Avalanche Forecast
Issued: Feb 3rd, 2015 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
The problem east of the crest should be small lingering wind slab on Wednesday mainly above and near treeline.
Summary
Detailed Forecast
A weak warm front should lift south to north over the Northwest on Wednesday. Little if any rain or snow is indicated by the latest model runs during the daylight hours on Wednesday but there will be a warming trend.
Watch for possible leftover wind slab on steep lee slopes mainly above treeline. Surface hoar from the end of January could act as a weak layer where it survived and was buried intact.
Deeper persistent weak layers should be unlikely for human triggering in the northeast and central east zone. It will probably be necessary to dig snowpits to determine if this layer is present in these areas.
It should remain cool enough Wednesday east of the crest to limit the problem of loose wet avalanches - unlike west of the crest.
An increasing avalanche danger should be seen starting Wednesday night and Thursday as the first in a series of wet and warm fronts moves to the Northwest.
Snowpack Discussion
Recent Weather: The weekend of January 24th and 25th a warm and wet weather system caused high snow levels, rain, avalanches and snowpack settlement in the Cascades.
Mild weather with sunny days or minor rain or snow was seen from about January 26th to about January 31st. This caused more consolidation, stabilizing and the formation of a thick strong stable surface crust in most areas of the Cascades. Surface hoar formed on shaded and north slopes in some areas of the Cascades in fair weather during this time.
Wetter weather from about February 1 through today produced about 0-8 inches of snow east of the crest.
Recent Observations: NWAC pro-obs and North Cascade Guides reports last week in the northeast and central east zones have the January 15th facet/crust layers at 40 cm-1 m below the surface with the facets starting to turn to rounded grains and not reactive or not releasing in pit tests. So the January 15th facet/crust layers expected in the northeast and central east zones appeared to be strengthening and stabilizing by late January. This problem will be removed soon for east of the crest unless any reports of activity are received in the near future.
NWAC pro-observer Tom Curtis was at 7500 ft on Mt Cashmere in the Wenatchee Mountains on January 28th and found a local facet/crust combo buried on a lee slope next to a ridge crest under 35 cm of thick, surface crust and 1F-P snow. This sort of slab could be very hard for a skier to trigger, but could be possible with a large sudden load, such as a snowmobile.
The North Cascades Guides on February 1st reported slick and often breakable crusts near Washington Pass. Here surface hoar was seen nearer the valley bottoms.
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.
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: Feb 4th, 2015 10:00AM
