Avalanche Forecast
Issued: Mar 10th, 2018 10:33AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Loose Wet, Wind Slabs and Persistent Slabs.
NWAC,
Northwest Avalanche Center
Dangerous avalanche conditions will develop Sunday as warming air temperatures and sunny skies make it easier for you to trigger avalanches. You can best avoid avalanches by staying out off of slopes where avalanches start especially if these slopes show signs of wet snow or recent wind loading. Be patient and dial down your terrain travel as this change in weather changes avalanche conditions.
Summary
Detailed Forecast
You will be able to trigger wet snow avalanches Sunday as warming air temperatures and sunny skies create wet surface snow. These conditions will develop first on steep rocky slopes recieiving direct sunshine. Expect new roller balls, pinwheels, and loose wet avalanches to occur. Unstable snow will spread as air temperatures warm, effecting even shaded slopes. Stay off of any steep slope where you see wet surface snow. As water reaches deeper layers in the snowpack, wet slab avalanches may occur. If you see any signs of new wet slab avalanches, step back your terrain use, and avoid all avalanche terrain.
You are most likely to find and trigger lingering wind slabs on shaded slopes near and above treeline where wind deposited snow Thursday and Friday. Use visual clues such as fresh cornices and snow drifts to identify and avoid wind loaded slopes.
Persistent slabs continue to exist in along the eastern regions of the Cascades. The exact depth and type of persistent weak layer depends on location. These present a low likelihood-high consequence scenario that resulted in three fatalities last week. Just because you see tracks on a slope does not mean it is safe. To avoid persistent slabs, stay off of large open slopes where these larger avalanches may occur. While these avalanches may be difficult for a person to trigger, a small avalanche such as a loose wet or cornice fall may trigger these deeper layers.
Limit your exposure to overhead cornices as you travel. They may fail without warning due to daytime warming and sunshine threatening slopes below.
Snowpack Discussion
Sunny skies and mild temps Saturday allowed the sun to effect surface snow on sun exposed aspects. Cold snow likely remained on shaded slopes.
Generally 6-14" of new snow fell along the east slopes of the Cascades Thursday through Thursday night, with lesser amounts further east. Winds heavily transported the snow forming wind slabs and other wind-affected snow surfaces. This resulted in several wind slab avalanches being reported from the Washington Pass area in the last two days.
Weak persistent layers exist within the snowpack. In the past two weeks three avalanche fatalities have occurred because of persistent slabs. The exact persistent weak layer and depth is dependent on location.
Three commonly seen persistent weak layers are:
- Surface hoar and facets buried on 3/8. This layer is generally found near and below treeline just below the recent strom snow.
- On slopes that received direct sun, a thin facet-sun crust can be found. This layer is generally 1-2 feet below the snow surface.
- A more wide spread persistent weak layer is found just above a firm crust layer buried on 2/5. This layer of weak sugary facets (2/13) can be found 2-4 feet below the snow surface.
- Other shallow and very weak snowpacks may be found further east of the crest or in areas with less snow.
Observations
North
On Friday and Saturday, North Cascade Heli reported several slab avalanches near and above treeline. These avalanches occurred on a variety of aspects and were up to size D2.
On Thursday, North Cascades Heli reported a D1.5 (small/large) avalanche that occurred naturally where a wind slab relased on a 40 degree SE aspect at 6800 feet near Washington Pass.
On Thursday during the storm, NCMG generally found the 2/5 crust down 30-35" in the Hairpin valley with a 0.5 cm layer of 1.5 mm rounding facets. They did not see the Feb 23 facets, but found a layer of 5 mm surface hoar buried on March 8th down 3-12" due to variable wind transport. Wind slabs were reactive on N-E-S aspects.
Mid-week, observers reported a mix of sun crusts and surface hoar/near surface facets prior to snow on 3/8.
Central
On Friday, Mission Ridge Pro Patrol reported 2" of new snow with extremely high winds forming slabs that were not reactive on test slopes or column tests. The recent 2" of snow sits on a layer of 3-4 mm surface hoar buried 3/8. The 2/5 crust was down 20" and was still reactive in snowpack tests.
Problems
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.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
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
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 11th, 2018 11:33AM
