Avalanche Forecast
Issued: Jan 23rd, 2016 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Persistent Slabs.
NWAC,
Northwest Avalanche Center
Watch for recent shallow wind slabs that may have formed on the lee, northerly aspects below ridges, mainly near and above treeline.
Continued conservative decision-making remains wise travel advice, until the persistent slab problems can be ruled out.
Summary
Detailed Forecast
Further cooling overnight Saturday and Sunday with a weak system, giving a chance of light snowfall in the south areas along with light winds, should all lead to an overall stabilizing snowpack. The main concern are areas of shallow wind slabs on lee slopes near ridges. Any previously wet or moist surface snow should have formed a strengthening crust by Sunday, locally decreasing danger. Further continued snowpack settlement should also allow for an improving trend.
Snowpack Discussion
Weather
Two fair weather periods earlier this month allowed surface hoar and near surface faceting to occur. These persistent weak layers were buried intact on January 3rd and 11th. One or both of these layers have been seen in snow pits by NWAC pro observers and avalanche professionals the past few days ranging from Mt Baker to the Crystal Mountain area. These persistent weak layers are most likely to be found on non-solar (northerly) aspects of the below and near-treeline band. However, we cannot rule out the above treeline elevation bands with no recent observations up high, so the problem is listed for all three bands. We will cautiously wait to see how the PWLs survived the most recent storm cycle over the past few days.
A series of storms deposited several feet of recent snow along the west slopes. The most recent frontal system brought up to a foot of snow late Wednesday night through early Thursday morning, before very warm and wet weather arrived through the day Thursday.
The rapid arrival of warmer air brought a quick change to rain for all areas away from the Passes like Mt. Baker, Paradise and Crystal. 2 to nearly 7 inches of water fell over the west slopes in 48 hours through 5 pm Friday, with the heaviest rainfall in the Mt. Baker area. The Passes were slower to warm on Thursday, but a thick, mostly breakable freezing rain crust was reported above about 4000 feet both in the Snoqualmie and Stevens Pass areas Friday and Saturday. While limiting the avalanche danger, it has also ruined the skiing for most areas near the passes.
Snow and Avalanche Observations
The snowpack has now undergone significant changes since the warming and rain Thursday, causing a widespread natural avalanche cycle of both wet-loose and slab releases.
The most recent observations by Jeff Ward Friday the 22nd in the Stevens Pass area indicated a widespread avalanche cycle has occurred with numerous wet-loose slides on all aspects as well as storm slab releases, possibly releasing on the persistent layer, though not confirmed, all size one.
Several NWAC pro observers found both the 1/3 and 1/11 buried surface hoar layers throughout the region earlier in the week, however, by Saturday after the rain, warming, numerous avalanches, and freezing rain crust, found no such layer in extensive searching in the Stevens Pass area within the below treeline elevation band. It is felt at this point, the Persistent Weak Layer, at least for most west slope areas, is no longer a prevalent avalanche problem.
We will continue to list the persistent slab problem in the forecast, but lower the likelihood until further observations can confidently put this to bed.
About 3-6 inches of new snow fell Saturday at moderate temperatures adding some shallow storm snow layers.
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
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
Valid until: Jan 24th, 2016 10:00AM
