Avalanche Forecast
Issued: Mar 28th, 2016 11:53AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
Moderate and persistent crest level winds out of the northeast Monday night and Tuesday will redistribute recent snowfall to unusual aspects mainly above treeline. At lower elevations and in wind sheltered terrain, the main avalanche problem will shift from wind slab to loose wet due to sunny skies and warming temperatures.
Summary
Detailed Forecast
Moderate and persistent crest level winds out of the northeast Monday night and Tuesday will redistribute recent snowfall to unusual aspects mainly above treeline. At lower elevations and in wind sheltered terrain, the main avalanche problem will shift from wind slab to loose wet due to sunny skies and warming temperatures.
A mix of older wind slab on lee easterly aspects and new wind slab on south through westerly aspects will make wind slab avalanches possible on a variety of aspects, and ultimately for tricky travel above treeline Tuesday. The east slopes forecast above treeline is Moderate versus Considerable for the west slopes because there is less new snow available for transport. Watch for cracking and firmer or chalky wind transported snow. Yesterday's windward aspect may be today's loaded lee slope.
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. Moderate winds above treeline will limit the loose wet potential except on direct solar slopes. Be aware of terrain traps where even a small loose wet avalanche could have unintended consequences.
Near treeline should be a mix of loose wet avalanche and wind slab concerns.
Cornices have grown large recently. 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 last Wednesday to Friday 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.
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
On Wednesday 3/23 NWAC pro-observer Tom Curtis was in the Blewett Pass/Table Mountain area and found dust on a mostly supportive crust prior to the incoming frontal system. Also of note, many windward and solar slopes were nearly or completely melted out up to about 6000 feet.
Mission Ridge pro-patrol reported strong west winds Thursday morning 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 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.
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: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
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
Valid until: Mar 29th, 2016 11:53AM
