Avalanche Forecast
Regions: Cascades - North East.
The main problem Friday is expected to be wind slab in the above and near treeline zones as well as storm slabs of heavy, dense snow or wet surface snow stressing weaker underlying snow at all elevations. Storm slabs may me masked by the drier, colder surface snow.
Detailed Forecast
Another front should arrive later Friday, bringing additional moderate snowfall at lower snow levels. Winds should be strong, especially above treeline. The main concern Friday will be newly forming wind slab on lee exposed slopes, especially NW-N-E facing near and above treeline.Â
With cooling temperatures, the new snow should have a favorable profile, but may mask older unstable storm slabs.
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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 were reported throughout the Cascade east slopes.
A warm front last Thursday caused moderate amounts of snow, up to about a foot, with some mid and lower elevations changing to freezing rain or rain, creating a thin crust.
Warm air arrived Wednesday with freezing levels climbing to near 7000 feet. A strong front moved across the region Thursday, dropping near 1.25 inches of water equivalent by Thursday evening at Washington Pass. Significant warm air associated with this system caused snow levels to likely fluctuate greatly before cooling arrived late Thursday.Â
This recent warm and wet weather should hopefully have cleaned out the earlier January persistent layers, however, until confirmation, we will still list this as an unlikely problem. Â
Snow and Avalanche Observations
NCMG guides travelling adjacent to Delancey Ridge, a week ago 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.
Additional observations in the northeast WA Cascades Monday, looking for the buried January layers, indicated they may have been destroyed by rain a week ago Thursday. Though it's still too early to rule out this layer altogether, though in specific areas it may no longer be reactive.
Extensive professional observations in the Washington Pass area Wednesday, the 27th, identified concern for recent wind and storm slabs.
The heavy loading event Thursday, may have caused a natural cycle, helping to reduce some of these more sensitive layers. However, new wind and storm slab layers have likely been deposited, so the message is extra caution will be required for these new layers to have time to settle and stabilize.
The new snow falling with cooling temperatures Thursday, should form a favorable bond to the older moist or wet surface snow. Â
Avalanche 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: Likely
Expected Size: 1 - 1
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: Possible
Expected Size: 1 - 1
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.
Elevations: Treeline, Below Treeline.
Likelihood: Unlikely
Expected Size: 1 - 2