Avalanche Forecast

Issued: Mar 24th, 2016 11:00AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

The avalanche danger will vary along the west slopes based on recent snowfall and wind affects seen since late Wednesday night. Fresh wind slab will continue to be a primarily hazard on Friday near and above treeline. Loose snow avalanches will be likely on steeper slopes. Human triggered storm slabs are possible as new snow becomes more cohesive with daytime warming on Friday. 

Summary

Detailed Forecast

Orographic showers in cool NW flow aloft Thursday night will wind down Friday morning followed by with partial clearing in the afternoon. Moderate W-NW transport winds will also ease off late Thursday night and Friday morning. The avalanche danger will vary along the west slopes based on recent snowfall and wind affects seen since late Wednesday night.      

Fresh wind slab will continue to be a primarily hazard on Friday near and above treeline. By avoiding obviously wind loaded slopes, you can enjoy some fine late season conditions. Firm wind transported snow and snowpack cracking can be good signs of wind slab layers.

We will flip which loose snow avalanche we identify as the primary problem with more sunshine expected Friday afternoon versus Thursday. Loose wet avalanches will be likely on steeper solar slopes by mid-day or afternoon. Loose dry avalanches should be small but likely on steeper non-solar slopes. Be aware of terrain traps where even a small loose avalanche could have unintended consequences in the wrong terrain.    

Human triggered storm slabs are possible as new snow becomes more cohesive with daytime warming on Friday. Watch for graupel layers received Thursday night to become potential bed surfaces for storm slabs. 

Recently formed cornices have grown large. If traveling along ridgelines, be aware that cornices break much further back than expected and you don't want to go for a ride with a chunk of cornice. Despite the cool weather, cornice releases can be unpredictable during the spring.

Although not listed as an avalanche problem, glide avalanches can release unexpectedly so avoid areas below steep unsupported slopes or rock faces. 

Snowpack Discussion

Weather and Snowpack

A low pressure system crossed the Northwest on Monday and Tuesday. NWAC stations along the west slopes accumulated a few inches above 4000 feet during this time. Light rain was seen on Wednesday in the below treeline band before a cold front swept through Wednesday night with moderate westerly transport winds continuing through Thursday for the south and central Washington Cascades.

Post-frontal snow accumulations varied based on elevation, with only a few inches below 4000 feet and 6-14 inches above. The most snow was received at Paradise and Mt. Baker NWAC stations through Thursday afternoon. New snow since Monday is generally well-bonded to a moist crust buried March 22nd.  

Frequent March storms have built large cornices along ridges in many areas.

The mid and lower snowpack along the west slopes should generally be a stable mix of crusts and layers of moist and rounded snow crystals.

Recent Observations

Snoqualmie Pass avalanche professionals reported loose wet avalanches triggered by light rain running far on Wednesday in paths below treeline. Glide avalanches releasing to the ground off of rock surfaces were also reported near the same Snoqualmie Pass paths Thursday morning. NWAC professional observer Dallas Glass was in Kendall Peak area of Snoqualmie Pass Thursday, and found small but sensitive wind slabs near treeline. Dallas also found loose snow concerns transitioning to a shallow storm slab problem by mid-day with settlement and daytime warming.

The Mt. Baker pro-patrol reported only scattered and small soft storm slabs in area Thursday morning.  Stevens Pass pro-patrol reported strong west winds had redistributed the new snow, but that the new wind slab was stubborn and generally nonreactive to ski cuts. 

Powerful glide avalanches remain possible in isolated terrain features such as on steep slopes with smooth bed surfaces and especially where the slabs are unsupported from below. These avalanches are not predictable in the sense that they are not tied to short term warming or rain events. We want to highlight the low likelihood-high consequence of this sporadic but ongoing avalanche hazard during this wet winter.  

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.

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

1 - 1

Loose Wet

An icon showing Loose Wet

Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

 

Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.

 

Several loose wet avalanches, and lots of pinwheels and roller balls.

Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.

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

Elevations: All elevations.

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

Valid until: Mar 25th, 2016 11:00AM