Avalanche Forecast
Issued: Mar 21st, 2016 11:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Persistent Slabs and Cornices.
NWAC,
Northwest Avalanche Center
A mix of spring avalanche problems exists with no one problem dominating. Evaluate the local snow and terrain carefully on Monday.
Summary
Detailed Forecast
Light to moderate west winds and a couple inches of new snow should be seen in the near and above treeline zone along the east slopes Monday night and Tuesday. Temperatures should continue to slightly cool from the previous couple days.
This weather may build new, isolated shallow wind slab on lee slopes in the near and above treeline through the day light hours on Tuesday. This is most likely on N-SE slopes. Firm wind transported snow and snowpack cracking can be good signs of wind slab layers.
Continued wet snow conditions can't be ruled out on Tuesday. The cooler temperatures should limit this problem to the below treeline. Watch for wet snow deeper than a few inches and pinwheels or natural releases.
Recently formed cornices have grown large and can become sensitive following nights with above freezing temperatures. Avoid areas on ridges or summits where there may be a cornice and avoid slopes below cornices. Cooler temperatures Tuesday may help to temporarily strengthen cornices Tuesday.
Due to recent sensitive storm slab releases on the suspected spotty 3/12 buried surface hoar layer, we are currently listing it as a persistent slab. Though it may be spotty and not widespread it should nonetheless get attention from backcountry travelers in the NE zone.
Snowpack Discussion
Weather and Snowpack
A combination of sun and cloud with mild temperatures for several days gave way to mild temperatures and spotty light rain Sunday, March 20th. The recent weather over the past five days has allowed for the 2 to 3 feet of storm snow that accumulated along the east slopes from about March 9-15 to settle and mostly stabilize. Recent sun, warmth, freezing and some light rain have changed snow conditions over the past five days. Surface snow conditions have become more variable, ranging from still good settled cold snow on steep shaded slopes and a mix of surface crusts, wind buffed surfaces or shallow moist to wet snow on other aspects.
Earlier avalanche problems of wind and storm slab have been settling and stabilizing in most areas but more triggered wind and storm slabs were reported in the northeast Cascades Friday and Saturday. Most east slope sites have seen the snowpack settle 6-12 inches over the past five days since the strong storms ended March 15th.
Little if any new snow reached the east slopes on Sunday night.
Recent storms did build large cornices along ridges in many areas with some recent natural releases noted.
A spotty layer of surface hoar likely formed and was buried on about 3/12 mainly the northeast Cascades with the terrain most suspected being N-E facing slopes.
No avalanches have been noted for quite some time on a persistent buried surface hoar layer from February 27th in the Washington Pass area and to a lesser extent the central-east zone. Avalanches on this layer are now unlikely.
We are no longer tracking layers of concern formed earlier this winter in the mid or lower snowpack due to lack of recent activity at these interfaces and confirming field observations.
Recent Observations
The North Cascades Heli Skiing operation skied extensively on a non-solar slope in Cedar Creek on Monday 3/21 with no avalanches.
The North Cascade Mountain Guides at Washington Pass area Friday to Sunday 3/18-20 reported no results from ski cuts and no signs of instability. On Friday a loose wet avalanche that triggered a small 40 cm deep slab. Cornices in this area area sagging greatly and snow in the below treeline was moist and heavy in the afternoons.
NWAC pro-observer Tom Curtis tested conditions along the Nason Ridge area to Rainy Pass Saturday, March 19th. No significant layers were found in test pits or on slopes. In general, shallow wet snow was found below treeline with shallow stubborn wind slabs on some isolated features near ridges that were gaining strength.
Reports via guides and a private email to the NWAC indicate 2 recent cornice releases in the Washington Pass area with a large size 3 storm slab and car sized blocks running to the valley floor.
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: North, North East, East, South East.
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: North, North East, East.
Elevations: All elevations.
Likelihood
Expected Size
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind lips of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
Cornices can never be trusted and avoiding them is necessary for safe backcountry travel. Stay well back from ridgeline areas with cornices. They often overhang the ridge edge can be triggered remotely. Avoid areas underneath cornices. Even small Cornice Fall can trigger a larger avalanche and large Cornice Fall can easily crush a human. Periods of significant temperature warm-up are times to be particularly aware.
A corniced ridgeline. A large cornice has formed at the top of the ridge. A smaller cornice has formed to the left of the trees from crossloading.
Cornices are easy to identify and are confined to lee and cross-loaded ridges, sub-ridges, and sharp convexities. They are easiest to trigger during periods of rapid growth (new snow and wind), rapid warming, and during rain-on-snow events. Cornices often catch people by surprise when they break farther back onto flatter areas than expected.
Aspects: North, North East, East, South East, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Mar 22nd, 2016 11:00AM
