Avalanche Forecast

Issued: Mar 23rd, 2017 1:21PM

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 Cornices.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

There should be just enough new snow and wind to elevate storm related hazards on Friday and create locally dangerous conditions primarily near and above treeline. Watch for loose wet avalanches below treeline as snow levels creep up during the day. Cornices have recently proven dangerous and unpredictable and capable of triggering very large avalanches.

Summary

Detailed Forecast

A frontal system will slowly move across the Cascades Thursday night with higher new snow accumulations for the volcanoes and potentially Crystal area with lower amounts likely for the Cascade Passes. Skies should remain cloudy on Friday with showers increasing in the afternoon. After a subtle cooling trend Thursday night, snow levels should rise slightly during the day on Friday.  

There should be just enough new snow and wind to elevate storm related hazards on Friday and create locally dangerous conditions primarily near and above treeline.

Recent winds have been mostly S-SW, so firmer wind slab should be found mainly on NW-SE slopes near and above treeline. However, near the Cascade Passes and in the southwest Cascades there have been periods of E-SE winds, so watch for wind slab on all aspects, especially in areas of complex terrain.

New storm snow is expected to bond well to the existing snow surface Thursday night but storm slab may still become locally sensitive in areas that receive rapid accumulation Thursday night or Friday.  

Snow levels will creep up 4000-4500 feet in the northwest Cascades and 4500-5000 feet in the southwest Cascades Friday. Loose wet avalanches won't make the cut to be listed as an avalanche problem but are still possible in the below treeline band. Watch for surface wet snow deeper than a few inches, pinwheels and increasing natural releases. Avoid steeper slopes with terrain traps where small loose wet avalanches could have unintended consequences. 

You need to avoid areas on ridges where there may be a cornice and slopes below cornices! Natural cornice releases and resulting slab avalanches are unpredictable and as seen above in the special note, cornices have triggered major slab avalanches as of late. See a blog post regarding cornices here.

Snowpack Discussion

Weather and Snowpack

Special note:

A major load has been placed on our snowpack over the past two weeks, especially in the north Cascades. This has resulted in many large avalanches and a few very large to historic avalanches. For example, from March 9-19, the NWAC station at Mt Baker received nearly 15 inches of water equivalent! In the alpine above 7000 feet, much or all of this fell as snow along with very strong winds. We are in a situation where hard to predict and low probability but very high consequence avalanches are possible. This situation may stick with us for awhile during the spring. Most recently:

Deep hard slab triggered by a massive natural cornice failure on Ruby Mountain, Sunday afternoon, 3-19. Photo via the NWAC Observations page.

The first week or so of March was very cool and snowy. NWAC stations along the west slopes of the Cascades piled up about 3 to 8 ft of snow with the most at Mt Baker.

The 2nd week of March was equally active with non-stop Pacific frontal systems pummeling the PNW. Unfortunately, these systems delivered far more rain than snow. At least two regional avalanche cycles occurred during the stretch. Significant snowpack consolidation occurred over this period due to rainfall and warmer temperatures. 

Another strong low pressure system brought several inches of rain to the west slopes of the Cascades outside the Cascade Passes Friday 3/17. 2.5 to 4 inches of water was seen at Paradise, Crystal and Mt. Baker respectively, over the 24 hours ending Saturday morning. Another wet snow avalanche cycle was seen in the Mt. Baker area Friday night into early Saturday morning. Rapid cooling with the frontal passage produced 2-8" of snow along the west slopes Saturday 3/18 afternoon and evening. 

Weaker fronts crossed the Northwest on Tuesday and Wednesday at moderate snow levels. Mt. Baker was slightly cooler and wetter during this period, picking up 11 inches of new snow Wednesday night. Most areas saw sunshine Thursday ahead of the next incoming Pacific frontal system. 

Recent Observations

North

The massive natural cornice triggered avalanche in the photo above was on the north side of Ruby Mountain on Sunday 3/19. The avalanche ran over 1400' vertically and a far greater distance laterally. The crown was just below ridge-crest and varied in height from 7-25 ft. This avalanche narrowly missed multiple parties and easily could have been a major avalanche accident! See the link to the observation in the special note above.

NWAC pro-observer Lee Lazzara was in the Mt. Baker backcountry on Thursday. Lee found the newest 10" (25 cm) of snow bonding well to the underlying moist snow. He observed evidence of two natural wind slab releases on east aspects of Mt. Herman near treeline. Storm slabs were possible in isolated areas of steep terrain but not widespread. Mt. Baker pro-patrol had minimal results during control work Thursday morning with a few pockets of stubborn wind slab noted.

Central

NWAC forecasters were on Denny Mountain at Alpental on Tuesday. At about 5000 ft found they found about 8 inches of moist recent snow over the thick and strong rain crust from Saturday 3/18.

The Alpental patrol on Wednesday reported about 2 additional inches of new moist or wet snow on the thick crust with triggered small loose wet avalanches remaining possible.

South

NWAC pro-observer Jeremy Allyn was out in the Paradise area on Thursday and found wind distributed snow variably spread across the compass near treeline. Wind slab was not particularly sensitive but up to 80 cm deep in places. Some windward aspects were scoured to the most recent rain crust. Sunshine on Thursday did not cause loose wet avalanches. Also of note but above our forecast area, serac fall had triggered a larger slab avalanche in the vicinity of the Furher Finger, likely within the last 24-48 hours.    

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, West, 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

Cornices

An icon showing Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind lips of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.

 

Cornices can never be trusted and avoiding them is necessary for safe backcountry travel. Stay well back from ridgeline areas with cornices. They often overhang the ridge edge can be triggered remotely. Avoid areas underneath cornices. Even small Cornice Fall can trigger a larger avalanche and large Cornice Fall can easily crush a human. Periods of significant temperature warm-up are times to be particularly aware.

 

A corniced ridgeline. A large cornice has formed at the top of the ridge. A smaller cornice has formed to the left of the trees from crossloading.

Cornices are easy to identify and are confined to lee and cross-loaded ridges, sub-ridges, and sharp convexities. They are easiest to trigger during periods of rapid growth (new snow and wind), rapid warming, and during rain-on-snow events. Cornices often catch people by surprise when they break farther back onto flatter areas than expected.

Aspects: North, North East, East, South East, North West.

Elevations: Alpine, Treeline.

Likelihood

Unlikely

Expected Size

1 - 2

Valid until: Mar 24th, 2017 1:21PM