Avalanche Forecast

Issued: Mar 25th, 2018 12:14PM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

The avalanche danger will rise during the day on Monday. Be prepared to dial back your terrain selection as you see blowing snow, a sign that the danger is increasing. Stay safe by steering around steep, unsupported slopes where firm snow produces cracking or recent snow drifts and cornices exist in the terrain. Persistent Slabs can still be found in the snowpack, although these avalanches are difficult to trigger.

Summary

Detailed Forecast

The avalanche danger will increase throughout the day on Monday. Be prepared to dial back your terrain selection if you observe more than 6" of fresh snowfall or witness signs of significant snow transport.

Today, heightened Wind Slab danger is building on steep slopes near ridge tops or in cross-loaded areas including: convex rollovers, steep unsupported slopes (ending in rocks or cliffs), or below fresh cornices. Clues such as variable snow height, drifts, cornices, stiff snow surfaces that produce cracking are signs that you should stay off of steep, unsupported slopes. 

It has been more than two weeks since the last reported avalanche involving older weak layers. While many of these layers have not gone away, they are now difficult to trigger, but as a series of storms adds weight to the snowpack, Monday through Tuesday night, they may become active again if we add more than 1" of snow or liquid water equivalent. During the daylight hours on Monday, new storm loading will remain below this threshold and Persistent Slab avalanches will remain unlikely, though not impossible. Snowpack tests can provide useful information about buried weak layers, they are not a decision making tool.

 

Snowpack Discussion

Mostly cloudy and cool conditions the past several days have preserved dry snow on many aspects, particularly the North Cascades although breakable crusts are expected to have formed on Southerly aspects.

Up to a foot of snow has fallen in the Eastern Cascades near Washington Pass since March 21st with about half that amount accumulating in the East Central zone. Lyman Lakes is the big standout with nearly 1.5' of settled snow added to the snowpack. New Wind Slabs formed on lee slopes and cross-loaded features near and above treeline. The new snow fell on a variety of old snow surfaces. On sunny aspects, firm melt-freeze crusts have formed during the recent clear weather. In shaded locations, near surface facets and surface hoar were observed earlier in the week. This may form a new buried weak layer within the snowpack. Rain fell below treeline and likely melted any weak surfaces and helped bonding of the new snow.

 Persistent weak layers were responsible for 3 avalanche fatalities in the past month. Many of these have rounded and are no longer an issue. The remaining weak layers are generally inactive or dormant. Drastic changes to the snowpack, such as significant amounts of new precipitation or a major rain on snow event could make these layers a threat, again. Currently, the persistent weak layers that you may find include:

  • Surface hoar and facets were buried on 3/8. This layer is typically found 18-24” (45-60 cm) below the snow surface on shaded aspects near and below treeline.
  • Weak sugar-like facets (2/13 or 16) are 3-5 feet (90-150cm) below the snow surface. This is the most widespread persistent weak layer. It can be found just above a firm crust buried on 2/5. 
  • Other shallow and very weak areas of snowpack may be found further east of the crest or in areas with less snow.

Observations

North

On Saturday, 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 below the snow surface.

Central

On Thursday Mission Ridge Ski Patrol reported 4 inches of generally right-side-up storm snow by mid afternoon. 

NWAC professional observer Matt Primomo traveled in the North Fork of the Teanaway River on March 15th. (just north of Cle Elum). The 3/8 buried surface hoar layer was found about 12 inches below the snow surface. It was reactive in some snowpack tests. The 2/5 layer was also found in this area about 3 feet below the snow surface.

 

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

Persistent Slabs

An icon showing 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: All aspects.

Elevations: All elevations.

Likelihood

Unlikely

Expected Size

1 - 1

Valid until: Mar 26th, 2018 12:14PM