Avalanche Forecast
Regions: Cascades - North West.
Uncertainty lingers above treeline where large wind slabs may have formed Monday or Tuesday during the last storm cycle and could still be reactive. Near treeline, generally stabilizing conditions have been observed, but look for signs of wind-blown snow above a refreezing crust to identify and avoid wind-loaded slopes capable of producing an avalanche on a steep, unsupported slope.
Discussion
Mt. Baker Pro Patrol reported 5†of new snow after the rain/snow transition Tuesday night, with a convergence zone Wednesday morning adding 3 more inches of fresh snow. The snow bonded well to wet snow underneath. Light snow early Thursday should have minimal impact on the existing hazard.
Take a moment to appreciate the magnitude of a major storm cycle culminating in widespread size 2-2.5 and even some size 3 avalanches which ran naturally Monday and Tuesday. Numerous large avalanches also released in mitigation at the ski area. Significant features on Shuksan Arm, Hemispheres, and in Bagley Lakes all ran long distances.
Two photos from Rumble Gully avalanche (R2,D3, ran Monday) near Mt. Baker Ski area. The slide filled in the valley bottom. (top) 5 ft deep crown, 500 ft wide photographed from Chari 5 and (bottom) 20 ft debris pile in the deposition zone just above Herb’s Berm. Photo Credits: Pete Durr
Since the last rain crust formed at the end of the evening last Friday 01/03, Mt. Baker (Heather Meadows) added about 40†to their snowpack, before the snow changed to rain as the snow line crept up to 6000 ft on Tuesday. This addition of snowpack came with about 6†of water with two inches of rain received on Tuesday; making for an impressive 13†of water equivalent since 4 AM on 12/31. The primary driver of the intense avalanche cycle was the volume of snow, but secondarily, the avalanches were more reactive because Monday’s low-density snow was rapidly loaded by an increasingly heavy slab. Rain ultimately completed the widespread cycle, changing the character of the avalanches. By Tuesday, the wet avalanches were gauging 3-4 ft deep through the entire volume of snow since last Friday.
Snowpack Discussion
New Regional Synopsis coming soon. We update the Regional Synopsis every Thursday at 6 pm.
Avalanche Problems
Wind Slabs
We have no direct observations above treeline since a major storm cycle wound down by early Wednesday. If you go above treeline, expect a high degree of uncertainty in wind slab size and distribution, and snowpack structure. If you travel above treeline on Thursday, look for signs that the snowpack is still adjusting to recent loading, including whumping, cracking of snow surfaces, or denser layers and be aware that these layers may be masked by low-density snow. We expect that given mild temperatures near the end of the storm cycle, weaknesses in the snow should be healing rapidly with the current cooling temperatures and you are more likely to trigger a shallow wind drift than a deep slab. Near treeline and above look for wind-textured snow surfaces or fresh cornices as signs that shallow wind slabs may form in lee terrain.
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: Possible
Expected Size: 1 - 1