Avalanche Forecast
Issued: Feb 13th, 2015 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Loose Wet and Persistent Slabs.
NWAC,
Northwest Avalanche Center
It is a stretch to find a primary avalanche problem Saturday, but some sunshine later in the day, especially in the southern areas may make shallow wet snow avalanches possible.
Summary
Detailed Forecast
A few showers early Saturday should give way to mostly cloudy conditions later Saturday and cooler temperatures. The cooler temperatures should begin to refreeze surface snow and form a crust at mid and upper elevations. This should put a lid on the potential for any wet snow avalanches.
Loose wet avalanches are unlikely Saturday due to the cooling, some some shallow wet snow may persist.
As a result of the overall low snowpack, especially below treeline, watch for terrain hazards such as open creeks, partially covered rocks and vegetation.
Snowpack Discussion
A series of fronts in a warm and wet southwesterly flow crossed the Northwest late last week. The east slopes have seen quite a variety of weather and snow conditions. The Harts Pass Snotel and the NWAC Washington Pass weather station's total snow gages indicate about a foot of snow during this period through late Saturday night. Other areas east of the crest have had less snow or rain.
Warm temperatures and a little light rain at times through the week has allowed for about 6-10 inches of snow settlement or melt.
The snowpack consists of mostly stable melt freeze grains or well settled snow with variable thickness, surface crusts, depending upon elevation and aspect.
Regarding the potential January 15th persistent slab combo in the northeast Cascades zone; NWAC pro-observers and North Cascade Guide reports as recently as Feb. 10th have found the January 15th facet/crust layers at 1 m below the surface on a north aspect of Silverstar Mt 6600 ft with the facets, starting to turn to rounded grains and not reactive or not releasing in pit tests. The January 15th facet/crust layers are stabilizing and becoming very unlikely for a human to trigger. A very large natural avalanche that may have run on this layer east of Washington Pass was likely tied to the heavier precipitation/warming from last week's event.
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: South East, South, South West.
Elevations: All elevations.
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, West, North West.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Feb 14th, 2015 10:00AM
