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.
Continued cautious routefinding and conservative decision-making remain the proper travel advice, until the persistent slab problems can be ruled out.
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 Jan 3rd and 11th and have been reported through Monday from the Methow, the Icicle Creek area, the Blewett Pass area, the Nason Ridge area and the Chiwaukum range east of Stevens Pass. It's safe to say this layer is extensive throughout the east slopes of the WA Cascades as well as most areas to our north where Avalanche Canada is highlighting these layers throughout B.C.
A progressive weather pattern this week added about 1-4 over feet of snowfall to the east slope zones. Temperatures warmed above freezing to near or above 6000 feet along the east slopes during the day Thursday. WSW ridgetop winds stayed strong during storms at Mission Ridge as well.
Snow and Avalanche Observations
NWAC pro observer Jeff Ward on Sunday found the reactive persistent weak layer on shaded slopes up to about 5000 feet in the Early Winters drainage. A video by Jeff shows a similar persistent weak layer and a remotely triggered avalanche in the Icicle Creek Drainage near Leavenworth on Monday.
On Monday NWAC pro observer Tom Curtis was on Diamond Head near Blewett Pass in the 4-6000 foot range and found a persistent weak layer at 30-35 cm on north to east aspects with propagation indicated via PST and ECT tests. Tom's video is here.
NCMG on Monday found several persistent weak layers near Goat Peak at 6400' on a north aspect, reactive to column test and sensitive to human triggering.
Remotely triggered avalanches on persistent weak layers were reported in TAY in the Mission Ridge area Monday.
NWAC pro observer Tom Curtis visited the Rainy Creek drainage of the central-east zone (near Jove Peak) Tuesday and traveled from 2000-5800 feet and did not find any signs of lingering persistent weak layers.
NCMG guides travelling adjacent to Delancey Ridge this Friday, saw widespread evidence of the recent natural cycle, with numerous crowns visible throughout the region. Many crowns were estimated to be about 1 meter (3 ft.)
With the recent precipitation and warming temperatures experienced late this week, Thursday and Friday, it remains an uncertainty about just how sensitive the persistent layer currently is. It seems likely there are warring factions working to in opposition with a new moist or denser slab adding stress to that layer, while overall snowpack settlement works to stabilize that layer. Best to treat this as a real threat in tour planning and err on the side of caution.
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, 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
