Heat related avalanche problems will continue on Sunday. Due to the warm weather, loose wet avalanches will again be possible primarily on steeper solar slopes. Also, be aware of low likelihood/high consequence problems such as cornice failures and glide avalanches.
Summary
Detailed Forecast
One more day of warm weather is in store for the Pacific Northwest Sunday before we briefly return to winter-like conditions early next week.
Increasing clouds and crest level winds should begin to curb the loose wet problem in the afternoon for the near and above treeline bands.
Melt-freeze crusts can quickly break down in 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.
Many areas have massive cornices along ridge-lines so avoid slopes below these overhead hazards. Unlike the loose wet problem you will not get advance 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.
Although not listed as an avalanche problem, large, powerful glide avalanches can release unexpectedly so avoid areas below steep unsupported slopes or rock faces.
In most areas previous wind slab will have become assimilated into the upper snowpack in the warm weather. The likelihood of triggering older wind slabs should be minimal.
Snowpack Discussion
Weather and Snowpack
A large upper ridge and warm air mass has been over the Northwest several days causing very warm weather. Temperatures on Friday reached into the 50's and 60's F at many NWAC sites on both sides of the Cascade crest. Temperatures on Saturday were slightly cooler but overall conditions remained mild. This weather has been causing a spring avalanche cycle in most areas and has shifted concerns to heat related avalanche problems the past couple days.
The frequent March storms built unusually large cornices along many ridges.
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.
Recent Observations
NWAC professional observer Tom Curtis was out on Wenatchee Ridge near Lake Wenatchee Wednesday, 3/30 in the 2-4000 foot range and found many slopes melted out. On slopes still holding snow he found natural and easily triggered loose wet avalanches on all solar slopes in the top 15-20 cm of snow.
Tom was out yet again on Mt Lichtenberg near Stevens Pass on Thursday and found widespread large natural loose wet avalanches on nearly all solar aspects with easy to trigger loose wet avalanches on non-solar slopes.
A report on Turns All Year for the Chiwaukums described big rollerballs and big resulting debris piles on Thursday.
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
Valid until: Apr 3rd, 2016 11:00AM
