Avalanche Forecast

Archived

Mar 29th, 2018–Mar 30th, 2018

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Regions

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You may still trigger wind slabs avalanche Friday on NW-NE aspects, steep convex rollovers, unsupported slopes, and below cornice. You can use visual clues such as firm surface snow with cracking, snow drifts, and cornices to identify and avoid these steep wind loaded features. Expect wet snow surface conditions to quickly develop Friday if the sun comes out. Stay off of slopes greater than 35 degrees if you experience wet surface snow deeper than your ankle, see new roller balls, or observe new loose wet avalanches.

Detailed Forecast

Isoalted wind slabs still linger near and above treeline especially in areas closer to the Cascade crest You are most likely to trigger these avalanches on NW-NE aspects, steep convex rollovers, unsupported slopes, and below cornice. Use visual clues such as snow drifts, cornices, wind textured snow, and firm snow surface conditions that produce cracks to identify and avoid wind loaded areas.

If the sun comes out or the clouds thin, expect the strong March sun to quickly effect the surface snow. This is most likely in areas further from the Cascade crest where sunny skies will be more common. This occurs very quickly this time of year and does not take full bright sunshine to ensue. Avoid slopes greater than 35 degrees if you see new rollerballs, observe new loose wet avalanches, or experience wet surface snow deeper than your ankle.

Older weak layers can still be found in the snowpack in some locations. While these layers have not gone away, it is now difficult to trigger a Persistent Slab avalanche. We do not have any recent observations regarding the current state of these older layers. Snowpack tests can provide useful information, however they are not a decision making tool. If you see any cracking, experience collapses, or hear whumphs, stay off of nearby avalanche startzones.

Snowpack Discussion

Rain and snow showers this week have been generally confined to areas near the Cascade crest. Snow levels hovered between 4500 and 5000 feet through much of the precipitation. Above these elevations a few inches of new snow may have accumulated. Drier weather has prevailed in other areas of the Easter Cascade regions. Moderate winds transported soft snow at higher elevations likely forming wind slabs on lee slopes.

On slopes receiving direct sunshine expect new surface crust to have formed. The thickness of these new melt-freeze crusts depends on aspect and location within the region.

Below the surface snow, a high degree of variability can be found in the snowpacks east of the Cascade crest. Various melt-freeze crusts, strong rounded snow, and older weak snow layers may be observed.

Old persistent weak layers were involved in 3 avalanche fatalities over the past month. Many of these weak snow grain types have gained strength and rounded. 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 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 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 (45cm) below the snow surface.

Central

No recent observations from this region.

Problems

Wind Slabs

Wind Slab avalanches are the 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.

Loose Wet

Loose Wet avalanches are the 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. 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.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of 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 Slab.