Avalanche Forecast
Regions: Cascades - North West.
The Avalanche Danger will rise to High by Tuesday afternoon. Be prepared to avoid avalanche terrain near and above treeline as the danger increases. Watch for natural avalanche activity as a sign that the danger is on the rise. You can stay safe by avoiding avalanche terrain with recent snow drifts, cornices, and firm snow that produces cracking. Stay away from areas where large, steep slopes could be above you.
Detailed Forecast
The avalanche danger will increase to High by Tuesday afternoon. Expect very dangerous conditions due to heavy snow, wind, and rain at low elevations. Large natural avalanches are likely at upper elevations. Be aware of overhead hazard from steep slopes above you. Avoid terrain where avalanches can start or run from above. It will be easy to trigger Wind Slab avalanches on slopes over 35 degrees near ridges or in cross-loaded terrain features. Watch for clues like blowing snow, variable snow height, drifts, cornices, and stiff snow that produces cracking. These are all indicators that you should avoid avalanche terrain today.
You can trigger Storm Slabs on steep slopes near and below treeline where the storm snow slides easily. Watch for cracking and stiff snow over softer snow near the surface. Rain will fall Tuesday afternoon bringing potential for Loose Wet avalanches below 4,500 feet. For indicators to either of these types of avalanches use quick tests and small test slopes to check how easily the new snow slides.
It has been more than two weeks since the last reported avalanche involving older weak layers. Deep Persistent Slab avalanches are now very unlikely. The weak snowpack structure could be become a problem later in the spring.
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Snowpack Discussion
The main concern for avalanches for Tuesday will lie with incoming snow, wind, and rain. Most areas picked up 6-9" of snow by 5p Monday. This past weekend, about 1 foot (35cm) of new snow fell near Mount Baker. Observers reported a range of avalanche activity over the weekend including triggered avalanches in wind loaded terrain near Mt Baker and the Crystal backcountry and numerous small natural slab avalanches in the upper storm snow near Snoqualmie Pass.
There were reports of surface hoar prior to Monday's snowfall. Quick snow profiles will help identify this weak layer (now about 1 foot below the surface). Snow from the past week sits on the 3/22 interface. On sunny and lower elevation slopes you will find a crust at this interface. Below the top 2 feet (60cm) the snowpack is generally well-rounded, strong, and lacking interfaces of concern.
An old, thin layer of sugary facets (2/13) can still be found in some locations just above a very firm and wide spread crust (2/8). The overall depth (over 6 feet) and significant rounding (melt-freeze cycles in some locations) has allowed this structure to drop off the official list of avalanche problems. NWAC professionals will continue to track this weak layer as it could be a problematic later in the spring.
Observations
Mt Baker and North West Cascades
On Sunday, NWAC Observer Lee Lazzara traveled near Hidden Lake. Lee found about 1 foot of recent snow topped with widespread surface hoar up to treeline.
Lee traveled in the Mt Baker backcountry Friday and reported 1’ (30cm) of new snow over a growing melt freeze crust. He observed several small skier triggered wind slab avalanche near ridges and on cross-loaded terrain.
Avalanche 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: Very Likely
Expected Size: 1 - 1
Storm Slabs
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-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.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Elevations: Treeline, Below Treeline.
Likelihood: Likely
Expected Size: 1 - 1