Avalanche Forecast
Issued: Mar 22nd, 2016 11:00AM
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
Spring is springing! Watch for rapid changes especially if the sun comes out. Continue to evaluate snow, terrain and cornices carefully on Wednesday.
Summary
Detailed Forecast
A bit of a break in the weather should be seen Tuesday night and Wednesday morning. Southwest flow should begin to carry another front to the Northwest beginning in the Olympics and northwest Cascades Wednesday afternoon and to the rest of the Cascades Wednesday night. Significant new snow layers are generally not expected in the Cascades through most of the daylight hours on Wednesday. Remember that you may need to adjust your plans if the next front arrives with wind and rain or snow sooner than expected.
Continued loose wet snow conditions can't be ruled out on Wednesday especially if there are sun or solar effects. Surface crusts in the morning don't mean that loose wet snow conditions won't develop by the afternoon. This is mostly likely on solar slopes but can't be ruled out on other aspects. Watch for wet snow deeper than a few inches and pinwheels or natural releases.
Recently formed cornices have grown large and with another release reported at Mt Hood on Tuesday. Avoid areas on ridges or summits where a cornice may be present and avoid slopes below cornices. Cornice releases can be unpredictable during the spring.
Small areas of previous wind slab may remain present on lee slopes. This should be mainly northwest to southeast aspects near ridges and summits. Firm wind transported snow and snowpack cracking can be good signs of wind slab layers.
Due to recent sensitive storm slab releases on a 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
About 2-3 feet of snowfall accumulated along the east slopes March 9-15th.
A combination of sun, mild temperatures and then some rain was seen for several days ending about Sunday 3/20. This weather caused about 6-12 inches of consolidation of the recent snow along the east slopes. Surface snow conditions became highly variable, ranging from cold snow or wind buffed surfaces on non-solar slopes to morning crusts and wet snow on solar slopes.
Recent storms did build large cornices along ridges in many areas.
A low pressure system crossed the Northwest on Monday and Tuesday. NWAC stations along the east slopes had about 0-4 inches of new snowfall ending on Tuesday morning.
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 also 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
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: East, South East, South, South West, West.
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, South East, North West.
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
Valid until: Mar 23rd, 2016 11:00AM
