Avalanche Forecast

Issued: Mar 31st, 2018 12:09PM

The alpine rating is moderate, the treeline rating is moderate, and the below treeline rating is low. Known problems include Wind Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

With a decent overnight freeze and incoming snow and wind, you can expect the Avalanche Danger to increase to Moderate by late Sunday. Steer around any Wind Slabs near upper elevation ridges and leeward terrain features. 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. Use caution on steep, firm slopes where stopping a fall may be difficult and could have serious consequences.

Summary

Detailed Forecast

Generally safe avalanche conditions Sunday morning will give way to Moderate Danger as Wind Slabs build later in the day. Fresh Wind Slab may form immediately on the leeward side of terrain features and ridges near and above treeline. Steer around freshly drifted slabs to avoid triggering avalanches today.

Slick, frozen snow surfaces could present the days biggest hazard. A fall on steep, firm slopes could be difficult to stop and could have serious consequences. With cool temperatures, you can expect an end to wet avalanche problems, for now. There are some lingering layers in the top two feet of the snowpack. This includes surface hoar on shaded northwest through northeast aspects and a number of crusts on sun exposed slopes. Digging below the surface and using snowpack tests can help you gain information about these layers. Old, weak snow layers exist 5-6 feet below the surface. It's been weeks since any reported avalanches involved these deeper weak layers.

Snowpack Discussion

The next storm is moving in with colder temperatures. Expect light to moderate snow and wind into Monday. Most slopes are capped with crusts between 6 inches and 2 feet thick at the snow surface.

At higher elevations dry soft snow can still be found. Moderate winds formed wind slabs as recently as Wednesday near and above treeline. Older Wind Slab avalanches are now hard to trigger in most zones except in the Mt Baker area.

Several avalanches 3/24-3/27 in the Crystal and Stevens areas failed on a layer of weak older snow buried on 3/22. This week, observers are still reporting test results on this layer, but it has become difficult to trigger. In some locations you may find a layer of small sugary facets above a firm melt-freeze crust. On shaded northerly slopes this layer may show up as buried surface hoar. This layer has generally been reported 12-30” (30-70cm) below the snow surface and may be most prevalent above 5,500ft.

The much older 2/8 melt-freeze layer can still be found over 6 feet (120-200cm) deep in the snowpack. While this layer isn't listed in our current avalanche problem set, it may reawaken if it becomes wet.

Observations

North

On Friday, NWAC Observers Lee Lazara and Simon Trautman traveled in the Bagley Lakes area. They reported 3 triggered or natural avalanches about 1 foot deep on north aspects near treeline on slopes with recently drifted snow. The observers found a thin weak layer of surface hoar buried 3/25 at the interface of concern.

Central

On Thursday NWAC professional observer Jeremy Allyn traveled in the Snoqualmie Pass area. Jeremy reported numerous wet avalanche below and near treeline that over the last few days. He found moist to wet surface snow up to 4600 feet.

South

On Saturday, NWAC Forecaster Dallas Glass traveled in the Crystal backcountry. Dallas found a surface crust up to 6" thick on most slopes. Cloud cover kept most slopes from softening. Dallas reported that the 3/22 interface was about 1 foot below the surface and that it was still reactive in snowpack tests. On north through east aspects the 3/22 layer is buried surface hoar, while on a southeast slope Dallas found rounding facets.

On Saturday, NWAC professional observer Jeremy Allyn toured from the Paradise trailhead at Mt Rainier. Jeremy reported a number of older slab avalanches at high elevations. Otherwise, Jeremy found a generally consolidated and stable snowpack. He did not find the 3/22 interface.

Problems

Wind Slabs

An icon showing 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

Possible

Expected Size

1 - 1

Valid until: Apr 1st, 2018 12:09PM