Avalanche Forecast
Issued: Mar 13th, 2018 11:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Persistent Slabs and Wind Slabs.
NWAC,
Northwest Avalanche Center
A refreezing snowpack will cause decreased danger Wednesday in most areas. A Cornice failure could trigger a large wind slab or a Persistent Slab avalanche. Limit your exposure to overhead hazard Wednesday, staying away from large avalanche paths. Watch for fresh but shallow new Wind Slabs near ridges at higher elevations.
Summary
Detailed Forecast
A strengthening surface crust will limit the avalanche danger Wednesday. Shallow new snow may be transported near ridges, especially near and above treeline, where small wind slabs may be possible to trigger on steep lee slopes. Avoid steep open slopes showing signs of recent wind loading.
Older wind slabs have been stabilizing but you are most likely to find and trigger lingering wind slabs on shaded slopes 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 along the East slopes 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 weekend (3/3-3/4). 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 heating and the sun.
Snowpack Discussion
Sunshine and temperatures in the 40's to 50's over the past few days has allowed for significant snowpack settlement and for a variety of surface conditions to develop. Several melt-freeze cycles over the past few days has allowed for a strengthening snowpack. Steep shaded north facing slopes are hanging on to some settled old snow that fell Thursday.
Clear weather over the past few nights has allowed for surface hoar or near surface faceted snow to develop on many slopes. These surfaces are easily destroyed by wind, sun, warming or rain, so may end up being widely variable when future snowfall occurs. Precipitation falling Tuesday afternoon has begun as rain in most areas and hopefully destroyed much of these weak surface crystals.
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.Generally 6-14" of new snow fell along the east slopes of the Cascades Thursday through Thursday night, with lesser amounts further east. Strong winds transported snow forming wind slabs and other wind-affected snow surfaces. This resulted in several wind slab avalanches being reported from the Washington Pass area Friday and Saturday.
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 Monday, North Cascade Heli reported two very small wind slabs, one triggered and one natural on steep shaded slopes at 6000 and 7000 feet.
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 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
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
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: North, North East, East, West, North West.
Elevations: Alpine.
Likelihood
Expected Size
Valid until: Mar 14th, 2018 11:00AM
