Avalanche Forecast
Issued: Apr 7th, 2018 11:39AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Storm Slabs.
NWAC,
Northwest Avalanche Center
Winter-like stormy weather will create dangerous avalanche conditions by Sunday. Wind and Storm Slabs will grow on a variety of terrain features and lee slopes, possibly forming lower on slopes than typical, due to very strong winds. You will be likely to trigger avalanches Sunday so stay well away from avalanche start zones or areas where avalanches could travel to. Use visual clues such as blowing snow, fresh cornices, and cracks in the snow all indicating that you could trigger a Wind Slab avalanche. Give fresh Cornices a wide margin of safety.
Summary
Detailed Forecast
A period of moderate to heavy rain Saturday morning transitioned to snow by late Saturday afternoon following a strong frontal passage. Very strong winds and further snowfall in moderate showers will continue Saturday night and Sunday.
Continued winter-like stormy weather will create dangerous avalanche conditions Sunday at most mid and upper elevation terrain.
Wind Slabs will build on lee slopes and a variety of cross loaded terrain features, possibly lower on slopes than typical, due to very strong winds where you could trigger a Slab avalanche. Use visual clues such as blowing snow, fresh Cornices, and cracks in the snow all indicating that you could trigger a Wind Slab avalanche.
While the bonding of new snow to the old wet snow surface should be favorable, unstable Storm Slabs will build on a variety of steep terrain, especially in those areas receiving the greatest new snow amounts. Watch for deeper stiffened slabs developing Sunday on a variety of aspects.
Fresh Cornices are likely to build along ridges, so 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
A vigorous storm Saturday caused periods of heavy rain and warm temperatures early Saturday that transitioned to snow or wet heavy snow by Saturday afternoon with shallow new snow beginning to accumulate by Saturday afternoon. The old rain soaked snowpack has begun to refreeze and form a crust under continued cooling. Cooler temperatures in the NE Washington Cascades limited the rain to lower elevations.
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 Primomo 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
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: All elevations.
Likelihood
Expected Size
Storm Slabs
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-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.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Apr 8th, 2018 11:39AM
