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Avalanche Forecast

Archived

Jan 29th, 2016–Jan 30th, 2016

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

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The main problem Saturday, is expected to be wind slab in the near and above 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

Cool weather with light snow showers and moderate westerly winds Saturday should allow for an overall slowly decreasing danger. Weak lower density snow that may exist above any crust present in the upper snowpack, especially near or below treeline, will slowly settle, but remain reactive to human triggers Saturday. 

The greatest avalanche problem should be sensitive wind slabs on lee slopes, mainly NW-N-SE facing slopes, especially below ridges, near and above treeline. 

Watch for newly formed cornices along ridges as well. These may be sensitive to human triggers.

Shallow storm slabs may be masked by the lower density surface snow. 

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. 

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.

Another strong, but significantly cooler front, arrived Friday with strong winds and additional moderate snowfall. This latest front has likely built new wind slab conditions near and above treeline with some new storm slab as well.

Snow and Avalanche Observations

Observations in the northeast WA Cascades Monday, looking for the January persistent 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.

There remains a great deal of uncertainty in this zone as to just how the recent warming actually affected the snowpack and how the new colder storm snow is reacting with that interface. A lot of speculation until folks get out and get some good on the ground observations.

Wind slab near and above treeline remain the primary concern as well as isolated storm slab conditions. 

Problems

Wind Slabs

Wind Slab avalanches are the 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.

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). 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.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of 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 Slab.