Avalanche Forecast
Issued: Jan 31st, 2016 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Storm Slabs and Persistent Slabs.
NWAC,
Northwest Avalanche Center
The greatest avalanche problem should be wind slab on lee slopes near and above treeline mainly NW-N-SE facing slopes. Watch for signs of firmer wind transported snow. Snow pits might be the only way to determine if persistent weak layers from earlier in January are still present or reactive in your area.
Summary
Detailed Forecast
Light winds, partly or mostly cloudy weather, a few light snow showers at higher elevations and cool temperatures should be seen along the east slopes on Monday.
This weather should bring a further gradually decreasing avalanche danger on Monday.
The greatest avalanche problem should be wind slab on lee slopes near and above treeline mainly NW-N-SE facing slopes. Watch for signs of firmer wind transported snow.
Watch for storm slab in areas that had rapidly accumulating snowfall. Storm layers that may exist above the new crust will slowly settle but could remain reactive to human triggers mainly near and above treeline.
Continue to evaluate snow and terrain carefully on Monday.
Snow pits might be the only way to determine if persistent weak layers from earlier in January are still present or reactive in your area.
Snowpack Discussion
Weather and Snowpack
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 about January 21st caused up to about a foot of snow in the northeast zone. Freezing rain or rain created a crust in the central east and southeast zones.
Another wet warm front was seen last Wednesday-Thursday. This brought up to another 1.5 feet of snow in the northeast zone and probably created another crust in the central and southeast zones.
This recent warm and wet weather should have cleaned out the January persistent weak layers in some areas, however, until confirmation, we will still list this as an unlikely problem along the east slopes.
A strong occluded front with strong winds crossed the Northwest on Friday. NWAC stations along the east slopes had about 4-14 inches of new snow by Saturday morning.
Some areas have reported a strong bond of the new snow to the new crust and some a poor bond depending on if the new snow arrived after cooling began. There have also been sensitive storm layers reported within the new snow which will take a bit of time to gradually stabilize.
Recent Observations
A report via the NWAC Observations page for Stemilt Basin last Monday near Mission Ridge strongly indicated that persistent weak layers at about 40 and 60 cm were still present east of the crest with test columns failing on isolation and evidence of previous widespread persistent slab avalanches.
NWAC pro-observer Jeff Ward in the Washington Pass vicinity on Thursday reported limited natural avalanches despite the stormy weather.
Another report via the NWAC Observatons page for Friday for the Clara Lake area near Mission Ridge reported strong west winds transporting recent snow.
The North Cascades Guides reported 50 cm of storm snow over a rain crust near Washington Pass on Friday. The storm snow lacked storm layers but test columns failed on isolation at the crust.
NWAC pro-observer Tom Curtis was at Blewett Pass on Saturday and found remnants of the January 3rd and 11th layers but they were not reactive.
The January 11th and 3rd layers were seen at 50 cm and 60 cm in a test pit reported via the NWAC Observations page for Saturday for the Clara Lake area. The January 11th layer gave no result but the January 3rd layer still indicated the potential for propagation.
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
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: 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 1st, 2016 10:00AM
