Quite a change in the weather is expected on Saturday. You should mainly need to watch for loose wet snow avalanches but read the forecast discussion for details.
Summary
Detailed Forecast
A front that taps sub tropical moisture should cause strong southwest winds and increasing moderate to heavy rain in the Cascades on Saturday. It looks like an initial wave on the front will somewhat evenly affect the Cascades from Mt Baker to Mt Hood.
The moderate to heavy rain is expected to reach above treeline on Saturday. On any slopes holding old powder or less consolidated surface snow this will likely cause loose wet avalanches. Watch for wet surface snow deeper than a few inches, natural avalanches, and pinwheels or rollerballs on steep north facing or previously shaded slopes mainly above and perhaps near treeline.
Below treeline rain is not expected to have as much effect since the Cascade snowpack on these slopes beforehand is so limited, consolidated and stable.
It seems possible that any cornices will get a bit of a test Saturday. Although cornices are not as big as usual this year give ridges with cornices and slopes below cornices a safety margin.
Some snow may also be seen in the upper above treeline in the Cascades on Saturday. This won't be listed as a problem since it only seems possible in the upper above treeline and loose wet snow should be the predominate problem. But watch for new storm slab in areas that accumulate more than a few inches of new snow during the day if you venture into the upper above treeline in the Cascades.
Quite a change in the weather is expected on Saturday so watch for any signs that conditions are turning out different than expected.
Due to the low snowpack, especially below treeline, watch for terrain hazards such as open creeks, partially covered rocks and vegetation. Many areas below treeline do not have enough snow to cause an avalanche danger.
Snowpack Discussion
The latest warm dry period of the winter was seen from the start of March through Tuesday. In the Cascades this caused more consolidation and stabilizing. Reports indicated thickening surface crusts on most slopes and some corn snow development on solar slopes. Lingering varied more winter-like surface conditions were still seen on some north slopes at higher elevations.
The North Cascade Mountain Guides were in the Harts Pass and Washington Pass area last weekend. They reported some cool varied surface snow on north slopes, corn snow on south slopes, and no stability concerns. The snow study plot on a north aspect at 6600 feet above Varden Creek measured a snow depth of nearly two and a half meters!
A weak front crossed the Northwest Wednesday to Thursday morning. NWAC sites west of the crest picked up anywhere from about .1 to 1.6 inches of rain with some snow above treeline on the volcanoes. But very little rain fell east of the crest.
There have not been any reports of significant avalanches in the Cascades for many days. There was nearly no snowfall January to so far in March near and below treeline. The snowpack at low elevations remains meager to non-existent.
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: North, North East, East, West, North West.
Elevations: Alpine, Treeline.
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, North West.
Elevations: Alpine.
Likelihood
Expected Size
Valid until: Mar 14th, 2015 11:00AM
