Avalanche Forecast

Issued: Mar 20th, 2016 11:00AM

The alpine rating is moderate, the treeline rating is moderate, and the below treeline rating is moderate. Known problems include Wind Slabs, Persistent Slabs and Cornices.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

A mixture of typical spring avalanche problems exists with no one problem dominating. Evaluate the local snow and terrain carefully on Monday.

Summary

Detailed Forecast

Cooler temperatures, cloudy and showery weather is expected Monday.  This weather should maintain areas of wet and weak snow below about 4-5000 feet in the Below Treeline elevation band. Moderate to strong winds at the higher elevations may build new shallow wind slabs on some leeward exposed ridges, mainly NW-E facing.  Firm wind transported snow and snowpack cracking can be good signs of wind slab layers.

Recently formed cornices have grown large and can become sensitive during mild weather, especially following nights with above freezing temperatures.  Avoid areas on ridges or summits where there may be a cornice and avoid slopes below cornices in the spring. Cooler temperatures Monday may help temporarily strengthen cornices Monday.

Due to recent sensitive storm slab releases on suspected buried surface hoar layer, March 12th, we are now listing this layer, though it may be spotty and not widespread, it should nonetheless draw attention from 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 from about March 9-15 to settle and mostly stabilize. Wind, 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 locales, but more triggered wind and storm slabs continue to be reported in the NE Washington Cascades as of 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. 

The recent storms have also built large cornices along ridges in many areas with several large cornice releases noted over the last few days.

A spotty layer of surface hoar likely formed and was buried by 3/12 in some areas, with the terrain most suspected N-E facing slopes in the upper elevations of the below treeline and near treeline band.  

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 any 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

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. 

Recent reports from the Washington Pass area Friday and again Saturday, March 18-19, indicated some shallow wind slabs were remained sensitive to human trigger as well as some shallow storm slabs in lower elevations, possibly on the 3/12 persistent layer. 

At least 2 recent cornice releases in the Washington Pass area caused large size 3 storm slab and car sized blocks running to the valley floor.

Tom Curtis was on Mt Cashmere Thursday 3/17 and found about 35 cm of storm snow well bonded to an underlying crust from last week. An extended column test did not indicate propagation and only small loose wet avalanches were seen on steep solar slopes. Temperatures did not greatly affect the snowpack.

The Mission Ridge pro-patrol reported limited wind transport on Friday, 3/18 from east winds and surface crusts below about 6000 feet from sun and warmth.

 

Problems

Wind Slabs

An icon showing 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, North West.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 1

Persistent Slabs

An icon showing 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.

Elevations: Treeline, Below Treeline.

Likelihood

Possible

Expected Size

1 - 1

Cornices

An icon showing 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

Unlikely

Expected Size

1 - 1

Valid until: Mar 21st, 2016 11:00AM