Avalanche Forecast
Issued: Apr 1st, 2018 12:23PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Storm Slabs.
NWAC,
Northwest Avalanche Center
Dangerous avalanche conditions exist due to new snow and wind. While you can trigger avalanches at all elevations, you are most likely to trigger a large avalanche on slopes where you see freshly drifted Wind Slabs. Steer around recent drifts and deep pillows of snow on steep, wind exposed slopes. Watch for obvious signs of instability like recent avalanches and cracks in the snow.
Summary
Detailed Forecast
You can trigger avalanches on Monday due to new snow and wind forming slabs over a variety of slick surfaces. Expect Wind Slabs at upper elevations on leeward sides of ridges and terrain features. Watch for cracking in the snow, wind-sculpted features, and thick pillows of snow as indicators that you could trigger a Wind Slab avalanche. Steer around these features on steep slopes to avoid triggering these avalanches.
Near and below treeline the new snow is falling on slick surface crusts. You can trigger storm slabs on slopes over 35 degrees where you find more than 6 inches of recent, cohesive snow that slides easily on the old surface.
Layers of buried surface hoar and crust facet combinations exist roughly 2 feet below the snow surface in some locations. This is most prominent in the Crystal and Mount Baker backcountry, but could be found elsewere.
Snowpack Discussion
Six to eight inches of snow fell by late Sunday, with over a foot possible by the end of the day Monday. Snow has fallen on a variety of surfaces including slick crusts and surface hoar. Freshly formed Wind Slabs could be adding to the stacks of slabs near and above treeline. Older slabs formed last Wednesday. Many slopes have crusts in the top 2-3 feet of the snowpack. At higher elevations and shaded slopes dry, soft snow can still be found.
Several avalanches 3/24-3/27 in the Crystal and Stevens areas failed on a layer of weak older snow buried on 3/22. This weekend, observers still reported test results on this layer, but it has become difficult to trigger related avalanches. In some locations you may find a layer of small sugary facets above a firm melt-freeze crust. On shaded northerly slopes this layer may show up as buried surface hoar. This layer has generally been reported 12-30” (30-70cm) below the snow surface and may be most prevalent above 5,500ft.
Below the top 2-3 feet, the snowpack is generally well rounded and lacking interfaces of concern. The much older 2/8 melt-freeze layer can still be found over 6 feet (120-200cm) deep in the snowpack. While this layer isn't listed in our current avalanche problem set, it may reawaken if it becomes wet.
Observations
North
On Friday, NWAC Observers Lee Lazara and Simon Trautman traveled in the Bagley Lakes area. They reported 3 triggered or natural avalanches about 1 foot deep on north aspects near treeline on slopes with recently drifted snow. The observers found a thin weak layer of surface hoar buried 3/25 at the interface of concern.
Central
On Sunday, Alpental patrol reported up to 7" of accumulated snow was sliding easily on a hard crust at the old snow interface.
On Thursday NWAC professional observer Jeremy Allyn traveled in the Snoqualmie Pass area. Jeremy reported numerous wet avalanche below and near treeline that over the last few days. He found moist to wet surface snow up to 4600 feet.
South
On Saturday, NWAC Forecaster Dallas Glass traveled in the Crystal backcountry. Dallas found a surface crust up to 6" thick on most slopes. Cloud cover kept most slopes from softening. Dallas reported that the 3/22 interface was about 1 foot below the surface and that it was still reactive in snowpack tests. On north through east aspects the 3/22 layer is buried surface hoar, while on a southeast slope Dallas found rounding facets.
On Saturday, NWAC professional observer Jeremy Allyn toured from the Paradise trailhead at Mt Rainier. Jeremy reported a number of older slab avalanches at high elevations. Otherwise, Jeremy found a generally consolidated and stable snowpack. He did not find the 3/22 interface.
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
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
Expected Size
Valid until: Apr 2nd, 2018 12:23PM
