Avalanche Forecast
Issued: Mar 26th, 2018 12:03PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Loose Wet and Persistent Slabs.
NWAC,
Northwest Avalanche Center
Dangerous avalanche conditions will develop Tuesday as strong winds build fresh Wind Slabs near and above treeline. You will be able to trigger Wind Slabs on steep slopes near or below ridgelines. Strong winds may build areas of Wind Slab further downslope than usual. Pay attention to stiff snow surfaces that produce cracking as a clear sign you should retreat to non wind-affected terrain. Expect small Loose Wet avalanches below treeline as snow turns to rain.
Summary
Detailed Forecast
Dangerous avalanche conditions will develop Tuesday as strong alpine winds build fresh Wind Slabs near and above treeline, transporting new and recent snowfall. You will be able to trigger Wind Slabs on steep slopes near or below ridgelines. Strong winds may build areas of Wind Slab further downslope than usual and transport snow onto a variety of aspects due to erratic loading patterns. Pay attention to stiff snow surfaces that produce cracking as a clear sign you should retreat to non wind-affected terrain.
Rising snow levels on Tuesday will produce small Loose Wet avalanches below treeline. Watch for warning signs like new roller balls, pinwheels, and natural Loose Wet avalanches as temperatures rise and snow turns to rain below treeline. Even small Loose Wet avalanches may carry you into terrain with high consequences such as over cliffs or into gullies.
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, it is now difficult to trigger a Persistent Slab avalanche. If either snowfall or rainfall amounts are heavier than predicted through Tuesday night, they may become active again. Especially keep this in mind if traveling in terrain near the Cascade crest where a steep precipitation gradient will exist over short distances; much heavier rain and snow is forecast near and just east of the Cascade crest. 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 in the northeast Cascades. Non-supportive crusts likely formed on southerly aspects.
Up to a foot of snow fell along the east slopes of the Cascades near Washington Pass from 3/21 through 3/24, with about half that amount accumulating in the East Central zone. During this period, 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 were buried. In shaded locations, near surface facets and surface hoar were observed prior to the storm cycle.
Persistent weak layers were involved in 3 avalanche fatalities over the past month. Many of these weak grain types 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 snow or a major rain on snow event could make these layers a threat once 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 2/16) 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 3/23, 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 3/22, Mission Ridge Ski Patrol reported 4 inches of generally right-side-up storm snow by mid afternoon.
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: Alpine, Treeline.
Likelihood
Expected Size
Loose Wet
Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Several loose wet avalanches, and lots of pinwheels and roller balls.
Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.
Elevations: Below Treeline.
Likelihood
Expected Size
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
Expected Size
Valid until: Mar 27th, 2018 12:03PM
