midday. These avalanches may run naturally. Watch for liquid water reaching buried crusts or softer layers of snow well-below the snow surface as a sign that Wet Slab avalanches could occur. Wet Slabs are dangerous because they are hard to predict, give little warning, and can be very destructive. If you see slab avalanches in areas of wet snowpack, avoid traveling in avalanche terrain. You may be able to trigger smaller Loose Wet avalanches more easily in the same types of terrain where the Wet Slab problem exists. Avoid slopes with terrain traps where even a small Loose Wet avalanche would have serious consequences.
Rain will change to snow Saturday afternoon near and above treeline. Wind and new snow will form Wind Slabs on lee slopes near and above treeline. Visual clues such as blowing snow, fresh cornices, and cracks in the snow all indicate that you could trigger a Wind Slab avalanche. Expect deeper Wind Slabs in the Washington Pass area where more snow is expected near and above treeline.
Recent rain and warm temperature have weakened Cornices. Give Cornices a wide berth if traveling along ridgelines as they often break back further than expected. Be aware of overhead hazard if traveling on slopes with Cornices above as they may fail naturally or be human-triggered.
Snowpack Discussion
Periods of light rain and mild temperatures have caused wet snow conditions to extend into the above treeline terrain along the east slopes of the Cascades. Up to 6 inches of snow fell earlier in the week. Strong W-NW winds redistributed recent snow onto lee slopes above treeline forming shallow wind slabs.
Older layers of weak snow can be found in the snowpack. We have 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. 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.
Rainfall and continued above freezing temperatures Friday night and Saturday morning may allow Wet Slabs to fail at these interfaces. Peristent Slabs may be reintroduced when we receive more snowpack information following this storm cycle.
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 4/6, professional observer Matt Promomo traveled up the Icicle to above Colchuck Lake. Matt found a moist and well settled snowpack below 5800' with no persistent weak layers. The 2/8 crust was 1 m down. At 6700' on a NNE aspect, Matt found preserved stellars 14 in (35 cm) down and showing a likelihood to propagate in snowpack tests. Dry snow was found at this elevation below the top 8 " (20 cm). Higher in the terrain, recent wind slab avalanches likely released on this layer. Matt also observed natural loose wet avalanches and one wet slab avalanche above treeline on a SE aspect.
On Saturday 3/31, an observer reported a large slab avalanche that likely failed on a persistent weak layer 4-5 feet below the surface in the Enchantment Mountians
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
Wet Slabs
Release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slab avalanches can be very destructive.
Avoid terrain where and when you suspect Wet Slab avalanche activity. Give yourself a wide safety buffer to handle the uncertainty
A Wet Slab avalanche. In this avalanche, the meltwater pooled above a dusty layer of snow. Note all the smaller wet loose avalanches to either side.
Wet slabs occur when there is liquid water in the snowpack, and can release during the first few days of a warming period. Travel early in the day and avoiding avalanche paths when you see pinwheels, roller balls, loose wet avalanches, and during rain-on-snow events.
Elevations: Treeline, Below Treeline.
Likelihood
Expected Size
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
Valid until: Apr 7th, 2018 1:05PM
