Avalanche Forecast
Issued: Jan 17th, 2016 10:44AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Persistent Slabs, Storm Slabs and Wind Slabs.
NWAC,
Northwest Avalanche Center
Even with an overall stabilizing trend on Monday, dangerous avalanche conditions will exist on lee wind-loaded slopes and where storm slabs become sensitive to human triggering. Slabs could step down to persistent weak layers producing large and deadly avalanches. Take it slow on Monday and evaluate the snowpack as you travel conservatively through the terrain.
Summary
Detailed Forecast
Scattered showers should quickly taper off Monday morning and sunbreaks are likely through mid-day before cloud cover increases Monday afternoon ahead of the next frontal system.
Persistent weak layers have had a decent test over the weekend from new loading. For slopes that have not released on these layers, the likelihood of natural triggering is decreasing, but the consequences are increasing as the slab depth increases. Storm slabs that step down to persistent weak layer depths could become large enough to bury, injure or kill.
Wind slab should be found mainly on lee northwest to east slopes near and especially above treeline. Generally avoid wind loaded slopes above treeline.
Storm slabs should more cohesive and likely to trigger with day-time warming and sunshine on Monday. Choose lower angled slopes if you find sensitive storm layers.
Small loose wet avalanches should be likely on steeper slopes below treeline involving small amounts of recent storm snow. Watch for loose wet avalanches near terrain traps, where even a small avalanche could have unintended consequences.
There have been numerous reports of large tree bombs with the new snowfall and fluctuating snow levels. Look down at the snowpack but also at the hazard above!
Snowpack Discussion
Two fair weather periods earlier this month allowed surface hoar and near surface faceting to occur. These persistent weak layers were buried on Jan 3rd and 11th intact and were reported around the east slopes. These weak layers are most likely to be found on non-solar (northerly) aspects and in the upper portion of the below treeline and the near-treeline elevation bands along the east slopes.
The latest report via the North Cascades Guides is from Delancey Ridge on Wednesday where easily triggered wind slab of about 20-25 cm was seen on north and south slopes at about 6000 feet which was releasing on a persistent layer crust sandwich buried on January 11th. Natural avalanches were also heard in the area.
A couple reports are available for Thursday via the NWAC Observations page. Skiers triggered 10 inch slabs on a crust which may involve a persistent weak layer on north slopes at 6000 ft on Silver Star Mt. Snow pits near Leavenworth had a easy to spot persistent buried surface hoar layer at 35-40 cm from the surface.
On Saturday Mission Ridge pro-patrol found easy 6-8" ski cuts on lee aspects near and above treeline involving new storm snow. It is thought that the buried surface hoar previously found in the Mission Ridge backcountry on northerly aspects was likely thoroughly tested earlier in the week and no longer a widespread issue in this specific area. However, just east of Blewett Pass a surface hoar layer was identified 1/13 in snowpack tests via the NWAC observation page.
On Sunday, Scottish Lake High Camps reported a natural 12" slab release on a southerly aspect around 5000' and easily identified two buried surface hoar layers in N-NW snowpits at 5500 feet.
Problems
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
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: All elevations.
Likelihood
Expected Size
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, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Jan 18th, 2016 10:44AM
