Avalanche Forecast
Issued: Mar 30th, 2016 11:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Loose Wet, Cornices and Wind Slabs.
NWAC,
Northwest Avalanche Center
We are in the midst of the first extended spring warm up. Very warm temperatures and the late March sun will focus the avalanche hazard on wet snow concerns Thursday, especially highlighted by significant recent storm snow in many areas. High freezing levels, light winds and sunny skies over the next several days will cause natural loose wet avalanches on solar aspects and increase the odds for low likelihood/high consequence problems such as cornice failures.
Summary
Detailed Forecast
On Thursday, alpine winds will by light and freezing levels should remain around 10000 feet. Very warm temperatures and the strong late March sun will maintain wet snow concerns as the main avalanche hazard Thursday. High freezing levels, light winds and sunny skies over the next several days will increase the hazard, causing natural loose wet avalanches on solar aspects and increasing the odds for low likelihood/high consequence problems such as cornice failures.
Melt-freeze crusts can quickly break down with intense spring sunshine. In areas that have received more recent snowfall, small loose wet avalanches have the potential to entrain deeper layers. Be aware of terrain traps where even a small loose wet avalanche could have unintended consequences. Plan to avoid steep solar slopes by late morning/mid-day to minimize the problem.
The likelihood of triggering older wind slabs continues to diminish, however, in isolated areas above treeline, a mix of older wind slab on lee easterly aspects and newer wind slab on the south through westerly aspects will maintain wind slab potential on a variety of aspects. Watch for cracking and firmer or chalky wind transported snow.
Many areas have massive cornices along ridge-lines, watch for those overhead hazards. Unlike the loose wet problem, you may not have much notice that a cornice is about to fail. Cornices can break much further back on ridges than expected and releases can be unpredictable during the spring.
Snowpack Discussion
Weather and Snowpack
Snow accumulations Wednesday 3/23 to Friday 3/25 varied based on proximity to the Cascade crest and higher terrain with about 12 inches at the NRCS Lyman Lake Snotel, 6 inches at Harts Pass and next to nothing at Blewett, Mission Ridge and Berne weather stations.
A strong front crossed the Northwest Sunday morning followed by a large upper trough and a cool unstable air mass Sunday afternoon and evening. Snowfall accumulations were light along the east slopes of the Cascades with only a few inches of new snow seen at the higher elevation stations near the crest. The highest totals were likely downwind of a Puget Sound convergence zone east of Snoqualmie Pass Sunday evening.
Sunny skies on Tuesday were accompanied by a slow warming trend and moderate NE crest level winds along the east slopes. While valley locations like Lake Wentachee and Mazama hit 60 F Tuesday, Mission Ridge summit temperatures stayed near freezing with wind speeds averaging in the 30s and gusts 55-70 mph out of the NE Monday night through Tuesday afternoon.
Frequent March storms have built large cornices along ridges in many areas.
A spotty layer of surface hoar was buried on about 3/12 mainly in the northeast Cascades with the terrain most suspect being N through E facing slopes. The last known human triggered avalanche on this layer was in the Washington Pass area on 3/19 and the persistent slab problem has been removed from the east slopes forecast moving forward.
No avalanches have been noted for quite some time on a persistent surface hoar layer buried February 27th in the Washington Pass area and to a lesser extent the central-east zone. Avalanches releasing on or down to this layer are unlikely.
Recent Observations
Mission Ridge pro-patrol reported strong west winds Thursday morning 3/24 but little in the way of avalanche concerns in area due to the lack of new snow. On Saturday they reported minor small point release loose wet avalanches and minor rollerballs.
Tom Curtis was out in the Table Mountain area of Blewett Pass on Monday and found about 6 cm of new snow in wind sheltered areas with up to 20 cm on lee north through east aspects near treeline. No recent avalanche activity was noted in the Blewett Pass area.
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
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
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
Valid until: Mar 31st, 2016 11:00AM
