Avalanche Forecast

Issued: Dec 28th, 2016 11:15AM

The alpine rating is considerable, the treeline rating is considerable, and the below treeline rating is moderate. Known problems include Wind Slabs, Storm Slabs and Persistent Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Recent or new wind and storm slab is expected on Thursday especially in the northwest zone. Continue to avoid steeper slopes in areas where you still find the 12/17 PWL in snow pits. 

Summary

Detailed Forecast

West southwest flow aloft will carry the next front across the Northwest on Thursday afternoon and night. Winds should increase in most areas on Thursday but significant new snow during the daylight hours is expected mainly in the Olympics and northwest Cascades. A warming trend should also be seen with temperature inversions possible Thursday in the central west and southwest Cascades.

Further snow should be seen in the Washington Cascades Thursday night.

Recent and renewed west to southwest winds make wind slab most likely on northwest to southeast aspects on Thursday. This is most likely by the end of the day in the northwest zone. Wind slab may build further anywhere there is still snow available for wind transport in wind exposed areas.

New storm slab is also most likely in the Northwest zone due to new snow and the warming trend on Thursday. Storm slab in less likely in the central west and southwest zone but lingering storm slab is possible there.

The 12/17 PWL persistent slab problem still warrants attention in the Cascades mainly in areas without the Solstice crust layer. Recent and new loading may make this layer more sensitive to triggering where it is still present. Remember that persistent weak layers are generally involved in larger avalanches. Avoid steeper slopes in areas where you still find this layer in snow pits.

Snowpack Discussion

Weather and Snowpack

Strong storms around the Solstice deposited generally 1 to 2 inches of water equivalent along the west slopes. A period of rain or freezing rain (Snoqualmie) during this storm cycle allowed crust layers to form in the Baker area from 4000-4500 feet, the Passes up to around 5000 feet (Stevens) to 6000 feet (Snoqualmie) and 6000-7000 feet in the Paradise area. The crust is very thin or non existent in the Crystal backcountry. 

A system Thursday and Friday 12/22-12/23 with low snow levels deposited about 20 inches of snow at Mt. Baker with about 2-8 inches accumulating elsewhere along the west slopes including the Passes. 

A strong front and strong west flow aloft was seen over the Olympics and Cascades on Monday and Tuesday. NWAC stations along the west slopes and crest had strong west winds Monday and Tuesday with 1-3 feet of new snow for the 48 hours ending Wednesday morning with a cooling trend.

Recent Observations

The pro-patrol at the Mt Baker ski area Tuesday reported widespread sensitive storm and wind slab releases both in the area during control work and being triggered by the public in the adjacent backcountry. Sounds like there was at least one close call. Initial storm slabs of 8-12 inches became larger as snow loads increased with triggered 2 foot storm slabs reported.

Reports from Alpental patrol Tuesday also reported sensitive storm slabs with good propagation and widespread distribution in mid and upper elevation start zones. These were storm slabs with numerous slides releasing down to the Solstice crust where a weak bond exists. Sensitive triggered slides ranging up to a foot were reported in the adjacent areas outside the ski area as well. Some of these slides initiated in storm layers about 6 inches down before stepping down to the crust another 6 inches or so.

A report via the NWAC Observations tab for Stevens Pass for Tuesday indicated new wind slab forming on most exposed slopes above 5000 feet. The 12/17 PWL was seen in a pit at 60 cm but was unreactive.

NWAC pro-observer Ian Nicholson was at Stevens Pass on Wednesday and reported no signs of instability on east slopes to 5800 feet. There was 2 feet of new snow, with a minor storm layer a Monday/Tuesday interface, over previous snow over the Solstice crust at 65-70 cm. The 12/17 PWL was going away and unlikely to be triggered.

NWAC pro-observer Dallas Glass was in the Bullion Basin and Pickhandle Basin area on Wednesday on most aspect to 6600 feet and reported 25-35 cm of storm snow generally right side up and minor shallow 2-3" storm slab and good skiing. The was visual evidence of wind slab on some cross loaded slopes near treeline.

The rangers at Paradise reported extensive new wind slab near and above treeline. The Solstice crust was seen at 135 cm with good bonds.

Wind effects seen near Paradise on Wednesday. Photo by Seth Waterfall.

 

   

Problems

Wind Slabs

An icon showing 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

Likely

Expected Size

1 - 1

Storm Slabs

An icon showing 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

An icon showing 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: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Dec 29th, 2016 11:15AM