Avalanche Forecast
Issued: Mar 27th, 2018 12:45PM
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
Moderate NW winds will continue to build fresh Wind Slabs above treeline Wednesday, continuing dangerous avalanche conditions at higher elevations. You will be able to trigger Wind Slabs on steep slopes near or below ridgelines. Small Loose Wet avalanches are possible on steep sunny slopes near and below treeline, especially during prolonged periods of sunshine.
Summary
Detailed Forecast
Moderate crest level NW winds will continue to transport recent snow onto lee slopes near and especially above treeline Wednesday. You will be able to trigger Wind Slabs on steep slopes near or below ridgelines. Watch for clues like variable snow height, drifts, cornices, and stiff snow that produces cracking. These are all indicators that you could trigger a Wind Slab. You can avoid triggering these avalanches by steering around steep roll-overs, unsupported features, and obvious start zones where you suspect Wind Slabs.
Small Loose Wet avalanches are possible on steep sunny slopes near and below treeline, especially during prolonged periods of sunshine. Watch for warning signs like new roller balls, pinwheels, and natural Loose Wet avalanches that indicate increasing hazard. Even small Loose Wet avalanches may carry you into terrain with high consequences such as over cliffs or into gullies. Cooler temperatures Tuesday night and Wednesday should help surface crusts form on more northerly aspects and shaded terrain.
It has been more than three 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. We do not have any recent information if the most recent storm awakened these older weak layers. Snowpack tests can provide useful information about buried weak layers, they are not a decision making tool.
Snowpack Discussion
Strong and gusty winds were seen at ridgecrest on Tuesday; DirtyFace Mt reported gusts into the 40s and Mission Ridge into the 80s for several hours. A stark drying trend was noted east of the crest with sunshine over the Wentachee Mountains and warm temperatures in all lower elevation stations. Higher elevation sites did not show any appreciable snowfall accumulation. Areas in close proximity to the Cascade crest and above 4500-5000 ft are likely the only areas the accumulated more than a few inches of snow during Tuesday's storm.
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. 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
None
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: Treeline, 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 28th, 2018 12:45PM
