Avalanche Forecast
Issued: Mar 9th, 2016 10:00AM
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
Very strong winds and snow showers should combine to form new wind slabs on lee slopes, especially NW-E facing. Watch for earlier storm slabs that may be masked by shallow colder surface snow. Dangerous avalanche conditions are expected near and above treeline Thursday.
Summary
Detailed Forecast
Yet another atmospheric river is slamming the Cascades Wednesday, bringing stormy wet weather with rising snow levels through Wednesday night.
East flow should locally maintain snowfall Wednesday night in the Stevens and Snoqualmie Pass areas.
The heavy precipitation at rising freezing levels should produce one or more natural avalanche cycles during the warmest periods, mainly Wednesday night.
By Thursday morning, cooling and light to moderate showers with continued strong winds are expected.
The cooling should help wet snow to begin consolidating and cause a lowering danger during the day with shallow new snow accumulations expected.
However, near and above treeline, very strong winds should build new areas of wind slab on a variety of aspects, but mainly on NW-E facing slopes below ridges.
Be especially aware that new shallow lower density snow may mask earlier storm slab formations, especially on slopes that did not avalanche overnight Wednesday.
Snowpack Discussion
Weather and Snowpack
Last week was wet and relatively mild with a series of active fronts transiting the region resulting in fluctuating freezing levels and periods of very strong winds. The heaviest snowfall and precipitation occurred late Sunday 2/28 and Tuesday 3/1. The latter half of the week featured more rain than snow at NWAC sites along the west slopes.
Mid and lower elevations late last week and over the weekend saw natural and triggered loose-wet avalanches most days due to increasing solar effects or during rain events.
A front Sunday and a cooler upper trough Monday brought some snow with 2 day storm totals of about 2-12 inches along the west slopes ending Tuesday morning.
A very strong storm arrived midday Wednesday bringing heavy snowfall/rain at rising temperatures and very strong south ridgetop winds.
Strong east winds through the passes maintained precipitation as snow Wednesday at Stevens and Snoqualmie Passes.
The mid and lower snow pack along the west slopes should generally be a stable mix of crusts and layers of moist and rounded snow crystals.
Recent Observations
Locally heavy rain at the Mt. Baker area led to sensitive loose wet ski cuts by the Mt. Baker pro-patrol Saturday morning. A natural loose wet cycle was observed in the Bagley Lakes area and one glide avalanche in unsupported terrain released nearby. Also in the Baker area, a glide avalanche occurred Saturday night off Shuskan Arm entraining moist surface snow and becoming very large while running to the valley bottom.
The pro-patrol at Alpental on Wednesday indicated the 6 inches of new snow received by Wednesday morning was combining with new snow and strong east winds to quickly form wind slabs that were sensitive to ski release.
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: North, North East, East, South East, North West.
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.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Mar 10th, 2016 10:00AM
